Dissertations / Theses on the topic 'Crystal Structure - Transition Metal Oxides'

Thesis (Ph. D.)--Ohio State University, 2003.
Title from first page of PDF file. Document formatted into pages; contains xx, 180 p.; also includes graphics. Includes bibliographical references (p. 139-145).

Owing to the scarcity of lithium, discovering alternatives for lithium in rechargeable batteries is a critical requirement. Sodium is an ideal candidate for this purpose. The absence of exceptional cathode materials in sodium-ion batteries is a bottleneck in realizing the above objective. This study focused on synthesizing novel transition metal oxide cathode materials for sodium-ion batteries and improving their electrochemical properties. The outcomes of this study emphasized the importance of novel material compositions as well as the role of smart cation substitution, anion redox reaction, electrochemical activation and the effect of the combination of strategies in achieving next-generation high-capacity cathodes.

The commercialization of lithium-ion batteries has played a pivotal role in the development of consumer electronics and electric vehicles. In recent years, much research has focused on the development and modification of the active materials of electrodes to obtain higher energies for a broader range of applications. High voltage spinel materials including LiNi_0.5Mn_1.5O_4-δ (LNMO) have been considered as promising cathode materials to address the increasing demands for improved battery performance due to their high operating potential, high energy density, and stable cycling lifetimes. In an effort to elucidate fundamental structure-property relationships, this thesis explores the tunable properties of single-crystal LNMO. Utilizing facile molten salt synthesis methods, the structural and electronic properties of LNMO can be well controlled. Chapter 2 of this thesis focuses on uncovering the effect of molten salt synthesis parameters including molten salt composition and synthetic temperature on the materials properties. A range of imaging, microscopic, and spectroscopic techniques are used to characterize structural and electronic properties which are investigated in tandem with electrochemical performance. Results indicate the Mn oxidation state is highly dependent on synthesis temperature and can dictate performance, while the molten salt composition strongly influences the particle morphology. In Chapter 3, we explore the concept of utilizing LNMO as a tunable support for heterogeneous metal nanocatalysts, where alteration of the support structure and electronics can have an influence on catalytic properties due to unique support effects. Ultimately, this work illustrates the tunable nature of single-crystal LNMO and can inform the rational design of LNMO materials for energy applications.
M.S.
The development of lithium-ion batteries has been fundamental to the expansion and prevalence of consumer electronics and electric vehicles in the twenty-first century. Despite their ubiquity, there is an ongoing drive by researchers to address the limitations and improve the quality and performance of lithium ion batteries. Much research has focused on altering the composition, structure, or properties of electrodes at the materials level to design higher achieving batteries. A fundamental understanding of how composition and structure effect battery performance is necessary to progress toward better materials. This thesis focuses on investigating the properties of LiNi_0.5Mn_1.5O_4-δ (LNMO). LNMO material is considered a promising cathode material to meet the increasing consumer demands for improved battery performance. Through the synthesis methods, the shape of individual particles and the global electronic properties of LNMO can be tuned. In this work, specific synthesis parameters are systematically tuned and the properties of the resultant LNMO materials are explored. Electrochemical testing also evaluates the performance of the materials and offers insights into how they may fair in real battery systems. In an effort to potentially recycle spent battery materials, LNMO is also utilized as a catalyst support. Alteration of shape and electronic properties of the LNMO support can influence the catalytic properties, or the ability of the material to enhance the rate of a chemical reaction. Overall, this thesis explores how LNMO can be tuned and utilized for different applications. This work provides insights for understanding LNMO properties and direction for the development of future battery materials.

Transition metal oxides adopt a wide variety of crystal structures and display a diverse range of physical phenomena from Mott insulating states to electron-nematics to unconventional superconductivity. Detailed understanding of these states and how they may be manipulated by structural modifications requires both precise structural knowledge and in-depth physical property measurements using as many techniques over as wide a range of phase space as possible. In the work described in this thesis a range of transition metal oxides were studied using high-resolution powder neutron diffraction and detailed low-temperature physical property measurements. The quaternary barium orthotellurates Ba₂NiTeO₆, Ba₂CuTeO₆ and Ba₂ZnTeO₆ belong to an almost unstudied family of materials. The development of procedures for synthesizing large single crystals has facilitated the investigation of interesting new anisotropic magnetic states in the Cu and Ni systems and the existence of a possible structural phase transition in the Zn-based compound. YMnO₃ is a multiferroic with improper ferrielectricity. The study of the high-temperature structural phases described in this thesis has led to the identification both of the transition path to the ferrielectric state and the identification of an isostructural phase transition within the ferrielectric phase. BiFe₀.₇Mn₀.₃O₃ is also a multiferroic material but with proper ferroelectricity. The investigation of the structural phases of this compound have provided confirmation of the high-temperature phases with the reassignment of the symmetry of the highest-temperature phase which is intriguingly different to that of the unsubstituted material. Finally, an investigation of the electronic structures of the high conductivity delafossites PdCoO₂ and PdCrO₂ using micro-cantilever torque magnetometry measurements of quantum oscillations is described. This has resolved the warping of the Fermi surface of PdCoO₂ and given insights into the complicated Fermi surface of the itinerant antiferromagnet PdCrO₂.

The electronic structure of the oxides Pb02, In203, PbO and Bh03 have been studied using high resolution X-ray photoelectron spectroscopy (XPS), ultra-violet photoemission spectroscopy (UPS), hard X-ray photoelectron spectroscopy (HXPS), and X-ray emission (XES). These techniques are supported by band structure calculations carried out within the framework of density functional theory (DFT). It has been demonstrated using UPS, XPS and HXPS that the metallic nature l Trinity Term 2008. David J. Payne Submitted for the degree of Doctor of Philosophy Trinity College, Oxford. ,. ~!II ItI \IfI [l I ofPb02 arises from the occupation of conduction band states above the Fermi level of stoichiometric Pb02, most likely arising from oxygen vacancy defects. XPS and HXPS studies of the Pb 4/ core line show that strong satellites are observed at an energy consistent with the plasmon frequency observed in electron energy loss spectra. These satellites are not present in UPS measurements of the Pb 5d core line. It has been shown using XPS, HXPS, XES and DFT that the fundamental band gap for In203 is much smaller than the often quoted value of 3.75 eV. The fundamental band gap is direct, but direct optical transitions give minimal intensity until 0.81 eV below the valence band maximum. The results are consistent with a fundamental bandgap in the region of2.67eV. Structural distortions in post-transition metal oxides are often explained in terms of the influence of sp hybrid 'lone pairs'. XPS and XES measurements on a- PbO and a-Bh03 show that this model must be revised. A high density of metal 6s states is observed at the bottom of the valence band, and would therefore be unable to directly participate in hybridization with metal 6p states which lie above the valence band. These measurements are consistent with the results of density functional theory calculations.

Compose etudiees : axmnoy, zh::(2)o a = na, k, rb, cs, mn (phyllomanganates) et mno::(1,93-1,98), zh::(2)o (type mno::(2) nongamma epsilon ). Pour les phyllomanganates, l'etude des variations des parametres cristallins et de la teneur en eau z en fonction de la temperature t et de l'hygrometrie rh confirme la structure lamellaire. Mesures de conductivite en fonction de t et rh. La structure des composes de type mno::(2) nongamma epsilon est basee sur une intercroissance de reseaux de type rutile et ramsdellite

