Добірка наукової літератури з теми "Oxychalcogenide system"

Our work shows a remarkable bandgap tuning range of 7.46 eV with AlOSe alloys, a type of III-oxychalcogenide class. Further analysis revealed large band offsets in conduction and valence bands, implying type-I band alignment in AlOSe/Al₂O₃ systems.

During the past two years, we have underlined the great potential of p-type oxychalcogenides, with parent compound BiCuSeO , for thermoelectric applications in the medium temperature range (400–650°C). These materials, which do not contain lead and are less expensive than Te containing materials, exhibit large thermoelectric figure of merit, exceeding 1 in a wide temperature range, mainly due to an intrinsically very low thermal conductivity. This paper summarizes the main chemical and crystallographic features of this system, as well as the thermoelectric properties. It also gives new directions to improve these properties, and discuss the potential of these materials for wide scale applications in thermoelectric conversion system in the medium temperature range.

Minerals of the Zn-Cd-S-Se system that formed by moderately reduced ~800–850 °C combustion metamorphic (CM) alteration of marly sediments were found in marbles from central Jordan. Their precursor sediments contain Se- and Ni-enriched authigenic pyrite and ZnS modifications with high Cd enrichment (up to ~10 wt%) and elevated concentrations of Cu, Sb, Ag, Mo, and Pb. The marbles are composed of calcite, carbonate-fluorapatite, spurrite, and brownmillerite and characterized by high P, Zn, Cd, U, and elevated Se, Ni, V, and Mo contents. Main accessories are either Zn-bearing oxides or sphalerite, greenockite, and Ca-Fe-Ni-Cu-O-S-Se oxychalcogenides. CM alteration lead to compositional homogenization of metamorphic sphalerite, for which trace-element suites become less diverse than in the authigenic ZnS. The CM sphalerites contain up to ~14 wt% Cd and ~6.7 wt% Se but are poor in Fe (means 1.4–2.2 wt%), and bear 100–250 ppm Co, Ni, and Hg. Sphalerite (Zn,Cd,Fe)(S,O,Se)cub is a homogeneous solid solution with a unit cell smaller than in ZnScub as a result of S2− → O2− substitution (a = 5.40852(12) Å, V = 158.211(6) Å3). The amount of lattice-bound oxygen in the CM sphalerite is within the range for synthetic ZnS1−xOx crystals (0 < x ≤ 0.05) growing at 900 °C.

AbstractThe emergence of insulating ferromagnetic phase in iron oxychalcogenide chain system has been recently argued to be originated by interorbital hopping mechanism. However, the practical conditions for the stability of such mechanism still prevents the observation of ferromagnetic in many materials. Here, we study the stability range of such ferromagnetic phase under modifications in the crystal fields and electronic correlation strength, constructing a theoretical phase diagram. We find a rich emergence of phases, including a ferromagnetic Mott insulator, a ferromagnetic orbital-selective Mott phase, together with antiferromagnetic and ferromagnetic metallic states. We characterize the stability of the ferromagnetic regime in both the Mott insulator and the ferromagnetic orbital-selective Mott phase forms. We identify a large stability range in the phase diagram at both intermediate and strong electronic correlations, demonstrating the capability of the interorbital hopping mechanism in stabilizing ferromagnetic insulators. Our results may enable additional design strategies to expand the relatively small family of known ferromagnetic insulators.

AbstractNematic fluctuations occur in a wide range physical systems from biological molecules to cuprates and iron pnictide high-Tc superconductors. It is unclear whether nematicity in pnictides arises from electronic spin or orbital degrees of freedom. We studied the iron-based Mott insulators La2O2Fe2OM2M = (S, Se), which are structurally similar to pnictides. Nuclear magnetic resonance revealed a critical slowing down of nematic fluctuations and complementary Mössbauerr spectroscopy data showed a change of electrical field gradient. The neutron pair distribution function technique detected local C2 fluctuations while neutron diffraction indicates that global C4 symmetry is preserved. A geometrically frustrated Heisenberg model with biquadratic and single-ion anisotropic terms provides the interpretation of the low temperature magnetic fluctuations. The nematicity is not due to spontaneous orbital order, instead it is linked to geometrically frustrated magnetism based on orbital selectivity. This study highlights the interplay between orbital order and spin fluctuations in nematicity.

