Journal articles on the topic 'High angle diffraction theory'
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1
Amali, Adam, and Peter Rez. "Theory of Lattice Resolution in High-angle Annular Dark-field Images." Microscopy and Microanalysis 3, no. 1 (January 1997): 28–46. http://dx.doi.org/10.1017/s1431927697970021.
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Abstract: The theoretical interpretation of lattice resolution in high-angle annular dark-field images produced in a scanning transmission electron microscope (STEM) has been a subject of controversy. A first-order perturbation theoretical analysis is presented here, which shows that the contrast in the image arises from large-angle multiphonon, incoherent scattering, which is atomic number dependent. The lattice resolution is a consequence of coherently filling the objective aperture, and dynamical elastic diffraction preceding the large-angle multiphonon scattering is not a necessary requirement. Elastic scattering to the higher order Laue zone (HOLZ) is also shown to be negligible, compared with the incoherent scattering. Calculations from application of the theory are also presented. They show that lattice images formed using the high-angle annular dark-field detector are sensitive to atomic number and are relatively insensitive to defocus. Although high-angle annular dark-field lattice imaging appears to be simple, scattering into the high-angle detector can only be approximately described by an incoherent imaging model.
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Zhang, Shan Wen, and Jian Xin Ying. "Optimization of the Two Parameters of Classical Blaze Grating in Littrow Mount." Advanced Materials Research 535-537 (June 2012): 1332–36. http://dx.doi.org/10.4028/www.scientific.net/amr.535-537.1332.
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On the base of electromagnetic theory of gratings, we analyse the effects of both of two important parameters of classical blaze grating in Littrow mount to diffraction efficiency, and the two parameters are optimized for obtaining highest diffraction efficiency. It is presented that not only blaze angle but apex angle affect diffraction efficiency. The investigation has been performed on the design and fabrication of classical blaze grating which has high diffraction efficiency.
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Ahn, B. R., and N. J. Kim. "Calculation of many-beam dynamic electron diffraction without high-energy approximation." Proceedings, annual meeting, Electron Microscopy Society of America 54 (August 11, 1996): 128–29. http://dx.doi.org/10.1017/s0424820100163101.
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High energy approximation in dynamic theory of electron diffraction involves some intrinsic problems. First, the loss of theoretical strictness makes it difficult to comprehend the phenomena of electron diffraction. Secondly, it is difficult to believe that the approximation is reasonable especially in the following cases: 1) when accelerating voltage is not sufficiently high, 2) when the specimen is thick, 3) when the angle between the surface normal of the specimen and zone axis is large, and 4) when diffracted beam with large diffraction angle is included in the calculation. However, until now the method to calculate the many beam dynamic electron diffraction without the high energy approximation has not been proposed. For this reason, the authors propose a method to eliminate the high energy approximation in the calculation of many beam dynamic electron diffraction. In this method, a perfect crystal with flat surface was assumed. The method was applied to the calculation of [111] zone axis CBED patterns of Si.
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Podorov, S. G., N. N. Faleev, K. M. Pavlov, D. M. Paganin, S. A. Stepanov, and E. Förster. "A new approach to wide-angle dynamical X-ray diffraction by deformed crystals." Journal of Applied Crystallography 39, no. 5 (September 12, 2006): 652–55. http://dx.doi.org/10.1107/s0021889806025696.
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A new approach is proposed for X-ray dynamical diffraction theory in distorted crystals. The theory allows one to perform dynamical diffraction simulations between Bragg peaks for non-ideal crystals, using a simple approach of two distorted waves. It can be directly applied for reciprocal-space simulation. The formalism is used to analyse high-resolution X-ray diffraction data, obtained for an InSb/InGaSb/InSb/InAs superlattice grown on top of a GaSb buffer layer on a (001) GaSb substrate.
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Dragoi, Danut, and Alexandru Dragoi. "Modeling of energy-dispersive X-ray diffraction for high-symmetry crystal orientation." Acta Crystallographica Section A Foundations and Advances 75, no. 1 (January 1, 2019): 63–70. http://dx.doi.org/10.1107/s2053273318013864.
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The methods for X-ray crystal orientation are rapidly evolving towards versatility, fewer goniometry measurements, automation, high accuracy and precision. One method that attracts a lot of attention is energy-dispersive X-ray diffraction (EDXRD) which is based on detecting reflections from crystallographic planes in a crystal at fixed angles of a parallel polychromatic X-ray incident beam. In theory, an EDXRD peak can move in a diffraction pattern as a function of a crystallographic plane d-spacing and its orientation relative to a fixed direction in space can change also. This is equivalent to the possibility of measuring the orientation of single crystals. The article provides a modeling for the EDXRD method whose main feature is the nonmoving crystal in the sense of traditional goniometry where the angle measurements of diffracting planes are a must. The article defines the equation of orientation for the method and shows the derivation in great detail. It is shown that the exact solutions of the equations can be obtained using the generalized reduced gradient method, a mathematical subroutine that is implemented in Excel software. The significance and scientific impact of the work are discussed along with the validated tested results.
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Feng, Ke, Jin Song Li, and Ling Guo. "Simulation of Flat-Top Focus Shaping Using Effect of Polarization in High Numerical Apertures Optical Systems." Advanced Materials Research 571 (September 2012): 175–79. http://dx.doi.org/10.4028/www.scientific.net/amr.571.175.
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Flat-top focus shaping using effect of polarization is investigated theoretically by vector diffraction theory. We demonstrate how to make a properly selected polarization component to achieve a flat-top focus in high numerical apertures optical systems, hence achieving a potential goal of beam shaping. The results show that the values of the polarization angle corresponding to different numerical apertures (NA)values for a flat-top focus and the polarization angle increases on increasing NA for achieving a flat-top focal spot, but the flat-top focal pattern will become -sensitive when NA is larger than 1. One will have to add a diffractive optical element to achieve a flat-top focal spot in NA larger than 1 system.
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Le, Dac Tuyen, and Dinh Lam Vu. "High-Order Angle and Polarization Resolved Reflection of Artificial Opal Photonic Crystal." Communications in Physics 28, no. 3 (November 14, 2018): 247. http://dx.doi.org/10.15625/0868-3166/28/3/10580.
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We present angle resolved reflection measurements showing the polarization dependence of photonic band gap in artificial opal photonic crystals. The SiO2 opals were prepared using thermal-assisted cell method. The observation of well-defined diffraction pattern indicates the samples with high quality. The reflection measurements were analyzed in the high energy region up to a/l = 1.6. It is shown that the diffraction peaks depend on s- and p-polarized light illumination. The polarization anisotropy effect due to symmetric properties of opal structure. The experiment results agree fairly well with calculated photonic band structure and are also discussed with predictions based on group theory. Angular reflection has implications in polarized light scattering in plasmonic structures and metamaterials and is also useful in applications like nano scale polarization splitters and lasers.
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Toraya, H., and J. Yoshino. "A Polycrystalline Thin Film Diffractometer for Asymmetric Diffraction Using Parallel Beam and High Resolution Parallel Slits." Advances in X-ray Analysis 39 (1995): 165–70. http://dx.doi.org/10.1154/s0376030800022576.
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A polycrystalline thin film diffractometer for asymmetric diffraction at a fixed incident angle using parallel beam and high-resolution-parallel slits has been developed. Well collimated CuK α1 incident beam was obtained by coupling of a rotating anode generator with a channel-cut crystal monochromator. The diffracted beam was analyzed with parallel slits with 0.17° angular aperture. The observed profiles of CeO2 standard powder sample were analyzed by using an individual profile fitting technique. The minimum full-width at half-maximum (FWHM) was 0.15° in the 20 range of 40° ∼ 70°, and the variation of FWHM with 20 was almost independent of the incident angle. Intensity in asymmetric diffraction was enhanced compared to that in the symmetric 0-20 scan, and its variation was well in accordance with the theory. The use of the parallel slits compromizes the intensity and resolution for polycrystalline thin film diffraction in the present setup, but gives the high quality of diffraction data for structural study.