Spin and orbital orderings are amongst the most important phenomena in the solid state chemistry of oxides. Physical property and powder neutron and X-ray diffraction measurements are reported for a range of mostly low dimensional ternary transition metal oxides which display spin or orbital order. Extensive studies of the physical properties and crystal structure of In2VO5 are reported. The structure of this material consists of one dimensional zig-zag chains of orbitally ordered S = 1/2 V4+. Magnetic susceptibility measurements show an unusual crossover from dominant ferromagnetic (θ = 17 K) to antiferromagnetic (θ = -70 K) exchange at 120 K, which is attributed to ferromagnetic dimerisation driven by magnetic frustration. The magnetic moment also increases from 1.81 to 2.2μB at the 120 K crossover. Heat capacity measurements confirm this scenario as the magnetic entropy tends towards 1/2 Rln3 below 120 K before approximating to Rln2 at high temperature. Synchrotron x-ray diffraction and high resolution neutron powder diffraction show no bulk structural changes, but the b axis, along which the VO6 chains run, shows an anomalous expansion below 120 K. At low temperatures, a downturn in the magnetic susceptibility is seen at 2.5 K, signifying a spin freezing transition. Heat capacity and powder neutron diffraction measurements show no evidence for long range magnetic order down to 0.42 K. The low dimensional brannerite materials MV2O6 (M = Mn, Co, Ni) were synthesised by a sol-gel method. Magnetic properties were investigated by magnetisation, powder neutron diffraction and in the case of CoV2O6, heat capacity measurements. The structure of these materials consists of linear chains of edge sharing MO6 octahedra. Monoclinic MnV2O6 is an isotropic antiferromagnet with TN = 20 K and a reduced magnetic coherence length due to 3 % Mn/V antisite disorder. The magnetic structure consists of ferromagnetic edge-sharing chains with k = (0,0,1/2) and a refined Mn moment of 4.77(7) μB. The triclinic materials CoV2O6 and NiV2O6 are also antiferromagnetic with TN = 7 and 14 K respectively and both show metamagnetic type transitions. Unusually, M(H) isotherms recorded below 5 K for CoV2O6 show a plateau at 1/3 of the saturation magnetisation. This feature, together with a long period modulated magnetic structure, is attributed to strong single ion (Ising) type anisotropy and nearest neighbour ferromagnetic exchange. Preliminary high pressure experiments on NiV2O6 have confirmed a previously reported transition to a columbite phase at 6 GPa and 900 °C. The high pressure polymorph is also antiferromagnetic with TN = 2.5 K. The previously uncharacterised perovskite, PbRuO3 has been prepared using high pressure/temperature synthesis techniques (10 GPa, 1000 °C). Synchrotron powder X-ray diffraction measurements show that the room temperature structure is orthorhombic, Pnma. A first order orbital ordering transition occurs at 75 K with an associated metal insulator transition. Below 75 K, the dxz orbitals are preferentially occupied and the structure is orthorhombic Imma. The transition may be driven by an increase in antiferroelectric Pb2+ displacements, whcih reach a peak at ~ 125 K. A further structural transition to a larger monoclinic cell is also identified at 9.7 K. The physical properties and crystal structures of two low dimensional lead manganese oxides have also been investigated. Acentric Pb2MnO4, which has a structure consisting of edge sharing chains, is antiferromagnetic with TN = 18 K. Powder neutron diffraction shows the magnetic structure consists of antiferromagnetic chains with k = (0,0,0) and a refined Mn moment of 2.74(2) μB. The crystal point group allows piezoelectricity and the magnetic point group symmetry allows piezomagnetism. We speculate that coupled magnetic and electric properties may be observed in this material. The layered material, Pb3Mn7O15, with a structure consisting of 1/2 filled Kagomé layers has also been studied. Single crystals were prepared by a flux growth method and polycrystalline material was prepared by the ceramic method. Powder neutron and synchrotron x-ray diffraction studies show that the single crystals are hexagonal and that the polycrystalline material is orthorhombic. Furthermore, heat capacity measurements show that the hexagonal single crystal material undergoes a glassy magnetic transition. In contrast, powder neutron diffraction shows that the orthorhombic polycrystalline material has coherent long range magnetic order. These differences are attributed to an oxygen deficiency in the polycrystalline magnetic order. These differences are attributed to an oxygen deficiency in the polycrystalline material.

Neutron and x-ray diffraction has been used to study charge, orbital and magnetic ordering in some transition metal oxides. The long standing controversy regarding the nature of the ground state (Verwey structure) of the canonical charge ordered material magnetite (Fe3O4) has been resolved by x-ray single crystal diffraction studies on an almost single domain sample at 90 K. The Verwey structure is confirmed to have Cc symmetry with 56 unique sites in the asymmetric unit. Charge ordering is shown to be a useful first approximation to describe the nature of the ground state, and the conjecture that Verwey made in 1939 has finally been confirmed. However, three-site distortions which couple to the orbital ordering of the Fe2+ ordered states (trimerons) are shown to provide a more complete description of the low temperature structure. Trimerons explain the rather continuous distribution of the valence states observed in magnetite below Tv, anomalous shortening of Fe-Fe distances and the off-centre distortions resulting in ferroelectricity. DFT+U electronic structure calculations on the experimental coordinates support the conclusion of this crystallographic study, with the highest electron densities calculated for those Fe-Fe distances predicated to participate in the trimeron bonds. The 6H-perovskites of the type Ba3ARu2O9 have been reinvestigated by high resolution neutron and x-ray power diffraction. The charge ordered state of Ba3NaRu2O9 has been characterised at 110 K (P2/c, a =5.84001(2) Å, b = 10.22197(4) Å, c = 14.48497(6) Å, β = 90.2627(3) °) and shown to consist of a structure with near integer charge ordering of Ru5+ 2O9 / Ru6+ 2O9 dimers. The ground state has been shown to be very sensitive to external perturbations, with a novel melting of charge ordering observed under x-ray irradiation below 40 K (C2/c, a =5.84470(2) Å, b = 10.17706(3) Å, c = 14.45866(5) Å, β = 90.2151(3)-° at 10 K). High pressure studies reveal that the Ru-Ru intra-dimer distance may dictate the response of the system to pressure. Empirical trends in the Ba3ARu2O9 series of compounds have shown that change in ‘chemical pressure’ in these systems may be rationalised in terms of Coulomb’s law. In A = La and Y the magnetic ordering is shown to be FM within the Ru2O9 dimers (1.4(2) μB and 0.5(1) μB, respectively per Ru), representing the first case of intra dimer FM coupling reported in a system containing face-sharing RuO6 octahedra . The overall AFM coupling of the dimers implies an as yet unobserved breaking of the parent symmetry. In A = Nd, a complex competition between the crystal field effect of Nd3+ and the magnetic ordering of the Ru2O9 FM moments has been observed, leading first vi to FM order of Nd at 25 K (1.56(7) μB) followed by ordering of Ru moments (0.5(1) μB) and a spin reorientation transition of Nd moments at 18 K. In A = Ca, the formation of a singlet ground state is observed in Ru2O9 rather than the expected AFM coupling and below 100 K Ba3CaRu2O9 is diamagnetic. All five systems indicate that the Ru2O9 dimer is the physically significant unit in these systems when considering structural trends and the ordering of charge, spin and orbital degrees of freedom.