ABSTRACTOxygen uptake by the rare earth chalcogenides takes place through a series of ordered compounds. At high oxygen concentrations the distinct and nearly stoichiometric oxychalcogenides, Ln202X (X = S, Se, Te) appear. For the chalcogenides of the larger rare earths there appears an ordered oxygen-containing beta structure, Ln10X14OxX1-x (X = S, Se). The vibrational spectrum of the trigonal oxysulfide structure contains four infrared and four Raman bands (2 A2u + 2 Eu + 2 Alg + 2 Eg). Band wavenumbers across the La to Lu series vary linearly with unit cell volume. The Raman bands are sharp indicating a high degree of order in the intermediate compounds. The Raman bands of the beta structure are remarkably sharp indicating that this compound also has a highly ordered structure. Known data plus synthesis data are combined to form not-impossible phase diagrams for the larger rare earth sulfide systems. The effect of oxygen in both series of compounds is to produce small wavenumber shifts rather than high wavenumber oxygen impurity bands.

AbstractThe insulator-metal transition in Mott insulators, known as the Mott transition, is usually accompanied with various novel quantum phenomena, such as unconventional superconductivity, non-Fermi liquid behavior and colossal magnetoresistance. Here, based on high-pressure electrical transport and XRD measurements, and first-principles calculations, we find that a unique pressure-induced Mott transition from an antiferromagnetic Mott insulator to a ferromagnetic Weyl metal in the iron oxychalcogenide La2O3Fe2Se2 occurs around 37 GPa without structural phase transition. Our theoretical calculations reveal that such an insulator-metal transition is mainly due to the enlarged bandwidth and diminishing of electron correlation at high pressure, fitting well with the experimental data. Moreover, the high-pressure ferromagnetic Weyl metallic phase possesses attractive electronic band structures with six pairs of Weyl points close to the Fermi level, and its topological property can be easily manipulated by the magnetic field. The emergence of Weyl fermions in La2O3Fe2Se2 at high pressure may bridge the gap between nontrivial band topology and Mott insulating states. Our findings not only realize ferromagnetic Weyl fermions associated with the Mott transition, but also suggest pressure as an effective controlling parameter to tune the emergent phenomena in correlated electron systems.

Search for clean and renewable energy resources has driven recent interest in designing thermoelectric materials that convert the waste heat to useful electricity. High performance thermoelectric materials require excellent electronic transport and favorable thermal transport, simultaneously. Given the interdependence of various transport parameters, it is daunting to achieve desirable performance. We attempt to address some of these challenges using density functional theory in combination with machine-learning based approaches. We first report the decoupling of Seebeck coefficient and electrical conductivity by tuning the distortion parameter of chalcopyrites leading to complete convergence of bands, thereby resulting in unprecedented enhancement of electronic transport properties. A combination of excellent electronic transport and low thermal conductivity in CdGeAs2 results into a high ZT of 1.67 at 1000K. To find a system with low thermal conductivity, we study the oxychalcogenide system AgBiTeO, demonstrating the unique collective rattling motion hosted by chemical bond hierarchy. The favorable electronic and thermal transport properties result in a maximum ZT of 1.99 at 1200K, which is highest among the existing bulk oxide-based thermoelectric materials. Owing to the complexity and resource extensive calculations involved in determining electron relaxation time (τel), we employed machine learning approach to estimate the τel. The machine learning model uses data available for experimental electrical conductivity and a collection of accessible elemental information. This model with a rmse of 0.22, outperforms the deformation potential model, and performs adequately on the unseen data to predict the relaxation time over a wide range of temperatures. Further, we develop an effective descriptor by using chalcopyrite class of compounds, to guide an accelerated screening of materials with desirable degree of anharmonicity. The high-throughput study corroborates the role of a very simple parameter, “phonon band center”. This can be calculated within the harmonic regime, yet having profound impact on the anharmonicity of the compounds. Since, the performance of thermoelectric devices is limited by the quality of the interface, we explore the role of fundamental parameters, such as surface termination of interface, electronegativity difference and lattice mismatch that influence the interface. Optimization of these parameters will have a significant role in preserving the thermoelectric performance of the materials in devices. The results of our study pave way to overcome some of the critical challenges related to thermoelectrics by effectively addressing electronic and thermal transport problems.