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Wang, Z. L. "Coupled thermal diffuse-atomic inner shell scattering in electron diffraction." Proceedings, annual meeting, Electron Microscopy Society of America 52 (1994): 994–95. http://dx.doi.org/10.1017/s042482010017270x.
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In electron diffraction patterns, diffuse scattering at high angles is primarily generated by phonon, or thermal diffuse, scattering (TDS). Techniques were introduced to acquire the electron energy-loss spectra (EELS) of high-angle thermal-diffuse-scattered electrons (TDS-EELS) in a transmission electron microscope (TEM). With regards to the scattering mechanism, the TDS-EELS core ionization edge intensity was believed to be generated primarily by TDS - single electron, double-inelastic electron scattering processes. It was concluded from experimental data that the signal from coupled phonon - atomic inner shell excitations is stronger than that from atomic inner shell excitation alone. A formal dynamical theory is presented in this paper to illustrate the theoretical basis of the experimental observations. The theory can be applied to calculate the diffraction patterns of inelastically double-scattered electrons and the signal intensity observed in TDS-EELS.TDS is actually a statistically averaged, quasi-elastic scattering of the electrons by the crystal lattice of different thermal vibration configurations.
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Muldarisnur, M., and F. Marlow. "Polarization angular-resolved transmission spectroscopy of opal films." Journal of Nonlinear Optical Physics & Materials 25, no. 02 (June 2016): 1650015. http://dx.doi.org/10.1142/s0218863516500156.
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In this paper, we investigated the angular behavior of light transmission through opal films over a broad range of wavelengths and angles. The opal films were prepared using the capillary deposition method (CDM). The observation of many well-defined diffraction peaks indicates that the CDM results in opal films with high quality. Peaks overlapping at normal incidence split when samples are rotated. The angular shift of these peaks was found to satisfy the kinematical diffraction theory very well. Furthermore, the variation of intensity with incident angle can be interpreted in terms of a simplified dynamical diffraction theory. Moreover, the presence of two differently oriented domains in CDM-made opal films is essential in discussing the measured spectra. These domains can be assigned to parallel microscopic stripes of ABC and ACB type fcc structures. Angular-resolved spectroscopy with polarized incident light shows interesting polarization dependence of light propagation inside opals.
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Haun, M. J., Y. H. Lee, H. A. McKinstry, and L. E. Cross. "High Temperature X-Ray Diffraction Study of Sol-Gel Derived Pb(ZrxTi1-x)O3 Powders." Advances in X-ray Analysis 30 (1986): 473–81. http://dx.doi.org/10.1154/s0376030800021637.
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The lattice parameters of sol-gel derived Pb(ZrxTi1-x)O3 powders were determined as a function of temperature using a microcomputer automated Picker x-ray diffractometer. The spontaneous strain was calculated from the lattice parameters, and used with other experimental data to develop a thermodynamic phenomenological theory for the PZT solid solution system. A second tricritical point, where the transition changes front first to second order, was found to occur near or possibly at the morphotropic phase boundary between the tetragonal and high temperature rhombohedral phases. Using an equation derived from the theory, the spontaneous tilt angle of the oxygen octahedra in the low temperature rhombohedral phase was calculated from the experimental spontaneous strain data.
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Bleloch, A. L., A. Howie, and M. Y. Lanzerotti. "Convergent-Beam RHEED of GaAs(110) in a STEM: Theory and Experiment." Proceedings, annual meeting, Electron Microscopy Society of America 48, no. 2 (August 12, 1990): 394–95. http://dx.doi.org/10.1017/s0424820100135575.
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A number of approaches to the theory of reflection high energy electron diffraction (RHEED) have been published over the years. There have been, however, few direct comparisons with experiment. In particular, the authors are not aware of calculations of the type of convergent beam RHEED (CBRHEED) pattern with very large convergence angle presented here. (By the principle of reciprocity, parallel illumination and a large collection angle in STEM is equivalent to a large angle convergent probe in a conventional transmission electron microscope (CTEM)) Large angle CBRHEED patterns provide a convenient method for investigating the origin and sensitivity of the so called surface resonance lines. These lines are the prominent features marked on Figure 1. There have been a number of attempts to explain this phenomenon. Initial results of calculations on GaAs patterns are presented together with experimental results.The approach is an extension of that of Collela in that Bloch wave amplitudes in three dimensions are computed with suitable reflection boundary conditions.
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Benmore, C. J. "A Review of High-Energy X-Ray Diffraction from Glasses and Liquids." ISRN Materials Science 2012 (November 14, 2012): 1–19. http://dx.doi.org/10.5402/2012/852905.
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This paper summarizes the scientific trends associated with the rapid development of the technique of high-energy X-ray diffraction over the past decade pertaining to the field of liquids, glasses, and amorphous materials. The measurement of high-quality X-ray structure factors out to large momentum transfers leads to high-resolution pair distribution functions which can be directly compared to theory or combined with data from other experimental techniques. The advantages of combining highly penetrating radiation with low angle scattering are outlined together with the data analysis procedure and formalism. Also included are advances in high-energy synchrotron beamline instrumentation, sample environment equipment, and an overview of the role of simulation and modeling for interpreting data from disordered materials. Several examples of recent trends in glass and liquid research are described. Finally, directions for future research are considered within the context of past and current developments in the field.
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Mendenhall, Marcus H., David Black, Donald Windover, and James P. Cline. "Polarization effects of X-ray monochromators modeled using dynamical scattering theory." Acta Crystallographica Section A Foundations and Advances 77, no. 4 (May 27, 2021): 262–67. http://dx.doi.org/10.1107/s2053273321003879.
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The difference in the diffracted intensity of the σ- and π-polarized components of an X-ray beam in powder diffraction has generally been treated according to equations based on dipole scattering, also known as kinematic X-ray scattering. Although this treatment is correct for powders and post-sample analyzers known to be of high mosaicity, it does not apply to systems configured with nearly perfect crystal incident-beam monochromators. Equations are presented for the polarization effect, based on dynamical diffraction theory applied to the monochromator crystal. The intensity of the π component relative to the σ component then becomes approximately proportional to |cos 2θm| rather than to cos22θm, where θm is the Bragg diffraction angle of the monochromator crystal. This changes the predicted intensities of X-ray powder diffraction patterns produced on instruments with incident-beam monochromators, especially in the regions far from 2θ = 90° in the powder pattern. Experimental data, based on well known standard reference materials, are presented, confirming that the dynamical polarization correction is required when a Ge 111 incident-beam monochromator is used. The dynamical correction is absent as an option in the Rietveld analysis codes with which the authors are familiar.
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Viladot, Désirée, Joaquim Portillo, Mauro Gemí, Stavros Nicolopoulos, and Núria Llorca-Isern. "Hafnium-Silicon Precipitate Structure Determination in a New Heat-Resistant Ferritic Alloy by Precession Electron Diffraction Techniques." Microscopy and Microanalysis 20, no. 1 (October 30, 2013): 25–32. http://dx.doi.org/10.1017/s1431927613013627.
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AbstractThe structure determination of an HfSi4 precipitate has been carried out by a combination of two precession electron diffraction techniques: high precession angle, 2.2°, single pattern collection at eight different zone axes and low precession angle, 0.5°, serial collection of patterns obtained by increasing tilts of 1°. A three-dimensional reconstruction of the associated reciprocal space shows an orthorhombic unit cell with parameters a = 11.4 Å, b = 11.8 Å, c = 14.6 Å, and an extinction condition of (hkl) h + k odd. The merged intensities from the high angle precession patterns have been symmetry tested for possible space groups (SG) fulfilling this condition and a best symmetrization residual found at 18% for SG 65 Cmmm. Use of the SIR2011 direct methods program allowed solving the structure with a structure residual of 18%. The precipitate objects of this study were reproducibly found in a newly implemented alloy, designed according to molecular orbital theory.
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Mitura, Zbigniew, and Sergei L. Dudarev. "Algorithms for determining the phase of RHEED oscillations." Journal of Applied Crystallography 48, no. 6 (November 28, 2015): 1927–34. http://dx.doi.org/10.1107/s1600576715020415.