A temperature-programmed desorption (TPD) study of CO and NH3 adsorption on MnO(100) with complimentary density functional theory (DFT) simulations was conducted. TPD reveals a primary CO desorption signal at 130 K from MnO(100) in the low coverage limit giving an adsorption energy of -35.6 ±2.1 kJ/mol on terrace sites. PBE+U gives a more reasonable structural result than PBE, and the adsorption energy obtained by PBE+U and DFT-D3 Becke-Johnson gives excellent agreement with the experimentally obtained ΔEads for adsorption at Mn2+ terrace sites. The analysis of NH3-TPD traces revealed that adsorption energy on MnO(100) is coverage-dependent. At the low-coverage limit, the adsorption energy on terraces is -58.7± 1.0 kJ/mol. A doser results in the formation of a transient NH3 multilayers that appears in TPD at around 110K. For a terrace site, PBE+U predicts a more realistic surface adsorbate geometry than PBE does, with PBE+U with Tkatchenko-Scheffler method with iterative Hirshfeld partitioning (TSHP) provides the best prediction. DFT simulations of the dehydrogenation elementary step of the ethyl and methyl fragments on α-Cr2O3(101̅2) were also conducted to complement previous TPD studies of these subjects. On the nearly-stoichiometric surface of α-Cr2O3(101̅2), CD3- undergoes dehydrogenation to produce CD2=CD2 and CD4. Previous TPD traces suggest that the α-hydrogen (α-H) elimination of methyl groups on α-Cr2O3(101̅2) is the rate-limiting step, and has an activation barrier of 135 ± 2 kJ/mol. DFT simulations showed that PBE gives reasonable prediction of the adsorption sites for CH3- fragments in accordance with XPS spectra, while PBE+U did not. Both PBE and PBE+U failed to predict the correct adsorption sites for CH2=. When the simulation is set in accordance with the experimentally observed adsorption sites for the carbon species, PBE gives very accurate prediction on the reaction barrier when an adjacent I adatom is present, while PBE+U failed spectacularly. When the simulation is set in accordance with the DFT-predicted adsorption sites, PBE is still able to accurately predict the reaction barrier (<1% to 8.7% error) while PBE+U is less accurate. DFT is also used to complement the previous study of the β-H elimination an ethyl group on the α-Cr2O3(101̅2) surface. The DFT simulation shows that absent surface Cl adatoms, PBE predicts an activation barrier of 92.6 kJ/mol, underpredicting the experimental activation barrier by 28.7%, while PBE+U predicts a barrier of 27.0 kJ/mol, under-predicting the experimental barrier by 79.2%. The addition of chlorine on the adjacent cation improved the prediction on barrier by PBE+U marginally, while worsened the prediction by PBE marginally. Grant information Financial support provided by the U.S. Department of Energy through grant DE-FG02- 97ER14751.
Doctor of Philosophy
Nowadays, density functional theory (DFT), a computational approach to chemistry has become increasingly more popular due to it being less computationally expensive than other traditional computational approaches. One major shortcoming of DFT is its inability to explain the electronic interactions within transition metal oxides, where the electronic configuration within one cation is intimately linked to those on adjacent cations. To address this, DFT+U, a variant of DFT, has been developed to better account for these special electronic interactions. However, not enough experimental comparisons have been established to verify the accuracy of DFT and DFT+U. Our lab focuses on providing high quality experimental benchmarks that can be readily compared to by the DFT community. To establish the experimental benchmarks, we use a technique called temperature-programmed desorption (TPD), which focuses on measuring the rate at which gas molecules leave a sample surface populated with a pre-determined amount of gas molecules as the temperature of the surface is raised at constant but slow temperature ramp rate. Through analysis of the results, the adsorption energy can be obtained for a desorption process, or an activation barrier if the desorption is the result of a surface reaction. Some simple calculations involving PBE, a popular functional used in the DFT community, and its variant PBE+U were conducted for comparison purposes. The transition metal oxide surfaces chosen in this study is MnO(100) and of α-Cr2O3(101̅2), because they both possess the special electronic interactions between their own cations. For adsorption studies, we determined adsorption energies of carbon monoxide (CO), and ammonia (NH3) on MnO(100) single crystal surface. For CO, TPD study revealed that CO undergoes weak adsorption on the surface, with no dissociation of CO detected. PBE predicts an unreasonable surface adsorption geometry while PBE+U predicts a reasonable one. When coupled with a particular dispersion correction method named DFT-D3 Becke-Johnson, PBE+U predicts a very accurate adsorption energy of CO on MnO(100). TPD shows that NH3 undergoes a stronger adsorption on MnO(100) with no dissociation of NH3. Similarly, PBE+U predicted a more reasonable adsorption geometry while PBE did not. Coupled with a dispersion correction named Tkatchenko-Scheffler method with iterative Hirshfeld partitioning (TSHP), PBE+U provides an accurate prediction of adsorption energy. In comparison to previous experimental works based on TPD results, the simple decomposition reactions of an ethyl group and a methyl group were also studied on α-Cr2O3(101̅2) surface using DFT. Overall, PBE gave better prediction on the activation barrier than PBE+U did in comparison to experimentally observed barriers.

This thesis describes crystallographic, magnetic and electrical studies of some mixed metal perovskites. In the first part, the charge distribution, magnetic and electrical properties of mixed metal rhodium-copper perovskite oxides were investigated. Various series with the general formula Ln1-yAyRh1-2xCuxBxO3 in which (Ln = La3+, Tb3+; A = Ca2+, Sr2+, Pb2+, Bi3+; B = Sc3+, Cu2+, Zn2+ ; y ≤ 0.3, x ≤ 0.25), have been synthesised by solid state reaction, and characterised by X-ray diffraction, scanning electron microscopy and physical property measurements and, as available, neutron diffraction and X-ray absorption near edge structure measurements. Structures were invariably orthorhombic with space group Pbnm with the Rh and Cu ions disordered on the same site. X-ray diffraction measurements of selected samples showed that the orthorhombic structure persisted over a wide temperature range, 30 to 900 ºC. All the samples are semiconductors and paramagnetic over the temperature range 4-300 K. Doping a divalent cation onto the B site appears to have a significant impact on charge delocalization between Rh3+/4+ and Cu2+/3+ ions due to the oxidation of Rh3+ to Rh4+ required to maintain the overall charge. That was most evident from the Rh L3 XANES measurements of the LaRh1-2xCu2xO3 and La1-xPbxRh0.5Cu0.5O3 series. Powder neutron diffraction measurements of La0.75Pb0.25Rh0.5Cu0.25Zn0.25O3, LaRh0.5Cu0.25Zn0.25O3 and La0.75Pb0.25Rh0.5Cu0.5O3 show no evidence for anion vacancies and it is postulated the oxides do not contain appreciable amount of oxygen vacancies. The magnetization curves show negative values for Weiss constants indicating weak antiferromagnetism may be present but there is no indication for long range coupling in the oxides. There are several factors that may influence the magnitude of the cell volume, octahedral distortion, octahedral tilting, magnetic interactions and electronic properties. These include ionic size, effective charge, electron configuration and electronegativity. In addition the charge delocalization and local ordering effects can play a role. The present work has demonstrated that: The changes in the unit cell volume and the octahedral distortion of the isovalent doped oxides such as La0.75A0.25Rh0.7Cu0.3O3 where A = Ca2+, Sr2+ and Pb2+ are consistent with the increase in the ionic radii, whereas the decrease in magnetic moments of these is correlated with the increase in the electronegativities of the dopant cation. The unit cell volumes for the terbium oxides are somewhat smaller than found in the analogous lanthanum oxides reflecting the small ionic size of Tb3+. The divalent cation doped oxides LnRh1-2xCu2xO3 and LnRh1-2xCuxZnxO3 display lower cell volumes and octahedral distortions but higher magnetic moments and electrical conductivities than the trivalent cation doped oxides LnRh1-2xCuxScxO3 as consequence of charge delocalization. The electrical conductivity of the oxides increases as the divalent dopant content increases possibly because of an increase in carrier concentration that occurs as consequences of the formation of ionic defects due to the oxidation of Rh3+ (3d6) to Rh4+ (3d5). The electron configuration influences the spin coupling and the band gap and this is most evident in the Pb2+ and Bi3+ (6s2) doped LaRh1-2xCu2xO3 oxides which exhibited the lowest magnetic moments and the highest activation energies among the oxides studied. Compared with the analogous lanthanum oxides, the magnetic susceptibilities of the terbium oxides increased as a consequence of the contribution of Tb3+ 4 f8 electrons. Changing the A site composition resulted in anomalous changes in the cell volumes, octahedral distortions, electrical resistivity and magnetic susceptibility of the La1-xPbxRh0.5Cu0.5O3 and La1-xBixRh0.5Cu0.5O3 perovskites. This is likely a consequence of charge delocalization and short-range local ordering effects. Increasing the doping on the B-site resulted in either a decrease or increase in the cell volumes and the magnetic moments, depending on the dopant type cation. The final part of this thesis describes the structure of some Ba2-xSr1+xBO5.5 (B = Nb5+ and Ta5+) perovskites. These were characterised by scanning electron microscopy, thermogravimetric analysis, X-ray and neutron diffraction. The preparation of these used solid state methods but the initial reactions were conducted under different media. Four compounds were prepared and these all have a face centred cubic structure with space group Fm3 ̅m. The two synthetic methods produce monophasic powders and these differ in color, particle size, and hardness. The cell edges of the oxides obtained by mixing the reactants with water are larger than these obtained when the mixing was conducted with acetone. The neutron diffraction profiles demonstrate that the A cation and oxygen ions are disordered in the BaSr2NbO5.5 and BaSr2TaO5.5 structures. The unusual thermal expansion of the unit cell is due to the presences of water and anion deficiency into the oxides structure. The oxides were found to absorb CO2 atmosphere during storage.