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Oscillations of reflection high-energy electron diffraction (RHEED) intensities are computed using dynamical diffraction theory. The phase of the oscillations is determined using two different approaches. In the first, direct, approach, the phase is determined by identifying the time needed to reach the second oscillation minimum. In the second approach, the phase is found using harmonic analysis. The two approaches are tested by applying them to oscillations simulated using dynamical diffraction theory. The phase of RHEED oscillations observed experimentally is also analysed. Experimental data on the variation of the phase as a function of the glancing angle of incidence, derived using the direct method, are compared with the values computed using both the direct and harmonic methods. For incident-beam azimuths corresponding to low-symmetry directions, both approaches produce similar results.
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Klahn, Emil Andreasen, Emil Damgaard-Møller, Lennard Krause, Iurii Kibalin, Arsen Gukasov, Shalini Tripathi, Abinash Swain, Maheswaran Shanmugam, and Jacob Overgaard. "Quantifying magnetic anisotropy using X-ray and neutron diffraction." IUCrJ 8, no. 5 (September 1, 2021): 833–41. http://dx.doi.org/10.1107/s2052252521008290.
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In this work, the magnetic anisotropy in two iso-structural distorted tetrahedral Co(II) complexes, CoX 2tmtu2 [X = Cl(1) and Br(2), tmtu = tetramethylthiourea] is investigated, using a combination of polarized neutron diffraction (PND), very low-temperature high-resolution synchrotron X-ray diffraction and CASSCF/NEVPT2 ab initio calculations. Here, it was found consistently among all methods that the compounds have an easy axis of magnetization pointing nearly along the bisector of the compression angle, with minute deviations between PND and theory. Importantly, this work represents the first derivation of the atomic susceptibility tensor based on powder PND for a single-molecule magnet and the comparison thereof with ab initio calculations and high-resolution X-ray diffraction. Theoretical ab initio ligand field theory (AILFT) analysis finds the d xy orbital to be stabilized relative to the d xz and d yz orbitals, thus providing the intuitive explanation for the presence of a negative zero-field splitting parameter, D, from coupling and thus mixing of d xy and d_{x^2 - y^2}. Experimental d-orbital populations support this interpretation, showing in addition that the metal–ligand covalency is larger for Br-ligated 2 than for Cl-ligated 1.
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Nagarajan, Anitharaj, Shusuke Hara, Hiroaki Satoh, Aruna Priya Panchanathan, and Hiroshi Inokawa. "Angle-Sensitive Detector Based on Silicon-On-Insulator Photodiode Stacked with Surface Plasmon Antenna." Sensors 20, no. 19 (September 28, 2020): 5543. http://dx.doi.org/10.3390/s20195543.
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We present a pixel-level angle sensitive detector composed of silicon-on-insulator (SOI) photodiode (PD) stacked with a gold surface plasmon (SP) antenna to affect the direction of the incoming light. The surface plasmons are excited in the grating-type SP antenna and enhance the diffraction efficiency of the grating. The diffracted light is coupled strongly with the propagation light in the SOI waveguide when the phase matching condition is satisfied. The phase matching takes place at a specific angle of light incidence, and the discrimination of the light based on the incident angle is achieved. As spatial patterns in the polar coordinate of the elevation-azimuth angles (θ, ϕ) of the incident light, we present the phase matching condition theoretically, the absorption efficiency in the SOI by simulation, and also the quantum efficiency of the SOI PD experimentally for different SP antennas of one-dimensional (1D) line-and-space (L/S) and two-dimensional (2D) hole array gratings under various polarization angles. 1D grating offers a polarization sensitive angle detection and 2D grating exhibits angle detection in two orthogonal directions, enabling a polarization independent angle sensitivity. A good agreement among the theory, simulation, and experiment are attained. The proposed device features relatively high quantum efficiency as an angle-sensitive pixel (ASP) and gives wider opportunities in applications such as three-dimensional (3D) imaging, depth-of-field extension, and lensless imaging.
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Jesson, D. E., and S. J. Pennycook. "Atomic Imaging of Crystals using Large-Angle Electron Scattering in STEM." Proceedings, annual meeting, Electron Microscopy Society of America 48, no. 1 (August 12, 1990): 74–75. http://dx.doi.org/10.1017/s0424820100179129.
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It is well known that conventional atomic resolution electron microscopy is a coherent imaging process best interpreted in reciprocal space using contrast transfer function theory. This is because the equivalent real space interpretation involving a convolution between the exit face wave function and the instrumental response is difficult to visualize. Furthermore, the crystal wave function is not simply related to the projected crystal potential, except under a very restrictive set of experimental conditions, making image simulation an essential part of image interpretation. In this paper we present a different conceptual approach to the atomic imaging of crystals based on incoherent imaging theory. Using a real-space analysis of electron scattering to a high-angle annular detector, it is shown how the STEM imaging process can be partitioned into components parallel and perpendicular to the relevant low index zone-axis.It has become customary to describe STEM imaging using the analytical treatment developed by Cowley. However, the convenient assumption of a phase object (which neglects the curvature of the Ewald sphere) fails rapidly for large scattering angles, even in very thin crystals. Thus, to avoid unpredictive numerical solutions, it would seem more appropriate to apply pseudo-kinematic theory to the treatment of the weak high angle signal. Diffraction to medium order zero-layer reflections is most important compared with thermal diffuse scattering in very thin crystals (<5nm). The electron wave function ψ(R,z) at a depth z and transverse coordinate R due to a phase aberrated surface probe function P(R-RO) located at RO is then well described by the channeling approximation;
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Darlington, C. N. W., and K. S. Knight. "High-temperature phases of NaNbO3 and NaTaO3." Acta Crystallographica Section B Structural Science 55, no. 1 (February 1, 1999): 24–30. http://dx.doi.org/10.1107/s010876819800963x.
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The high-temperature phases of the perovskites sodium niobate, NaNbO3, and sodium tantalate, NaTaO3, have been re-examined using the high-resolution powder diffractometer HRPD at the ISIS neutron spallation source; the two materials show the same sequence of phases with tilted octahedra. Diffraction patterns were measured every 5 K allowing structural changes with temperature within a single phase to be determined for the first time. Previous structure determinations within one phase had been performed at a single temperature only. The octahedra are tilted about pseudocubic \langle100\rangle directions and are also deformed; the magnitude of the deformation is shown to be proportional to the square of the angle of tilt as expected from a phenomenological theory applied to such transitions. The structures of NaNbO3 between 753 and 793 K and of NaTaO3 below 758 K are not as reported in the literature.
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Gajdardziska-Josifovska, M., J. K. Weiss, and J. M. Cowley. "Energy-filtered convergent beam RHEED rocking curves from cleaved (100) surface of MgO." Proceedings, annual meeting, Electron Microscopy Society of America 49 (August 1991): 626–27. http://dx.doi.org/10.1017/s0424820100087446.
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Reflection high energy electron diffraction (RHEED) has been used extensively to observe changes in surface reconstructions by analyzing the geometry of the RHEED pattern and to monitor growth of layers in MBE systems by measuring the changes of the intensity of the specular spot with time. RHEED is also capable of yielding the structure of the surface by using dynamical diffraction theory to analyze experimental reflection rocking curves. These rocking curves trace the change in the intensity of the RHEED spots as a function of the angle of incident illumination. They are equivalent to the intensity vs. voltage curves (I-V) obtained in low energy electron diffraction (LEED) which have been used to determine most of the known surface structures. The LEED I-V curves are energy filtered and the theoretical calculations consider only the elastically-scattered electrons. The RHEED theory is also developed only for elastic scattering, but the experimental measurements so far have not been energy filtered.
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Wu, Taihui, Jianshe Ma, Chengchen Wang, Haibei Wang, and Ping Su. "Full-Color See-Through Three-Dimensional Display Method Based on Volume Holography." Sensors 21, no. 8 (April 11, 2021): 2698. http://dx.doi.org/10.3390/s21082698.
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We propose a full-color see-through three-dimensional (3D) display method based on volume holography. This method is based on real object interference, avoiding the device limitation of spatial light modulator (SLM). The volume holography has a slim and compact structure, which realizes 3D display through one single layer of photopolymer. We analyzed the recording mechanism of volume holographic gratings, diffraction characteristics, and influencing factors of refractive index modulation through Kogelnik’s coupled-wave theory and the monomer diffusion model of photopolymer. We built a multiplexing full-color reflective volume holographic recording optical system and conducted simultaneous exposure experiment. Under the illumination of white light, full-color 3D image can be reconstructed. Experimental results show that the average diffraction efficiency is about 53%, and the grating fringe pitch is less than 0.3 μm. The reconstructed image of volume holography has high diffraction efficiency, high resolution, strong stereo perception, and large observing angle, which provides a technical reference for augmented reality.
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Wang, Chengchen, Jianshe Ma, Hongxu Kao, Taihui Wu, and Ping Su. "Wide-Band High Concentration-Ratio Volume-Holographic Grating for Solar Concentration." Sensors 20, no. 21 (October 26, 2020): 6080. http://dx.doi.org/10.3390/s20216080.
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Efficient and low-cost solar-energy collection has become the focus of many research works. This paper proposes a recording method and an experimental verification of a wide-band, large-angle, and high concentration-ratio volume-holographic grating for solar concentration. We applied the Kogelnik coupled-wave theory and photopolymer diffusion model to analyse the formation mechanism and influencing factors on the diffraction efficiency of monochromatic volume-holographic gratings. We design and construct a three-color laser-interference system to record three monochromatic volume-holographic gratings. The best recording conditions are determined by experiment and simulation. A trichromatic volume-holographic grating is obtained by gluing the three monochromatic gratings together. The experimental results show that the trichromatic volume-holographic grating with a working angle of 6.7° and a working band of visible light has a light concentration ratio of 149.2 under an illumination of the combined recorded three-color beams, and that under sunlight is 27.2. We find that the proposed trichromatic volume-holographic grating for light concentration offers the advantages of wide band and high light concentration ratio, which provide a reference for solar concentration.
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Peng, Ding, and Philip N. H. Nakashima. "Identification of the impurity phase in high-purity CeB6 by convergent-beam electron diffraction." Acta Crystallographica Section A Foundations and Advances 75, no. 3 (March 11, 2019): 489–500. http://dx.doi.org/10.1107/s2053273319000354.
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The rare earth hexaborides are known for their tendency towards very high crystal perfection. They can be grown into large single crystals of very high purity by inert gas arc floating zone refinement. The authors have found that single-crystal cerium hexaboride grown in this manner contains a significant number of inclusions of an impurity phase that interrupts the otherwise single crystallinity of this prominent cathode material. An iterative approach is used to unequivocally determine the space group and the lattice parameters of the impurity phase based on geometries of convergent-beam electron diffraction (CBED) patterns and the symmetry elements that they possess in their intensity distributions. It is found that the impurity phase has a tetragonal unit cell with space group P4/mbm and lattice parameters a = b = 7.23 ± 0.03 and c = 4.09 ± 0.02 Å. These agree very well with those of a known material, CeB4. Confirmation that this is indeed the identity of the impurity phase is provided by quantitative CBED (QCBED) where the very close match between experimental and calculated CBED patterns has confirmed the atomic structure. Further confirmation is provided by a density functional theory calculation and also by high-angle annular dark-field scanning transmission electron microscopy.
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Kirste, L., K. M. Pavlov, S. T. Mudie, V. I. Punegov, and N. Herres. "Analysis of the mosaic structure of an ordered (Al,Ga)N layer." Journal of Applied Crystallography 38, no. 1 (January 19, 2005): 183–92. http://dx.doi.org/10.1107/s0021889804030675.
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The mosaic structure of an (Al,Ga)N layer grown on (0001) sapphire showing natural ordering was studied by high-resolution X-ray diffraction (HRXRD) reciprocal-space mapping. The direction-dependent mosaicity of the layer has been elaborated using maps of symmetrical and asymmetrical reflections. The reciprocal-lattice points show significant broadening depending on the direction in reciprocal space, the diffraction order and the reflection type (fundamental or superstructural). The evaluation followed two paths: (i) a procedure based on the Williamson–Hall plot and (ii) a new approach based on the statistical diffraction theory (SDT). Here, the transformed Takagi equations were implemented for the simulation of the reciprocal-space maps (RSM) for symmetrical and asymmetrical reflections. The reconstruction comprised the mosaic block size, their average rotation angle and the spatial distribution of some components of the microdistortion tensor. The results based on the SDT modelling agree well with those obtained by the Williamson–Hall method, while providing a higher degree of precision and detail.
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FERRARIO, M., V. FUSCO, M. MIGLIORATI, and L. PALUMBO. "EMITTANCE DEGRADATION DUE TO WAKE FIELDS IN A HIGH BRIGHTNESS PHOTOINJECTOR." International Journal of Modern Physics A 22, no. 23 (September 20, 2007): 4214–34. http://dx.doi.org/10.1142/s0217751x07037779.
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Wake fields effects in addition to space charge forces may have an important impact during the emittance compensation process in a high brightness photo-injector. To study this effect we developed an upgraded version of the Homdyn code including off axis beam dynamics and wake fields. Homdyn describes a bunch as a uniformly charged cylinder, divided in cylindrical slices; in the upgraded version each slice's centroid can be transversally displaced from the nominal axis thus inducing wake fields. When the bunch is short as compared to the beam pipe radius, wake fields for a single cavity are calculated using methods of diffraction theory; instead we use, for a periodic collection of cavities, an asymptotic wake field obtained numerically at SLAC and then fitted to a simple function. As a first application we studied and verified a correction scheme for the SPARC photo-injector to control the bunch trajectory and angle at the entrance of the undulator. The correction scheme consists of a number of steering magnets and beam position monitors placed along the photo-injector. Two different steering approaches are analyzed and the emittance degradation is studied. The code demonstrates the steering positions and number do correct the bunch's orbit and angle and gives good results concerning the emittance degradation. The emittance and energy spread degradation due to wake fields in the emittance meter experiment is also discussed.
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Kotmool, Komsilp, Bing Li, Sudip Chakraborty, Thiti Bovornratanaraks, Wei Luo, Ho-kwang Mao, and Rajeev Ahuja. "High pressure-induced distortion in face-centered cubic phase of thallium." Proceedings of the National Academy of Sciences 113, no. 40 (September 21, 2016): 11143–47. http://dx.doi.org/10.1073/pnas.1612468113.
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The complex and unusual high-pressure phase transition of III-A (i.e. Al, Ga, and In) metals have been investigated in the last several decades because of their interesting periodic table position between the elements having metallic and covalent bonding. Our present first principles-based electronic structure calculations and experimental investigation have revealed the unusual distortion in face-centered cubic (f.c.c.) phase of the heavy element thallium (Tl) induced by the high pressure. We have predicted body-centered tetragonal (b.c.t) phase at 83 GPa using an evolutionary algorithm coupled with ab initio calculations, and this prediction has been confirmed with a slightly distorted parameter (2 × a − c)/c lowered by 1% using an angle-dispersive X-ray diffraction technique. The density functional theory (DFT)-based calculations suggest that s–p mixing states and the valence-core overlapping of 6s and 5d states play the most important roles for the phase transitions along the pathway h.c.p→f.c.c.→b.c.t.
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Chiang, An-Chung, Yuan-Yao Lin, Shou-Tai Lin, and Yen-Yin Lin. "A Broadband High-Diffraction-Efficiency Electro-Optic Bragg Deflector Based on Monolithic Dual-Grating Periodically-Poled Lithium Niobate." Photonics 8, no. 7 (June 28, 2021): 242. http://dx.doi.org/10.3390/photonics8070242.