Perovskite-type materials have been widely studied in the literature as a result of the plethora of properties they have been found to exhibit. This is largely down to their versatile nature, which allows the substitution of a wide variety of different elements into the crystallographic sites. In addition to this the presence of 4d and 5d transition metal elements enables an even wider range of potential properties to be considered. The solid solution Sr1-xBaxMoO3 (x = 0.000, 0.025, 0.050, 0.075, 0.100 and 1.000) has been synthesised. Examination of the X-ray diffraction data via Rietveld refinement showed the materials crystallised with cubic Pm-3m symmetry, and there was a miscibility gap from x = 0.1 – 1.0. Examination of the optical properties showed that increasing x from 0 to 1 reduced the measured band gap, which was attributed to the electronic transition from the Mo 4d t2g band to the eg band, from 2.20 eV to 2.07 eV, as the ligand field splitting energy is closely related to the extent of hybridisation between Mo dx2-y2 and dz2 and the O 2p orbitals and the larger radius of Ba2+ compared to Sr2+ leads to longer Mo-O bonds and therefore weaker orbital mixing. The materials were examined as potential water-splitting photocatalysts but no evidence of hydrogen or oxygen evolution was found. In a similar fashion the solid solution Sr1-xCaxMoO3 (x = 0.00, 0.05, 0.10, 0.13, 0.15 and 0.17) was synthesised, and structural phase transitions were found to occur as x increased, from cubic Pm-3m to tetragonal I4/mcm to orthorhombic Imma. Discontinuities were observed in the cell parameters, bond lengths and angles at the transition from tetragonal to orthorhombic as a result of the switching of the octahedral rotation axis at the tetragonal to orthorhombic transition. The band gap was also found to decrease from 2.20 eV to 2.10 eV as x increased, which was further attributed to the octahedral tilting. The magnetic, electrical and structural properties of the Ruddlesden-Popper material – a variation on the perovskite structure – Sr3CoRuO7 were examined, and showed no structural changes down to 5 K, and no evidence of long-range magnetic order. A broad antiferromagnetic transition was observed at ~160 K which was attributed to short-range magnetism. The material was found to be semiconducting, and displayed Mott variable-range hopping behaviour below 240 K. The novel hexagonal perovskite series Ba3AMo2O9 (A = Sr, Ca, Nd and Pr0.5Nd0.5) was successfully synthesised, and attempts were made to synthesise the material Ba4Mo2O9, which was obtained mostly phase pure, with some minor impurities which were identified as polymorphs of the material and small amounts of Ba6Nb3O13.5 and Ba5.75Nb2.25O11.38. Examination of the magnetic properties revealed what appeared to be a transition at ~100 K in the Ba4Mo2O9, Ba3SrMo2O9 and Ba3CaMo2O9 materials, and spin gap formation was suspected below 100 K. The reduction in susceptibility was a possible indicator of spin dimer formation. Curie-Weiss fits were obtained for Ba4Mo2O9, Ba3CaMo2O9 and Ba3Pr0.5Nd0.5Mo2O9.

There is no apparent, dominant interaction in heavy transition metal oxides (TMO), especially in 5d-TMO, where all relevant interactions are of comparable energy scales, and therefore strongly compete. In particular, the spin-orbit interaction (SOI) strongly competes with the electron-lattice and on-site Coulomb interaction (U). Therefore, any tool that allows one to tune the relative strengths of SOI and U is expected to offer an opportunity for the discovery and study of novel materials. BaIrO3 is a magnetic insulator driven by SOI whereas the isostructural BaRuO3 is a paramagnetic metal. The contrasting ground states have been shown to result from the critical role of the strong SOI in the iridate. This dissertation thoroughly examines a wide array of newly observed novel phenomena induced by adjusting the relative strengths of SOI and U via a systematic chemical substitution of the Ru4+(4d4) ions for Ir4+(5d5) ions in BaIrO3, i.e., in high quality single crystals of BaIr1-xRuxO3(0.0 < x < 1.0) . Our investigation of structural, magnetic, transport and thermal properties reveals that Ru substitution directly rebalances the competing energies so profoundly that it generates a rich phase diagram for BaIr1-xRuxO3 featuring two major effects: (1) Light Ru doping (0 < x < 0.15) prompts a simultaneous and precipitous drop in both the magnetic ordering temperature TC and the electrical resistivity, which exhibits metal-insulator transition at around TC. (2) Heavier Ru doping (0.41 < x < 0.82) induces a robust metallic and spin frustration state. For comparison and contrast, we also substituted Rh4+(4d5) ions for Ir4+(5d5) ions in BaIrO3, i.e. BaIr1-xRhxO3(0.0 < x < 0.10), where Rh only reduces the SOI, but without altering the band filling. Hence, this system remains tuned at the Mott instability and is very susceptible to disorder scattering which gives rise to Anderson localization.

In this study, the oxygen-deficient TiO2, SrTiO3 systems and transition metal ion (Cr or V) doped TiO2, SrTiO3 and SrZrO3 systems have been investigated. We prepared samples as polycrystals, single crystals and thin films for various desires. Their structural, physical and electronic properties were measured by bulk-sensitive techniques (X-Ray Diffraction, SQUID and Electro Paramagnetic Resonance) or surface-sensitive techniques (Photoemission spectroscopy and X-ray absorption spectroscopy). The measurement of SQUID and EPR showed not only their magnetic properties but also the valence state of Cr dopant. We verified the valence state of Cr ions in oxides and found the key parameters of sample synthesis which control the valence state of Cr ions. Segregated phases such as SrCrO4 were formed when the samples were synthesized under O2 rich environment. The surface properties of Cr doped SrZrO3 films are also discussed. We found the synthesis conditions which influence on not only the behavior of Cr ions but also the resistive-switching behaviors. Various resistive-switching behaviors seem to depend on the surface chemistry of films. We found that the accumulation of Cr3+ on film surface provides a clean interface without any non-stoichiometric oxides and that this sharp interface termination results in a good performance of resistive-switching.

Cette thèse porte sur les propriétés de transport anormal des oxydes de métaux de transition, en particulier de la surface de SrTiO₃ ou de l’interface entre SrTiO₃ et LaAlO₃. Dans ces systèmes on observe l’apparition de gaz d’électrons bidimensionnels. Des mesures d’Effet Hall non linéaire indiquent que ces gaz sont constitués de plusieurs sortes de porteurs de charge, et que leurs populations varient de manière non monotone sous l’effet du dopage électrostatique. L’effet des propriétés électrostatiques et des corrélations électroniques sur ces variations sont discutées. Celles-ci sont à l’origine de réponses remarquables en ce qui concerne la conversion du spin en charge dans ces systèmes à l’aide d’un modèle de liaisons fortes et de la théorie de la réponse linéaire. Les effets conjoints du spin-orbite atomique et de la brisure de symétrie d’inversion à l’interface verrouille les nombres quantiques de spin, de caractère orbital et d’impulsion des électrons, et induit des textures de spin complexe dans l’espace réciproque. Ces textures sont responsables de l’apparition des effets Edelstein et Hall de spin dans ces hétérostructures et sont caractéristiques de la nature multi-orbitale de ces systèmes électroniques. Enfin nous conduirons une étude ab initio des hétérostructures STO/LAO/STO pour expliquer les observations expérimentales de nouvelles manières de former un gaz d’électrons à ces interfaces d’oxydes. Nous discuterons des rôles respectifs de la chimie, de l’électrostatique et des défauts dans l’apparition de ce gaz
The anomalous transport properties of transition metal oxides, in particular the surface of SrTiO₃ or at the interface between SrTiO₃ and LaAlO₃ is investigated in this thesis. These systems host two-dimensional electron gases. Nonlinear Hall Effect measurements suggest that several species of carriers are present in these systems, and that their population is varying on a nontrivial manner upon electrostatic doping. The role of the electrostatics properties of the electron gas and of the electronic correlations are discussed in this light. Next we discuss the spin to charge conversion of these systems thanks to tight-binding modeling and linear response theory. The complex interplay between atomic spin-orbit coupling and the inversion symmetry breaking at the interface leads to a complex spin-orbital-momentum locking of the electrons, inducing spin textures. These spin textures are responsible for the appearance of the Edelstein and Spin Hall Effect in these heterostructures and are characteristic of the multi-orbital character of these electronic systems. Finally an ab initio study of STO/LAO/STO heterostructures is performed to explain experimental evidence of new ways to produce an electron gas at this interface. The respective roles of the chemistry, electrostatics and defects are discussed

Transition metal oxides have been extensively studied during past decades. The purpose of this research was to synthesize new or little characterised transition metal oxides using high-pressure/high-temperature (HPHT) techniques. Various ternary vanadium oxides have been synthesised at ambient and high pressure conditions. All compounds have been studied by neutron and laboratory X-ray powder diffraction and magnetisation measurements. In some cases resistivity and synchrotron X-ray powder diffraction measurements were also carried out. The MnVO3 perovskite containing localized 3d5 Mn2+ and itinerant 3d1 V4+ states has been synthesised at 8 GPa and 1100°C. MnVO3 crystallises in Pnma space group (a = 5.2741(6) Å, b = 7.4100(11) Å, and c = 5.1184(8) Å at 300 K) and is metallic at temperatures of 2 – 300 K and at pressures of up to 67 kbar. Synchrotron X-ray powder diffraction study on the combined sample of several high pressure products showed slight variation in the stoichiometry of MnVO3. Incommensurate Mn spin order was discovered in the neutron powder diffraction measurements, which reveal a (0.29 0 0) magnetic vector below the 46 K spin ordering transition, and both helical and spin density wave orderings are consistent with the diffraction intensities. Electronic structure calculations show large exchange splittings of the Mn and V 3d bands, and (kx 0 0) crossings of the Fermi energy by spin up and down V 3d bands may give rise to Ruderman-Kittel-Kasuya-Yosida coupling of Mn moments, in addition to their superexchange interactions. The new compound CoVO4 has been discovered in a high pressure synthesis experiment. Magnetic susceptibility measurement, synchrotron X-ray and neutron powder diffraction studies were carried out. Refinements of the synchrotron X-ray and neutron data show CoVO4 to crystallise in space group Pbcn (a = 4.5012(2) Å, b = 5.5539(3) Å, and c = 4.8330(2) Å at 300 K (synchrotron X-ray data)). The magnetic susceptibility measurement reveals that Co3+ is most likely in a low spin state in CoVO4. Monoclinic brannerite type CoV2O6 was synthesised in ambient pressure. Neutron powder diffraction measurements were carried out and an antiferromagnetic order with an a x b x 2c supercell was observed below TN = 15 K. High spin Co2+ moments of magnitude 4.77(4) μB at 4 K lie in the ac plane and are ferromagnetically coupled within chains of edge-sharing CoO6 octahedra parallel to b axis. No structural transition is observed down to 4 K, but a magnetostriction accompanying antiferromagnetic order at TN = 15 K was discovered. A field-induced 1/3 magnetisation plateau and corresponding changes in the magnetic structure were studied by carrying out neutron powder diffraction measurements at 2 K in applied magnetic fields of 0, 2.5 and 5.0 T. Three collinear magnetic phases were observed as field increases; the above antiferromagnetic state with propagation vector (0 0 ½), a ferrimagnetic (¯⅓ 1 ⅓) phase, and a (0 0 0) ferromagnetic order. Co2+ moments of 4.4 - 5.0 μB have a large orbital component and are aligned close to the c-axis direction in all cases. Spin-lattice coupling leads to a magnetostriction and volume expansion as field increases. The ferrimagnetic phase accounts for the previously reported 1/3 magnetisation plateau, and demonstrates that monoclinic CoV2O6 behaves as an accidental triangular antiferromagnetic lattice in which further frustrated orders may be accessible. Orthorhombic columbite-type NiV2O6 and CoV2O6 compounds were synthesised at 6 GPa and 900°C. Metamagnetism and magnetic transitions were found in magnetic measurements. Powder neutron diffraction studies in zero and applied field were carried out. Both compounds were refined in space group Pbcn and the following lattice parameters were obtained at 300 K, CoV2O6: a = 13.4941(20) Å, b = 5.5736(9) Å, and c = 4.8082(8) Å and NiV2O6: a = 13.3725(17) Å, b = 5.5344(7) Å, and c = 4.8162(7) Å. Neutron powder diffraction studies in zero field did not reveal any magnetic peaks for either of the compounds but magnetic order emerges in applied fields between 1 and 4 T.