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Electro-optic (EO) Bragg deflectors have been extensively used in a variety of applications. Recent developments show that bandwidths and deflection efficiencies, as well as angular bandwidths, would significantly limit the utilization of EO Bragg deflectors, especially for applications which need strong focusing, such as intra-cavity applications. In this paper, we introduce a broadband EO Bragg deflector based on periodically-poled lithium niobate with a monolithic dual-grating design. We analyzed the deflection properties of this device by using a modified coupled wave theory and showed that this device can be still efficient for a small beam radius under strong focusing, whereas a single-grating one becomes very inefficient. Using a 1064-nm laser beam with a 100-μm beam radius, we obtained a 74% deflection efficiency with a 190-V bias voltage with a 0.5-mm-thick and 7.5-mm-long dual-grating sample. The acceptance angle for the Bragg condition of this device is as large as a few tens of mrad. The potential bandwidth of this device exceeds 500 nm if the proper operation region is chosen.
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Michielon de Souza, Sergio, Hidembergue Ordozgoith da Frota, Daniela Menegon Trichês, Angsula Ghosh, Puspitapallab Chaudhuri, Marta Silva dos Santos Gusmao, Aercio Filipe Franklim de Figueiredo Pereira, Mariana Couto Siqueira, Kleber Daum Machado, and Joao Cardoso de Lima. "Pressure-induced polymorphism in nanostructured SnSe." Journal of Applied Crystallography 49, no. 1 (February 1, 2016): 213–21. http://dx.doi.org/10.1107/s1600576715023663.
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The pressure-induced phase transitions in nanostructured SnSe were investigated using angle-dispersive X-ray diffraction in a synchrotron source along with first-principles density functional theory (DFT) calculations. The variation of the cell parameters along with enthalpy calculations for pressures up to 18 GPa have been considered. Both the experimental and the theoretical approaches demonstrate a phase transition at around 4 GPa. Below 8.2 GPa the X-ray diffraction patterns were fitted using the Rietveld method with space groupPnma(No. 62). The lattice parameters and atomic positions for the above-mentioned symmetry were used in DFT calculations of thermodynamic parameters. The enthalpy calculations with the computationally optimized structure and the proposedPnmastructure of SnSe were compatible. The variations of the cell volume for the high-pressure phases are described by a third-order Birch–Murnaghan equation of state.
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Han, Meng, Xiaotong Guan, Moshe Einat, Wenjie Fu, and Yang Yan. "Investigation on a 220 GHz Quasi-Optical Antenna for Wireless Power Transmission." Electronics 10, no. 5 (March 9, 2021): 634. http://dx.doi.org/10.3390/electronics10050634.
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This paper investigates a 220 GHz quasi-optical antenna for millimeter-wave wireless power transmission. The quasi-optical antenna consists of an offset dual reflector, and fed by a Gaussian beam that is based on the output characteristics of a high-power millimeter-wave radiation source-gyrotron. The design parameter is carried on by a numerical code based on geometric optics and vector diffraction theory. To realize long-distance wireless energy transmission, the divergence angle of the output beam must be reduced. Electromagnetic simulation results show that the divergence angle of the output beam of the 5.6 mm Gaussian feed source has been significantly reduced by the designed quasi-optical antenna. The far-field divergence angle of the quasi-optical antenna in the E plane and H plane is 1.0596° and 1.0639°, respectively. The Gaussian scalar purity in the farthest observation field (x = 1000 m) is 99.86%. Thus, the quasi-optical antenna can transmit a Gaussian beam over long-distance and could be used for millimeter-wave wireless power transmission.
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Baba, Toshihiko, Tomohiko Asatsuma, and Takashi Matsumoto. "Negative Refraction in Photonic Crystals." MRS Bulletin 33, no. 10 (October 2008): 927–30. http://dx.doi.org/10.1557/mrs2008.201.
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AbstractPhotonic crystals are multidimensional periodic gratings, in which the light propagation is dominated by Bragg diffraction that appears to be refraction at the flat surfaces of the crystals. The refraction angle from positive to negative, perfectly or only partially obeying Snell's law, can be tailored based on photonic band theory. Negative refraction enables novel prism, collimation, and lens effects. Because photonic crystals usually consist of two transparent media, these effects occur at absorption-free frequencies, affording significant design flexibility for free-space optics. The photonic-crystal slab, a high-index membrane with a two-dimensional airhole array, must be carefully designed to avoid unwanted reflection and diffraction. Light focusing based on negative refraction forms a parallel image of a light source, facilitating optical couplers and condenser lenses for wavelength demultiplexing. A compact wavelength demultiplexer can be designed by combining the prism and lens effects.
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Barbee, Troy W. "Multilayer Optics for the Soft X-Ray and Extreme Ultraviolet." MRS Bulletin 15, no. 2 (February 1990): 37–45. http://dx.doi.org/10.1557/s0883769400060449.
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This article considers a particular class of optic microstructure — multilayers for soft x-ray (SXR, 100 eV[124 Å] to 3,000 eV[4.1 Å] and extreme ultraviolet (EUV, 15 eV[800 Å] to 100 eV[124 Å] optics. Multilayers are manmade vapor-deposited depth-periodic layered micro-stractures of high enough quality to be considered synthetic crystals. Layers of two materials, A and B, having significant differences in their scattering powers for x-rays (electron densities) and of uniform thicknesses ta and tb are combined to form a sample of uniform in-depth period d0 ( = ta + tb). These microstructures are of scientific and technological significance since the high angle of incidence SXR and EUV reflectivities of single film reflectors is ~10−4) and since only a limited number of naturally occurring or synthetic large lattice constant materials efficiently diffraction reflect long wavelength radiation (SXR and EUV).Multilayer microstructure based SXR and EUV optics are direct analogues to standard quarterwave stacks applied at longer wavelengths with the requirement that absorption must be included. They are also analogous to atomic crystalline lattices in that one multilayer period is equivalent to one lattice plane. Thus, the multilayer structure forms a superlattice that diffraction reflects incident radiation. Multilayer diffraction may be modeled using x-ray scattering theory and obeys the refraction-corrected Bragg's law of crystal lattice diffraction. As will be discussed later, many factors determine the character of the multilayer response to an incident spectrum.
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Berk, N. F., and K. A. Hardman-Rhyne. "Characterization of alumina powder using multiple small-angle neutron scattering. I. Theory." Journal of Applied Crystallography 18, no. 6 (December 1, 1985): 467–72. http://dx.doi.org/10.1107/s002188988501072x.
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Microstructural parameters of high-purity alumina powder are determined quantitatively throughout the bulk of the material using small-angle neutron scattering techniques. A unified theoretical and experimental approach for analyzing multiple scattering data is developed to obtain values for particle size, volume fraction and surface area. It is shown how particle size and volume fraction can be measured in a practical way from SANS data totally dominated by incoherent multiple scattering (`beam broadening'). The general phase-shift dependence of single-particle scattering is incorporated into the multiple scattering formalism, and it is also shown that the diffractive limit (small phase shift) applies even for phase shifts as large as unity (particle radii of order 1 μm). The stability of the Porod law against multiple scattering and the phase-shift scale are described, a useful empirical formula for analysis of beam broadening data is exhibited, and the applicability of the formulations to polydispersed systems is discussed.
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Martin, Andrew V. "The correlation of single-particle diffraction patterns as a continuous function of particle orientation." Philosophical Transactions of the Royal Society B: Biological Sciences 369, no. 1647 (July 17, 2014): 20130329. http://dx.doi.org/10.1098/rstb.2013.0329.
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A statistical model for X-ray scattering of a non-periodic sample to high angles is introduced. It is used to calculate analytically the correlation of distinct diffraction measurements of a particle as a continuous function of particle orientation. Diffraction measurements with shot-noise are also considered. This theory provides a general framework for a deeper understanding of single particle imaging techniques used at X-ray free-electron lasers. Many of these techniques use correlations as a measure of diffraction-pattern similarity in order to determine properties of the sample, such as particle orientation.
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Muniz, Francisco Tiago Leitão, Marcus Aurélio Ribeiro Miranda, Cássio Morilla dos Santos, and José Marcos Sasaki. "The Scherrer equation and the dynamical theory of X-ray diffraction." Acta Crystallographica Section A Foundations and Advances 72, no. 3 (April 21, 2016): 385–90. http://dx.doi.org/10.1107/s205327331600365x.
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The Scherrer equation is a widely used tool to determine the crystallite size of polycrystalline samples. However, it is not clear if one can apply it to large crystallite sizes because its derivation is based on the kinematical theory of X-ray diffraction. For large and perfect crystals, it is more appropriate to use the dynamical theory of X-ray diffraction. Because of the appearance of polycrystalline materials with a high degree of crystalline perfection and large sizes, it is the authors' belief that it is important to establish the crystallite size limit for which the Scherrer equation can be applied. In this work, the diffraction peak profiles are calculated using the dynamical theory of X-ray diffraction for several Bragg reflections and crystallite sizes for Si, LaB6and CeO2. The full width at half-maximum is then extracted and the crystallite size is computed using the Scherrer equation. It is shown that for crystals with linear absorption coefficients below 2117.3 cm−1the Scherrer equation is valid for crystallites with sizes up to 600 nm. It is also shown that as the size increases only the peaks at higher 2θ angles give good results, and if one uses peaks with 2θ > 60° the limit for use of the Scherrer equation would go up to 1 µm.
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Pint, B. A., A. J. Garratt-Reed, and L. W. Hobbs. "The effect of time and temperature on the segregation of foreign ions to grain boundaries in growing α-Al2O3 scales." Proceedings, annual meeting, Electron Microscopy Society of America 51 (August 1, 1993): 950–51. http://dx.doi.org/10.1017/s0424820100150587.
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The addition of small quantities of certain oxygen-active or “reactive” elements (REs) such as Y, Zr, Hf and Ce produces a tremendous improvement in the oxidation behavior of both chromia and alumina-forming alloys. However, few of the hypotheses that have been put forth to explain this phenomenon have been confirmed by careful characterization of the oxidation process.The segregation theory emphasizes the role of RE segregation to oxide grain boundaries and has been successful in correlating the presence of Y and Zr segregants to changes in the oxidation mechanism (thus reducing the oxidation rate) of chromia and alumina scales. Improvements in oxide adhesion have been attributed to the segregation of REs to the metal-alumina interface on doped β-NiAl and FeCrAl alloys. All of these studies employed FEG-STEM equipped with XEDS in order to detect the interfacial segregation.The RE is also found in the scale as RE-rich particles which have been observed after high temperature oxidation using XEDS on SEM and FEG-STEM and detected using glancing-angle (0.5° incident angle) x ray diffraction.
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Gang, Zhu, Xiong Xian-cai, Zhong Xian-xin, and Yang Yan. "A Novel Spectrometer for Measuring Laser-Produced Plasma X-Ray in Inertial Confinement Fusion." Science and Technology of Nuclear Installations 2012 (2012): 1–5. http://dx.doi.org/10.1155/2012/804217.
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In the experimental investigations of inertial confinement fusion, the laser-produced high-temperature plasma contains very abundant information, such as the electron temperature and density, ionization. In order to diagnose laser-plasma distribution in space and evolution in time, an elliptical curved crystal spectrometer has been developed and applied to diagnose X-ray of laser-produced plasma in 0.2~2.46 nm region. According to the theory of Bragg diffraction, four kinds of crystal including LiF, PET, MiCa, and KAP were chosen as dispersive elements. The distance of crystal lattice varies from 0.4 to 2.6 nm. Bragg angle is in the range of 30°~67.5°, and the spectral detection angle is in 55.4°~134°. The curved crystal spectrometer mainly consists of elliptical curved crystal analyzer, vacuum configuration, aligning device, spectral detectors and three-dimensional microadjustment devices. The spectrographic experiment was carried out on the XG-2 laser facility. Emission spectrum of Al plasmas, Ti plasma, and Au plasmas have been successfully recorded by using X-ray CCD camera. It is demonstrated experimentally that the measured wavelength is accorded with the theoretical value.
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Yang, Guang, and Ji Zhou. "The Annealing Effect on Optical Properties of Silver Copper Alloy Films." Applied Mechanics and Materials 320 (May 2013): 329–35. http://dx.doi.org/10.4028/www.scientific.net/amm.320.329.
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In this paper, the dielectric functions of Ag-30.3at.%Cu alloy films upon the annealing effect were investigated by variable angle spectroscopy ellipsometry (VASE). The silver copper alloy films were deposited onto p-type silicon (100) substrate by direct current (DC) magnetron sputtering. With the increase of annealing temperature from 100 °C to 300 °C, the image part of the permittivity for Ag-30.3at.%Cu is significantly decreased in the wavelength below ~500 nm. The structure and surface topography of the alloy films were characterized using high resolution scanning electron microscopy (HR-SEM) and X-ray diffraction (XRD). The effective medium theory (EMA) has been utilized for the treating of surface roughness. The dielectric functions can be manipulated by changing the annealing temperature. Key words: Dielectric functions; silver copper alloy films; magnetron sputtering
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Brzustowicz, Michael R., and Axel T. Brunger. "X-ray scattering from unilamellar lipid vesicles." Journal of Applied Crystallography 38, no. 1 (January 19, 2005): 126–31. http://dx.doi.org/10.1107/s0021889804029206.
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An improved small-angle X-ray scattering (SAXS) method for determining asymmetric lipid bilayer structure in unilamellar vesicles is presented. From scattering theory, analytic expressions are derived for the bilayer form factor over flat and spherical geometries, assuming the lipid bilayer electron density to be composed of a series of Gaussian shells. This is in contrast to both classic diffraction and Guinier hard-shell SAXS methods which, respectively, are capable only of ascertaining symmetric bilayer structure and limited-resolution asymmetric structure. Using model fitting and direct calculation of the form factor, using only one equation, an asymmetric electron density profile of the lipid vesicle is obtained with high accuracy, as well as the average radius. The analysis suggests that the inner leaflet of a unilamellar lipid vesicle is `rougher' than the outer one.
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Perrillat, J. P. "Kinetics of high-pressure mineral phase transformations using in situ time-resolved X-ray diffraction in the Paris-Edinburgh cell: a practical guide for data acquisition and treatment." Mineralogical Magazine 72, no. 2 (April 2008): 683–95. http://dx.doi.org/10.1180/minmag.2008.072.2.683.
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AbstractSynchrotron X-ray diffraction (XRD) is a powerful technique to study in situ and in real-time the structural and kinetic processes of pressure-induced phase transformations. This paper presents the experimental set-up developed at beamline ID27 of the ESRF to perform time-resolved angle dispersive XRD in the Paris-Edinburgh cell. It provides a practical guide for the acquisition of isobaric-isothermal kinetic data and the construction of transformation-time plots. The interpretation of experimental data in terms of reaction mechanisms and transformation rates is supported by an overview of the kinetic theory of solid-solid transformations, with each step of data processing illustrated by experimental results of relevance to the geosciences. Reaction kinetics may be affected by several factors such as the sample microstructure, impurities or differential stress. Further high-pressure kinetic studies should investigate the influence of such processes, in order to acquire kinetic information more akin to natural or technological processes.
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Jana, Sanjay Kr, Saptarsi Ghosh, Syed Mukulika Dinara, Apurba Chakraorty, and D. Biswas. "Comparative High-Resolution X-Ray Diffraction Analysis of GaN/AlGaN Heterostructure on Al2O3 and Si (111) Substrate Grown by Plasma Assisted Molecular Beam Epitaxy." MRS Proceedings 1754 (2015): 129–34. http://dx.doi.org/10.1557/opl.2015.401.