2011/2012
Nel vasto scenario dei materiali fortemente correlati gli ossidi dei metalli di transizione hanno attratto enorme interesse a causa delle loro interessanti proprietà fisiche, come ad esempio, la superconduttività nei cuprati e la magnetoresistenza gigante nelle manganiti. In particolare, il mio interesse è stato rivolto ad una specifica classe di materiali, per i quali la dimensionalità è il parametro più importante. Le attività sperimentali sono state focalizzate verso due sistemi: la manganite Pr0.5Ca1.5MnO4 dopata a metà e a strato singolo (hd-PCMO) e la famiglia dei rutenati Srn+1RunO3n+1 (n=1,2,3). Entrambi questi sistemi esibiscono fenomeni affascinanti strettamente legati ad una complicata interazione tra i gradi di libertà del reticolo cristallino, di spin, di carica, ed orbitale, dove la dimensionalità cristallina gioca un ruolo cruciale. Con il mio progetto di dottorato ho studiato alcune proprietà dei materiali sopracitati per mezzo di spettroscopie con raggi X, come l’emissione risonante di raggi X (RXES) e l’assorbimento di raggi X (XAS) statico e risolto in tempo. Tutte le misure sono state condotte utilizzando la linea di luce BACH (linea di luce per dicroismo avanzato) dell’anello di accumulazione Elettra della Elettra-Sincrotrone Trieste. Il sistema hd-PCMO presenta una transizione di ordinamento di carica ed orbitale (CO-O) ad una temperatura TCO relativamente elevate, i.e. 340 K, accompagnata da una distorsione strutturale ortorombica, dove i portatori di carica fortemente correlati eg del Mn si ordinano in sotto-reticoli cristallografici separati (stato di carica ordinato) con un carattere orbitale specifico (stato di ordinamento orbitale). Inoltre, hd-PCMO presenta anche una risposta reticolare anomala ad una temperatura 20 K sopra la temperatura di Neél TN, che è associata ad un inatteso accoppiamento spin-reticolo. Poiché mancava uno studio degli stati elettronici non occupati del PCMO, misure dipendenti dalla temperatura per mezzo del dicroismo lineare (XLD) sono state realizzate alle soglie K dell’ossigeno e L3 del Mn al fine di spiegare il ruolo della topologia orbitale dei Mn 3d – O 2p. I dati sperimentali, supportati da calcoli ab-initio LDA+U, ci danno informazioni sulla ridistribuzione di carica e sui cambiamenti delle p-DOS alla transizione CO-O e a quella antiferromagnetica (AFM). I risultati ottenuti mostrano che l’interazione competitiva tra la distorsione locale atomica, necessaria per permettere l’ordinamento CO, e le dinamiche di carica del meccanismo di hopping regolano lo stato orbitale dei portatori di carica. Inoltre, sulla base di studi teorici che predicono la formazione di fasi orbitali e strutturali transienti “nascoste” per mezzo della stimolazione ottica, abbiamo studiato le DOS non occupate dello stato metastabile indotto otticamente nel PCMO per mezzo della XAS risolta in tempo, che offre uno strumento unico per misurare le DOS proiettate in sito ed in simmetria degli stati metastabili della materia. Le misure XAS risolte in tempo alla soglia K dell’ossigeno sono state realizzate per mezzo di un nuovo apparato sperimentale disponibile a BACH, che si basa su un laser Ti:zaffiro (impulsi di pompa) con tasso di ripetizione variabile sincronizzato con gli impulsi a 500 MHz dei raggi X (impulsi di sonda). L’evoluzione temporale degli spettri XAS attraverso la transizione CO-O fotoindotta otticamente risulta differente rispetto alle misure XAS adiabatiche, dimostrando l’esistenza di una “fase nascosta” fotoindotta nel PCMO, la cui natura è ancora sconosciuta. I rutenati Srn+1RunO3n+1 (n=1,2,3) sono emersi come una famiglia importante di peroschiti a causa dell’evoluzione inattesa e senza precedenti dal comportamento anisotropico ferro- o metamagnetico del Sr4Ru3O10 (n=3) dipendente dalla direzione del campo magnetico, all’ aumentato paramagnetismo di Pauli vicino all’ordinamento magnetico del Sr3Ru2O7 (n=2) e, infine, alla superconduttività a bassa temperature in Sr2RuO4 (n=1). Nonostante vengano riportati numerosi studi sulle proprietà strutturali e magnetiche di questi composti, l’evoluzione delle strutture elettroniche occupate e non occupate non è stata investigata in dettaglio. Quindi, la dipendenza delle strutture elettroniche e l’ibridizzazione degli stati 2p dell’ossigeno sono state investigate combinando la spettroscopia XAS alla soglia K dell’ossigeno (transizione 2p-1s) dipendente dalla polarizzazione e la spettroscopia RXES. Una sezione del capitolo 3 è dedicata ad illustrare un setup sperimentale sviluppato recentemente per esperimenti XAS risolti in tempo sfruttando la struttura temporale “multibunch” dell’anello di accumulazione del sincrotrone. Sfruttando le potenzialità di questo setup, la transizione di superficie semiconduttore-metallo nel germanio cristallino è stata fotoindotta ed il set completo di dati viene discusso. Lo schema della mia tesi di dottorato è il seguente. Il primo capitolo presenta una panoramica dell’intero lavoro. Il secondo capitolo è diviso in due sezioni. La prima sezione introduce il lettore alla fisica orbitale ed alle transizioni di fase elettroniche nei metalli di transizione a ridotta dimensionalità, con un excursus sullo stato dell’arte dei composti 3d del manganese e la famiglia 4d dei rutenati. L’intento della seconda sezione è quello di spiegare l’importanza delle tecniche spettroscopiche nei raggi X molli come strumenti per investigare le proprietà elettroniche dei solidi. La descrizione delle spettroscopie XAS e RXES vengono riviste più in dettaglio nel capitolo 3, che include anche la descrizione dell’apparato sperimentale della beamline BACH e del laboratorio T-ReX al Sincrotrone Elettra. Il capitolo 4 è dedicato alla teoria funzionale di densità (DFT) ed alla approssimazione locale di densità più U (LDA+U) ed ai dettagli del modello del sistema hd-PCMO. Il capitolo 5, che presenta i casi studiati, è diviso in due sezioni: il caso del PCMO, che include le misure XAS statiche e risolte in tempo, ed il caso della serie Ruddlesden-Popper dei rutenati di Sr investigate per mezzo della RXES. Nel capitolo finale vengono presentati i commenti finali su questo lavoro.
In the vast scenario of strongly correlated-electron materials transition-metal oxides have attracted enormous interest because of their interesting physical properties, including for example, superconductivity in cuprates and colossal magnetoresistance in manganites. In particular, my interest was directed to a particular class of materials, whose dimensionality is the most defining material parameter. With my Ph.D. project I deepened into some physical properties of these materials by means of core-levels spectroscopies such as resonant x ray emission (RXES) and static and time-resolved x ray absorption (XAS). All the measurements have been carried out at the beamline BACH (Beamline for Advanced diCHroism) at the Elettra light source facility in Trieste. The experimental activities focused on two case-study systems: the single layered half-doped Pr0.5Ca1.5MnO4 (hd-PCMO) and the layered Srn+1RunO3n+1 (n=1,2,3) family. Both these systems exhibit fascinating phenomena intimately related to a complicated interplay between the crystal lattice, spin, charge, and orbital degrees of freedom, where crystal dimensionality plays a crucial role. hd-PCMO exhibits a charge-orbital ordering (CO-O) transition at a remarkably high TCO, slightly above room temperature, accompanied by an orthorhombic structural distortion, where the strongly correlated Mn eg charge carriers order onto separate crystallographic sub-lattices (charge-ordered state) with a specific orbital character (orbital ordered state). Furthermore, hd-PCMO also displays an anomalous lattice response at temperatures 20K above the Neél temperature TN, which is associated to an unexpected spin-lattice coupling. Since a study of the PCMO unoccupied electronic states was lacking, temperature dependence measurements by XAS linear dichroism (XLD) have been performed at the O-K and Mn-L3 thresholds in order to elucidate the role of Mn 3d - O 2p orbital topology. The experimental data, supported by ab-initio LDA+U, shed light on the charge redistribution and p-DOS changes at the CO-O and antiferromagnetic (AFM) transitions. The results obtained show that the competitive interplay between the local atomic distortion, necessary for accomodating the CO-ordering, and the charge dynamics of the hopping mechanism regulates the orbital state of the charge carriers. Furthermore, on the basis of theoretical studies that predict the formation of transient “hidden” orbital and structural phases by optical stimulation, we have studied the unoccupied DOS of the optically induced metastable state in PCMO by means of time resolved XAS, which offers a unique tool to measure site and symmetry projected DOS of metastable states in matter. Tr-XAS measurements at the O-K edge have been carried out by means of a novel experimental apparatus available at BACH, which is based on a variable repetition rate Ti:sapphire laser (pump pulse) synchronized with the ∼ 500 MHz X-ray photon pulses (probe pulses). The time evolution of the XAS lineshapes across the optically photoinduced CO-O transition results different respect to the adiabatic XAS measurements, demonstrating the existence of a photoinduced “hidden phase” in PCMO, whose nature is still unknown. The layered Srn+1RunO3n+1 (n=1,2,3) have emerged as an important family of perovskites because of the unexpected and unprecedented evolution from anisotropic ferro- or metamagnetic behavior of Sr4Ru3O10 (n=3) dependent on the direction of the magnetic field, enhanced Pauli paramagnetism close to magnetic order of Sr3Ru2O7 (n=2) and, finally, to low-temperature superconductivity in Sr2RuO4 (n=1). Although numerous studies have been reported on the structural and magnetic properties of these compounds, the evolution of the occupied and unoccupied electronic structures were not investigated in detail. Thus, the dependence of electronic structures and the hybridization of O 2p states have been investigated by combining polarization dependent O K (2p-1s transition) XAS and RXES spectroscopies. A section of the chapter 3 is dedicated to illustrate a newly developed experimental setup for time-resolved XAS experiments by exploiting the multibunch time structure of a synchrotron storage ring. By exploiting the capabilities of this setup, the surface semiconductor-metal transition in crystalline germanium has been photoinduced and the complete set of data discussed. The outline of my Ph.D. thesis is the following. The first chapter presents an overview of the entire work. The second chapter is divided into two sections. The first section introduces the reader into the orbital physics and the electronic phase transitions in low dimensional transition metal oxides, with an excursus on the state of the art of 3d manganese compounds and the family of 4d Ruthenates. The second section is aimed to explain the importance of soft x-ray spectroscopic techniques as tools to investigate the electronic properties of solids. The description of XAS and RXES are reviewed in more details in chapter 3, which includes also the description of the experimental apparatus of BACH beamline and T-ReX lab at the Elettra synchrotron light source. Chapter 4 is dedicated to the Density Functional Theory (DFT) and Local Density Approximation plus U (LDA+U) theories and to the details of the modelling of the hd-PCMO system. Chapter 5, which presents the cases studied, is divided into two sections: the case of PCMO, including static and time resolved XAS measurements, and the case of Ruddlesden-Popper series of Sr Ruthenates investigated by means of RXES. In the final chapter the concluding remarks on this work are presented.
XXV Ciclo
1983