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AbstractThe work presents a comparative study on GaN/AlGaN type-II heterostructures grown on c-plane Al2O3 and Si (111) substrates by Plasma Assisted Molecular Beam Epitaxy. The in-depth structural characterizations of these samples were performed by High-Resolution X-Ray Diffraction, X-ray Reflectivity and Field Emission Scanning Electron Microscopy. The in-plane and out-of plane strains were determined from measured c- and a-lattice parameters of the epilayers from reciprocal space mapping of both symmetric triple axis (002) and asymmetric grazing incidence (105) double axis mode. The mosaicity parameters like tilt and correlation lengths were also calculated from reciprocal space mapping. Moreover, the twist angle was measured from skew symmetric off axis scan of (102), (103), and (105) planes along with (002) symmetric plane. The defect density were measured from the full width at half maxima of skew symmetric scan of (002) and (102) reflection planes. Also, the strained states of all the layers were analyzed and corresponding Al mole fraction was calculated based on anisotropic elastic theory. The thicknesses of the layers were measured from simulation of the nominal structure by fitting with X-ray Reflectivity experimental curves and also by comparing with cross sectional Field Emission Scanning Electron Microscopy micrographs.
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Aubert, Emmanuel, Florence Porcher, Mohamed Souhassou, and Claude Lecomte. "Characterization of intra-framework and guest/host interactions in the AlPO4-15 molecular sieve by charge-density analysis." Acta Crystallographica Section B Structural Science 59, no. 6 (November 25, 2003): 687–700. http://dx.doi.org/10.1107/s0108768103017075.
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The electron-density distribution of AlPO4-15 has been determined using high-resolution single-crystal X-ray diffraction, and the topological properties of the charge density have been calculated using the `atoms in molecules' (AIM) theory. Analysis of the topological properties at the bond critical points has been used to characterize the interactions within the framework, and between the framework and the extra-framework species (ammonium ions and water molecules), and to define atomic properties, such as volume and net charges, uniquely. A comparison between procrystal and multipolar representations of the density was performed in order to explore to what extent the former representation is likely to reflect the interactions in the solid. Correlation with geometrical properties (P—O and Al—O bond lengths, and Al—O—P angle) is found for topological charges obtained from the multipolar model, but not for the results from the procrystal representation.
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Grieb, Tim, Knut Müller, Emmanuel Cadel, Andreas Beyer, Marco Schowalter, Etienne Talbot, Kerstin Volz, and Andreas Rosenauer. "Simultaneous Quantification of Indium and Nitrogen Concentration in InGaNAs Using HAADF-STEM." Microscopy and Microanalysis 20, no. 6 (September 30, 2014): 1740–52. http://dx.doi.org/10.1017/s1431927614013051.
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AbstractTo unambiguously evaluate the indium and nitrogen concentrations in InxGa1−xNyAs1−y, two independent sources of information must be obtained experimentally. Based on high-resolution scanning transmission electron microscopy (STEM) images taken with a high-angle annular dark-field (HAADF) detector the strain state of the InGaNAs quantum well is determined as well as its characteristic HAADF-scattering intensity. The strain state is evaluated by applying elasticity theory and the HAADF intensity is used for a comparison with multislice simulations. The combination of both allows for determination of the chemical composition where the results are in accordance with X-ray diffraction measurements, three-dimensional atom probe tomography, and further transmission electron microscopy analysis. The HAADF-STEM evaluation was used to investigate the influence of As-stabilized annealing on the InGaNAs/GaAs sample. Photoluminescence measurements show an annealing-induced blue shift of the emission wavelength. The chemical analysis precludes an elemental diffusion as origin of the energy shift—instead the results are in agreement with a model based on an annealing-induced redistribution of the atomic next-neighbor configuration.
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MA, YIQUN. "Why the Bloch Wave Solution for Reflection Fails on the Au(110) Surface." Proceedings, annual meeting, Electron Microscopy Society of America 48, no. 2 (August 12, 1990): 372–73. http://dx.doi.org/10.1017/s0424820100135460.
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The Bloch wave method has been widely used for interpreting reflection high energy electron diffraction (RHEED) patterns and the consistency between the theory and high energy electron reflection (HEER) experiments has been claimed by different authors. The recent rigorous investigation on the consistency between the Bloch wave method and the multislice approach due to Cowley and Moodie in the reflection case for Au(001) surface has also provided a clear theoretical proof for the validity of the Bloch wave method in reflection case. However, a severe deviation of the Bloch wave solution for the Au(110) surface in the reflection case from the stabilized solution of its multislicing via the multislice iteration has recently revealed by the BMCR method (Bloch wave + Multislice Combined for Reflection).Fig.1 shows the results calculated for the Au(110) surface using the BMCR method. The incident angle is 30mRad and the absorption is included by taking the imaginary potential as 10% of the real potential in both the Bloch wave and multislice calculation.
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Rozhdestvenskaya, Ira V., Enrico Mugnaioli, Marco Schowalter, Martin U. Schmidt, Michael Czank, Wulf Depmeier, and Andreas Rosenauer. "The structure of denisovite, a fibrous nanocrystalline polytypic disordered `very complex' silicate, studied by a synergistic multi-disciplinary approach employing methods of electron crystallography and X-ray powder diffraction." IUCrJ 4, no. 3 (March 8, 2017): 223–42. http://dx.doi.org/10.1107/s2052252517002585.
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Denisovite is a rare mineral occurring as aggregates of fibres typically 200–500 nm diameter. It was confirmed as a new mineral in 1984, but important facts about its chemical formula, lattice parameters, symmetry and structure have remained incompletely known since then. Recently obtained results from studies using microprobe analysis, X-ray powder diffraction (XRPD), electron crystallography, modelling and Rietveld refinement will be reported. The electron crystallography methods include transmission electron microscopy (TEM), selected-area electron diffraction (SAED), high-angle annular dark-field imaging (HAADF), high-resolution transmission electron microscopy (HRTEM), precession electron diffraction (PED) and electron diffraction tomography (EDT). A structural model of denisovite was developed from HAADF images and later completed on the basis of quasi-kinematic EDT data byab initiostructure solution using direct methods and least-squares refinement. The model was confirmed by Rietveld refinement. The lattice parameters area= 31.024 (1),b= 19.554 (1) andc= 7.1441 (5) Å, β = 95.99 (3)°,V= 4310.1 (5) Å3and space groupP12/a1. The structure consists of three topologically distinct dreier silicate chains,viz. two xonotlite-like dreier double chains, [Si6O17]10−, and a tubular loop-branched dreier triple chain, [Si12O30]12−. The silicate chains occur between three walls of edge-sharing (Ca,Na) octahedra. The chains of silicate tetrahedra and the octahedra walls extend parallel to thezaxis and form a layer parallel to (100). Water molecules and K+cations are located at the centre of the tubular silicate chain. The latter also occupy positions close to the centres of eight-membered rings in the silicate chains. The silicate chains are geometrically constrained by neighbouring octahedra walls and present an ambiguity with respect to theirzposition along these walls, with displacements between neighbouring layers being either Δz=c/4 or −c/4. Such behaviour is typical for polytypic sequences and leads to disorder along [100]. In fact, the diffraction pattern does not show any sharp reflections withlodd, but continuous diffuse streaks parallel toa* instead. Only reflections withleven are sharp. The diffuse scattering is caused by (100) nanolamellae separated by stacking faults and twin boundaries. The structure can be described according to the order–disorder (OD) theory as a stacking of layers parallel to (100).
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Dere, E. Gözde, Hemant Sharma, Richard M. Huizenga, Giusseppe Portale, Wim Bras, Vitaliy Bliznuk, Jilt Sietsma, and S. Erik Offerman. "Formation of (Fe,Cr) carbides and dislocation structures in low-chromium steel studiedin situusing synchrotron radiation." Journal of Applied Crystallography 46, no. 1 (December 21, 2012): 181–92. http://dx.doi.org/10.1107/s0021889812043531.