This thesis is an investigation into the accuracy of hybrid density functional theory to predict the properties of two transition metal oxides: Ilmenite (FeTiO3) and haematite (sigma-Fe2O3). The hybrid density functional theory examined is Becke's B3LYP functional, which is an empirical mix of density functional theory and exact nonlocal exchange from Hartree-Fock theory. For bulk ilmenite, results from the B3LYP functional are compared with Hartree-Fock and pure density functional theory calculations. The computed properties are found to be very sensitive to the treatment of electronic exchange and correlation, with the best results being achieved using the hybrid functional. Calculations performed using the hybrid functional benefit from its better treatment of the electronic self interaction and its reasonable estimate of the pair correlation energy of the doubly occupied Fe-d orbital. To assess the performance of the hybrid functional in simulating Fe2O3 and FeTiO3 with different cation-anion coordination than that found in ilmenite or haematite, studies were performed on their high pressure polymorphs, for which there are a range of experimental results for comparison. This tests the transferability of the functional before examining cases, such as the surfaces of these materials, where there are little or no experimental or theoretical results. For the currently known high pressure polymorphs of ilmenite and haematite, the structural and elastic parameters computed using the hybrid functional are found to be in good agreement with those observed, as is the predicted stability of the phases. In ilmenite, the calculations predict the stability of a new high-pressure polymorph with space group Cmcm, occurring at pressures above 44 GPa. Calculations of the high pressure polymorphs of haematite involve the examination of a range of charge, spin, and magnetic states for each of the polymorphs. Magnetic ordering was found to be important for all the polymorphs, and for each polymorph an antiferromagnetic ordering was found to be lower in energy than the ferromagnetic ordering. The predicted transition pressure from the corundum structure and the magnetic collapse of the Fe3+ cations were in good agreement with experiment. At high pressures the lowest energy configuration for the orthorhombic perovskite structure was computed to occur with mixed high-spin /low-spin Fe3+ cations, in contrast to predictions in the literature of a Fe2+/Fe4+ solution. The CaIrO3-type structure was also computed to be stable with a mixed high-spin/ low-spin Fe3+ configuration at high pressures, and is computed to be the most stable polymorph at pressures above 46 GPa at 0 K. The structure of the ilmenite (0001) surface is examined using the B3LYP functional, and for this surface twelve different terminations are considered, with surface energies and relaxed geometries calculated. The Fe terminated (0001) surface was found to have the lowest cleavage energy, and also to be the most stable surface at low oxygen partial pressures suggesting it is most likely to form when ilmenite is cleaved under high vacuum.

The subject of the present thesis is theoretical first principles electronic structure calculations on designer hard materials such as the transition metal carbides and oxides. The theoretical investigations have been made in close collaboration with experimental research and have addressed both bulk electronic properties and surface electronic properties of the materials.

Among the bulk studies are investigations on the effects of substoichiometry on the relative phase stabilities and the electronic structure of several phases of MoC and the nature of the resulting vacancy peaks. The changes in phase stabilities and homo-geneity ranges in the group IV to VI transition metal carbides have been studied and explained, from calculations of the T=0 energies of formation and cohesive energies. The anomalous volume behavior and phase stabilities in substoichiometric TiC was studied including effects of local relaxations around the vacancy sites. The vacancy ordering problem in this compound was also studied by a combination of electronic structure calculations and statistical physics.

The studies of the surface electronic properties include research on the surface energies and work functions of the transition metal carbides and an investigation on the segregation of transition metal impurities on the TiC (100) surface.

Theoretical studies with the aim to facilitate the realization of novel designer hard materials were made, among these a survey of means of stabilizing potentially super-hard cubic RuO₂, studying the effects of alloying, substoichiometry and lattice strains. A mechanism for enhancing hardness in the industrially important hard transition metal carbides and nitrides, from the discovery of multi-phase/polytypic alloys, has also been predicted from theoretical calculations.

Lithium-ion batteries have transformed energy storage and technological applications. They stand poised to convert transportation from combustion to electric engines. The discharge/charge rate is a key parameter that determines battery power output and recharge time; typically, operation is on the timescale of hours but reducing this would improve existing applications and open up new possibilities. Conventionally, the rate at which a battery can operate has been improved by synthetic strategies to decrease the solid-state diffusion length of lithium ions by decreasing particle sizes down to the nanoscale. In this work, a different approach is taken toward next-generation high-power and fast charging lithium-ion battery electrode materials. The phenomenon of high-rate charge storage without nanostructuring is discovered in niobium oxide and the mechanism is explained in the context of the structure–property relationships of Nb2O5. Three polymorphs, T-Nb2O5, B-Nb2O5, and H-Nb2O5, take bronze-like, rutile-like, and crystallographic shear structures, respectively. The bronze and crystallographic shear compounds, with unique electrochemical properties, can be described as ordered, anion-deficient nonstoichiometric defect structures derived from ReO3. The lessons learned in niobia serve as a platform to identify other compounds with related structural motifs that apparently facilitate high-rate lithium insertion and extraction. This leads to the synthesis, characterisation, and electrochemical evaluation of the even more complicated composition–structure–property relationships in ternary TiO2–Nb2O5 and Nb2O5–WO3 phases. Advanced structural characterisation including multinuclear solid-state nuclear magnetic resonance spectroscopy, density functional theory, X-ray absorption spectroscopy, operando high-rate X-ray diffraction, and neutron diffraction is conducted throughout to understand the evolution of local and long-range atomic structure and changes in electronic states.

First, the effect of transition metal oxide (e.g., V2O5, Co2O3, etc.) on the physical properties (e.g., density, glass transition temperature (Tg), optical properties and mechanical properties) and chemical durability of a simplified borosilicate nuclear waste glass was investigated. Adding V2O5 in borosilicate nuclear waste glasses decreases the Tg, while increasing the fracture toughness and chemical durability, which benefit the future formulation of nuclear waste glasses. Second, structural study of ZrO2/SiO2 substitution in silicate/borosilicate glasses was systematically conducted by molecular dynamics (MD) simulation and the quantitative structure-property relationships (QSPR) analysis to correlate structural features with measured properties. Third, for bioactive glass formulation, mixed-network former effect of B2O3 and SiO2 on the structure, as well as the physical properties and bioactivity were studied by both experiments and MD simulation. B2O3/SiO2 substitution of 45S5 and 55S5 bioactive glasses increases the glass network connectivity, correlating well with the reduction of bioactivity tested in vitro. Lastly, the effect of optical dopants on the optimum analytical performance on atom probe tomography (APT) analysis of borosilicate glasses was explored. It was found that optical doping could be an effective way to improve data quality for APT analysis with a green laser assisted system, while laser spot size is found to be critical for optimum performance. The combined experimental and simulation approach adopted in this dissertation led to a deeper understanding of complex borosilicate glass structures and structural origins of various properties.

Amorphous transition metal oxides [aTMOs], have emerged as innovative functional materials for wide-ranging electronic, optical and energy-related applications. However, no systematic and broadly applicable method exists to assess their atomic-scale correlations, and since the optical and electronic processes are local structure-dependent, still there are not well-stablished mechanisms that suitably explain the physical properties of aTMOs. This thesis presents experimental and theoretical studies of the atomic short-range order, optical and electronic properties, and state-defects induced by Li+-ion-intercalation and oxygen-vacancies in amorphous titanium aTiO2 and tungsten aWO3 thin-film oxides. Those properties play a key role for application in high energy-density Li+-ion batteries and in switchable dynamical modulation of solar-irradiation transmittance for energy efficient "smart windows", where the disorder-dependent Li+-ion-intercalation and oxygen-vacancy-induced defect-states influence charge-carrier transfer mechanisms. After introducing the scope of this thesis, the fundamental theoretical concepts describing the experimental findings on amorphous solids are reviewed. Thereafter, a comprehensive analysis on the optical absorption phenomena experimentally observed in oxygen-deficient and Li+-ion-intercalated aLixTiO2−y and aLixWO3−y thin-films and a discussion on the electrochromic properties are presented. The optical absorption is described in the framework of the small polaron absorption model. Finally, a state-of-the-art systematic procedure involving theory and experiment in a self-consistent computational framework is implemented to unveil the atomic-scale structure of aTiO2 and aWO3, and its role for the electronic properties. The procedure is based in Reverse Monte Carlo [RMC] and Finite Difference Method [FDM] simulations of X-ray-Absorption spectra to construct a disordered theoretical model having the same bonding and coordination distribution as the experimental system. Ab-initio molecular dynamics simulations and density functional theory are then used to assess defect-states induced by Li+-ion-intercalation and oxygen-vacancies in aTiO2 and aWO3 oxides. The schemes introduced in this study offer a consistent route to experimentally and theoretically assess the role of the atomic-scale structure on the optical and electronic properties of aTiO2 and aWO3 and could be extended to the study of other aTMOs. The final results provide crucial insight towards the understanding of optical and electronic mechanisms where disorder-dependent ion-intercalation and oxygen-vacancy-induced localized defect-states influence charge transfer mechanisms of crucial importance for wide ranging optical and energy-related application of aTiO2 and aWO3 oxides.

Future energy systems based on hydrogen as energy carrier require reliable ways for storing hydrogen gas in safe, clean and efficient ways. Metal hydrides absorb hydrogen gas reversibly, making them suitable for storage applications. Investigations of the crystal structures of these materials contribute to an understanding of the factors which can influence the absorption. Three systems, Ti3Sn-D, Nb4MSi-D (M=Co or Ni) and Pd-Ni-P, have been investigated in this thesis. Various solid state synthesis techniques have been used for sample preparation. The crystal structures have been studied using x-ray and neutron diffraction techniques. Three metal hydride phases were found in the Ti3Sn-D system upon hydrogenation. Deuterium occupies titanium octahedra and the applied deuterium pressure induces the phase transitions. The distances between the deuterium atoms increase from 2.47 Å in orthorhombic Ti3SnD0.80 to 4.17 Å in cubic Ti3SnD. The Nb4MSi-D system (M=Co or Ni) readily absorbs deuterium at room temperature and 90 kPa deuterium pressure to give a deuterium content of Nb4MSiD~2.5. Two interstitial voids, both coordinated by four niobium atoms arranged in a tetrahedral configuration, accommodate deuterium atoms. Two ternary phases and a solid solution of nickel in Pd3P have been synthesised and the crystal structures determined. PdNi2P is orthorhombic and crystallises in the MgCuAl2-type structure: an ordered derivative of the Re3B-type structure. Pd8Ni31P16 is a tetragonal high-temperature phase stable at 700°C with 110 atoms in the unit cell. Pd2.7Ni0.3P0.94 has the cementite-type structure with mixed occupancy of palladium and nickel at one of the two non-equivalent crystallographic metal positions.

La détection infrarouge, autrefois réservée aux applications militaires et spatiales, connait depuis une dizaine d’années une mutation importante et s’ouvre de plus en plus vers des marchés "grand public". Cette démocratisation est principalement liée aux développements rapides que connaissent les technologies utilisant des bolomètres "non refroidis", qui profitent de leurs compatibilités avec les filières de la microélectronique.La technologie utilisée au CEA/LETI repose sur l’utilisation d’un matériau thermomètre à base de silicium amorphe (également noté "a-Si"). Ce dernier comporte de nombreux avantages dont, principalement, son excellente compatibilité avec les outils "classiques" de la microélectronique. Cependant, l’intégration d’un matériau thermomètre plus performant que le a-Si est nécessaire pour répondre aux défis à venir.Conscient de l’importance de cette problématique "matériau" le laboratoire CEA/LETI développe depuis plusieurs années des matériaux à base d’oxydes de métaux de transition déposés en couches minces.Cette étude s’appuie sur l’ensemble des variantes d’oxydes de métaux de transition étudié dans ce cadre. Cette palette de matériaux, qui se sont révélés très différents dans leur structure et, corrélativement, les mesures de transport dans chacun de ces types, nous ont permis de relier structure et mécanismes de conduction spécifiques à chacun. Une attention particulière a été portée aux mesures de TCR, ou « Temperature Coefficient of Resistance », (facteur à maximiser) et de bruit en 1/f (source de bruit à minimiser), les deux paramètres de choix pour le matériau thermistor.Des grandes tendances qui pilotent la performance d’un matériau thermistor pour la bolométrie ont pu être déduites de ces investigations. Les travaux présentés dans cette thèse permettent d’évaluer le potentiel de tel ou tel compétiteur avant d’en entreprendre le développement
InfraRed detection, formerly reserved to defense and spatial applications, is currently undergoing deep changes which open new opportunities. Uncooled microbolometer technologies, compatible with classical semiconductors processes, are now able to produce low cost thermal imagers and this will open the door to customer markets in a close future.The technology developed in the CEA/LETI laboratory use the amorphous silicon (noted "a-Si") as the thermistor material. This material has many advantages, in particular, its excellent compatibility with the classical tools used in microelectronic industry. However, better performance in the thermistor material is still needed to address future applications.To handle this challenge, CEA/LETI laboratory is currently developing thermistors made of transition metal oxides thin films. The study presented hereby is based on various transition metal oxides samples deposited in the CEA/LETI Laboratory.Characterization of the structure and the electronic transport for each of these samples allowed us to put in evidence correlations between microscopic structure and conduction mechanisms. Two main figures of merit impacting the overall material performance were investigated : the TCR, Temperature Coefficient of Resistance (which must be maximized) and the 1/f noise (which must be minimized).Finally we conclude this work by highlighting majors outlines governing the performance of a thermistor

Future energy systems based on hydrogen as energy carrier require reliable ways for storing hydrogen gas in safe, clean and efficient ways. Metal hydrides absorb hydrogen gas reversibly, making them suitable for storage applications. Investigations of the crystal structures of these materials contribute to an understanding of the factors which can influence the absorption.

Three systems, Ti₃Sn-D, Nb₄MSi-D (M=Co or Ni) and Pd-Ni-P, have been investigated in this thesis. Various solid state synthesis techniques have been used for sample preparation. The crystal structures have been studied using x-ray and neutron diffraction techniques.

Three metal hydride phases were found in the Ti₃Sn-D system upon hydrogenation. Deuterium occupies titanium octahedra and the applied deuterium pressure induces the phase transitions. The distances between the deuterium atoms increase from 2.47 Å in orthorhombic Ti₃SnD_0.80 to 4.17 Å in cubic Ti₃SnD.

The Nb₄MSi-D system (M=Co or Ni) readily absorbs deuterium at room temperature and 90 kPa deuterium pressure to give a deuterium content of Nb₄MSiD_~2.5. Two interstitial voids, both coordinated by four niobium atoms arranged in a tetrahedral configuration, accommodate deuterium atoms.

Two ternary phases and a solid solution of nickel in Pd₃P have been synthesised and the crystal structures determined. PdNi₂P is orthorhombic and crystallises in the MgCuAl₂-type structure: an ordered derivative of the Re₃B-type structure. Pd₈Ni₃₁P₁₆ is a tetragonal high-temperature phase stable at 700°C with 110 atoms in the unit cell. Pd_2.7Ni_0.3P_0.94 has the cementite-type structure with mixed occupancy of palladium and nickel at one of the two non-equivalent crystallographic metal positions.

Application de l'etude du titre a deux grandes famille structurales: (1) les structures tetraedriques derivant de cele de la wuertzite (li::(1,33)ge::(1,67)on::(2); zn::(1,231)ge::(0,689)o::(0,782)n::(1,218); kgeon), (2) les structures de type perovskite (batao::(2)n, banbo::(2)n, lawo::(0,6)n::(2,4))