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The evolution of the size distribution of (Fe,Cr) carbides and the dislocation structure in low-chromium steel is studied during quenching and rapid heating byin situsmall-angle X-ray scattering (SAXS). The two-dimensional SAXS patterns consist of streaks on top of an isotropic SAXS signal. The evolution of the size distribution of the (Fe,Cr) carbides during heat treatment is determined from the isotropic component of the SAXS patterns. The isotropic part of the SAXS patterns shows that, after austenitization and quenching to room temperature, the average precipitate radius is 4.74 nm and the dispersion parameter for the lognormal size distribution is 0.33. Subsequent rapid heating to 823 K results in an average precipitate size of 5.25 nm and a dispersion parameter of 0.26. Bright-field transmission electron microscopy and high-resolution transmission electron microscopy reveal the nearly spherical morphology of the precipitates. The microstructural evolution underlying the increase in the average precipitate size and the decrease in the dispersion parameter after heating to and annealing at 823 K is probably that at room temperature two types of precipitates are present,i.e.(Fe,Cr)23C6and (Fe,Cr)7C3precipitates according to thermodynamic calculations, and at 823 K only (Fe,Cr)7C3precipitates are present. Additional measurements have been carried out on a single crystal of ferrite containing (Fe,Cr) carbides by combining three-dimensional X-ray diffraction (3DXRD) and SAXS during rotation of the specimen at room temperature, in order to investigate the origin of the streaks at low angles in the SAXS pattern. From simulations based on the theory of SAXS from dislocations, it is shown that the measured streaks, including the spottiness, in the two-dimensional SAXS patterns correspond to a dislocation structure of symmetric low-angle tilt boundaries, which in turn corresponds to the crystallographic orientation gradient in the single crystal of ferrite as measured by 3DXRD microscopy.
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Correia, Carlos M., Olivier Fauvarque, Charlotte Z. Bond, Vincent Chambouleyron, Jean-François Sauvage, and Thierry Fusco. "Performance limits of adaptive-optics/high-contrast imagers with pyramid wavefront sensors." Monthly Notices of the Royal Astronomical Society 495, no. 4 (June 9, 2020): 4380–91. http://dx.doi.org/10.1093/mnras/staa843.
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ABSTRACT Advanced adaptive-optics (AO) systems will likely utilize pyramid wavefront sensors (PWFSs) over the traditional Shack–Hartmann sensor in the quest for increased sensitivity, peak performance and ultimate contrast. Here, we explain and quantify the PWFS theoretical limits as a means to highlight its properties and applications. We explore forward models for the PWFS in the spatial-frequency domain: these prove useful because (i) they emanate directly from physical-optics (Fourier) diffraction theory; (ii) they provide a straightforward path to meaningful error breakdowns; (iii) they allow for reconstruction algorithms with $O (n\, \log(n))$ complexity for large-scale systems; and (iv) they tie in seamlessly with decoupled (distributed) optimal predictive dynamic control for performance and contrast optimization. All these aspects are dealt with here. We focus on recent analytical PWFS developments and demonstrate the performance using both analytic and end-to-end simulations. We anchor our estimates on observed on-sky contrast on existing systems, and then show very good agreement between analytical and Monte Carlo performance estimates on AO systems featuring the PWFS. For a potential upgrade of existing high-contrast imagers on 10-m-class telescopes with visible or near-infrared PWFSs, we show, under median conditions at Paranal, a contrast improvement (limited by chromatic and scintillation effects) of 2×–5× when just replacing the wavefront sensor at large separations close to the AO control radius where aliasing dominates, and of factors in excess of 10× by coupling distributed control with the PWFS over most of the AO control region, from small separations starting with an inner working angle of typically 1–2 λ/D to the AO correction edge (here 20 λ/D).
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Antao, Sytle M. "Quartz: structural and thermodynamic analyses across the α ↔ β transition with origin of negative thermal expansion (NTE) in β quartz and calcite." Acta Crystallographica Section B Structural Science, Crystal Engineering and Materials 72, no. 2 (April 1, 2016): 249–62. http://dx.doi.org/10.1107/s205252061600233x.
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The temperature variation,T, of the crystal structure of quartz, SiO2, from 298 to 1235 K was obtained with synchrotron powder X-ray diffraction data and Rietveld structure refinements. The polymorphic transformation fromP3221 (low-T, α quartz) toP6222 (high-T, β quartz) occurs at a transition temperature,Ttr= 847 K. TheTvariations of spontaneous strains and several structural parameters are fitted to an order parameter,Q, using Landau theory. The change in Si atom coordinate, Six, givesTtr−Tc= 0.49 K, which indicates an α ↔ β transition that is weakly first order and nearly tricritical in character (Q4∝T). Strains give higherTtr−Tcvalues (≃ 7 K). Other fitted parameters are the oxygen Ozcoordinate, Si—Si distance, Si—O—Si and φ angles, and intensity of the (111) reflection,I111. In α quartz, the Si—Si distance increases withTbecause of cation repulsion, so the Si—O—Si angle increases (and φ decreases) and causes the thermal expansion of the framework structure that consists of corner-sharing distorted rigid SiO4tetrahedra. The Si—Si distances contract withTand cause negative thermal expansion (NTE) in β quartz because of increasing thermal librations of the O atom in the Si—O—Si linkage that occur nearly perpendicular to the Si—Si contraction. In calcite, CaCO3, the short Ca—Ca distance expands withT, but the next-nearest Ca—Ca distance, which is of equal length to theaaxis, contracts withTand causes NTE along theaaxis. The thermal librations of the atoms in the rigid CO3group increase withTalong thecaxis.
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Somashekar, R., I. H. Hall, and P. D. Carr. "The determination of crystal size and disorder from X-ray diffraction photographs of polymer fibres. 1. The accuracy of determination of Fourier coefficients of the intensity profile of a reflection." Journal of Applied Crystallography 22, no. 4 (August 1, 1989): 363–71. http://dx.doi.org/10.1107/s0021889889004085.
Full textAbstract:
Methods which determine the number and disorder of lattice planes in a crystal from the Fourier cosine coefficients of the intensity profile of an X-ray reflection use only the low harmonics and require that the coefficients be normalized so that the zero harmonic is unity. Experimentally, the profiles can only be recorded over a smaller range of scattering angle than required by the theory, and it is necessary to subtract background, which is likely to be estimated with considerable error, before determining the coefficients. It is shown that with polymer fibres this causes serious errors in the normalization, and in the values of those low harmonics used in the size and disorder determination, and prevents reliable values being obtained. Methods which avoid normalization and use only high harmonics are needed. It is shown that disorder may be obtained in such a way, but not size, for which low-order normalized coefficients are essential. A method of extrapolation is described and tested which enables the accurate high harmonics to be used to improve the estimates of the low ones. Whilst this will yield more reliable values of crystal size than are obtainable from existing methods, the accuracy depends entirely on the validity of the extrapolation, which cannot be tested in many cases of interest.
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Gunka, Piotr, Yu-Sheng Chen, and Janusz Zachara. "Dispersed Lone Electron Pairs in Cubic Polymorph of Arsenic(III) Oxide." Acta Crystallographica Section A Foundations and Advances 70, a1 (August 5, 2014): C1347. http://dx.doi.org/10.1107/s2053273314086525.
Full textAbstract:
Stereoactive lone electron pairs (LEPs) situated on arsenic atomic cores in arsenic(III) compounds are responsible for a number of their interesting structural features. Thanks to their presence, arsenic may be involved in weak interactions such as As···O and As···X (X stands for halogen). It is the directional As···O interactions that cause the sphere-like As4O6molecules to pack in a diamondoid network in the cubic polymorph of As2O3, arsenolite, rather than in the closest-packed-sphere-type structure as P4O6molecules do in phosphorus(III) oxide. Recently, Gibbs and co-workers have determined the charge density distribution (CDD) in As2O3polymorphs by means of periodicab initiocalculations and have analysed its topological features.[1] Matsumoto et al. investigated the role of LEPs in As, Sb and Bi sesquioxides.[2] We have carried out a very precise high-angle diffraction experiments on arsenolite single crystals using both laboratory X-ray source and synchrotron X-ray radiation. The obtained diffraction data have been analysed utilising the Hansen-Coppens multipolar model and X-ray constrained wavefunction refinement. CDD resulting from both models has been analysed within the QTAIM (Quantum Theory of Atoms in Molecules) framework. The structural activity and localisation of LEPs has been compared with the predictions of bond valence vector model.[3] The computations performed by Gibbs et al. are critically evaluated by comparison with the experimental results extended by our own calculations in Gaussian basis sets. The obtained results suggest arsenic LEPs are dispersed into three distinct regions in arsenic atomic core vicinity.