Electronically driven tetragonal distortion in TiH2

A study of the fluorite structure stoichiometric TiH₂ by means of the augmented plane wave method reveals that the density of states at the Fermi level is very high, due mainly to a degenerate flat band in the ΛL direction. This result, in agreement with the experimental data of Ducastelle, Caudron and Costa confirms their interpretation of the tetragonal distortion in terms of a Jahn-Teller type effect. The Fermi surface geometry is discussed in connection with the sign change of the observed Hall coefficient. A comparison with Switendick's previous work on hydrides is also given.     

Pressure-induced structural phase transition in ReO3

We have investigated the pressure-induced structural phase transition in ReO₃ by neutron diffraction on a single crystal. We collected neutron diffraction intensities from the ambient and high pressure phases at P=7 kbar and refined the crystal structures. We have determined the stability of the high pressure phase as a function temperature down to T=2 K and have constructed the (P-T) phase diagram. The critical pressure is P_c=5.2 kbar at T=300 K and decreases almost linearly with decreasing temperature to become P_c=2.5 kbar at T=50 K. The phase transition is driven by the softening of the M₃ phonon mode. The high pressure phase is formed by the rigid rotation of almost undistorted ReO₆ octahedra and the Re-O-Re angle deviates from 180°. We do not see any evidence for the existence of the tetragonal (P4/mbm) intermediate pressure phase reported earlier.     

Lithium insertion to ReO3-type metastable phase in the Nb2O5–WO3 system

Lithium insertion to distorted ReO₃-type metastable solid solution Nb_xW_1−xO_3−x/2 (0≤x<0.25) has been studied by chemical and electrochemical methods. In the course of lithium insertion into tetragonal compounds, transition to a cubic phase was found to occur in the region where values of y (in Li_yNb_xW_1−xO_3−x/2) fall between 0.2 and 0.3, and the phase transition was found to depend on the conditions of the reaction. Changes in OCV and lattice parameters in tetragonal region (y<0.2) were discussed from the viewpoint of the ordering of lithium ions. Also, the component diffusion coefficient of lithium in tetragonal compounds Li_0.1Nb_xW_1−xO_3−x/2 (0≤x≤0.23) was found to increase with niobium content when x≤0.10, and to saturate at 4×10⁻⁹ cm²/s.     

Role of WO3 in structural and optical properties of WO3-Al2O3-PbO-B2O3 glasses

Glass samples of composition xAl₂O₃-20PbO-(80−x)B₂O₃ and xWO₃-xAl₂O₃-20PbO-(80−2x)B₂O₃ with x varying from 0% to 10% mole fraction are prepared by melt quench technique. The optical band gap decreases (from 3.21 to 2.37 eV) more for WO₃-Al₂O₃-PbO-B₂O₃ glasses with an addition of WO₃ content. The FTIR spectral studies have pointed out the conversion of structural units of BO₃ to BO₄ and WO₄ to WO₆ in these glasses. The increase in density from 4.51 to 5.80 g cm⁻³ for WO₃-Al₂O₃-PbO-B₂O₃ glasses is observed with an increase in WO₃ content. This is observed that the atomic structure changes more with the incorporation of WO₃. This is due to the formation of WO₆, WO₄ and BO₄ units.     

Grain size effect on GdFeO3-type lattice distortion and ferroelectric behavior in DyMnO3

The GdFeO₃-type lattice distortion and consequent ferroelectric behavior in polycrystalline DyMnO₃ with different grain sizes are studied by means of the Rietveld refinement of the X-ray diffraction at room temperature and ferroelectric measurements at low temperature. As evidenced by the variation in the in-plane MnOMn bond angle and the ferroelectric polarization, the distorted crystal structure and ferroelectric behavior of DyMnO₃ can be effectively modulated by varying grain size. This work sheds light on the size effect in type-II multiferroic manganites.     

Effects of anharmonic lattice distortion on orbital and magnetic orderings in KCuF3

Lattice, magnetic and orbital structures in KCuF3 are self-consistently determined by our cluster self-consistent field approach based on a spin-orbital-lattice Hamiltonian. Two stable structures are obtained and found to be degenerate, which confirms the presence of the coexistent phases observed experimentally. We clearly show that due to the inherent frustration, the ground state of the system only with the superexchange interaction is degenerate; while the Jahn-Teller distortion, especially the anharmonic effect, stabilizes the orbital ordered phase at about 23% in the x2-y2 orbit and at 77% in the 3z2-r2 orbit. Meanwhile the magnetic moment of Cu is considerably reduced to 0.56μB, and magnetic coupling strengths are highly anisotropic, Jz/Jxy ≈ 18. These results are in good agreement with the experiments, implying that the anharmonic Jahn-Teller effect plays an essential role in stabilising the orbital ordered ground state of KCuF3.     

Electrical properties of WO3 powders

The electrical properties of WO₃ powders have been investigated. For a “fresh” sample, at low electric fields, the ln I vs $\text{T}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>7</mtext>}}$ in the temperature range 60–360 K is consistent with a hopping mechanism. At higher fields the experimental results are explained in terms of a single centre Poole-Frenkel effect. The negative resistance regime and the switching processes can be due to a thermal mechanism.     

Electrochemical lithium incorporation into WO3 and MoO3 thin films

The generally accepted mechanism of electrochromic phenomena into WO₃ thin films involves the simultaneous injection of cations and electrons to form a tungsten bronze M_xWO₃, e.g. H_xWO₃,Li_xWO₃. Electrochromic cells of the type ITO MO₃/LiClO₄(M) in propylene carbonate/Pt, where M = W or MO and MO₃ is an amorphous thin film have been used. Different amounts of charges have been injected through these cells by electrochemical means at room temperature. The variations of the MO₃ electrode potential with the injected charge are in good agreement with the assumption of the formation of Li_xMO₃ compounds. The free enthalpies of formation of these bronzes have been calculated. The behavior of thin film electrodes seems to be intermediate between amorphous and crystallized bulky materials. Some measurements of electrode potential have also been carried out with crystallized thin films of sputtered WO₃.     

Al2O3:V3+晶体局域结构及其自旋哈密顿参量研究

提出了解释掺杂离子局域结构畸变的配体平面移动模型，建立了此模型下晶体微观结构与自旋哈密顿参量之间的定量关系.在考虑自旋与自旋、自旋与另一电子轨道和轨道与轨道作用等微小磁相互作用的基础上，采用全组态完全对角化方法，对Al₂O₃晶体中V³⁺的局域结构和自旋哈密顿参量进行了系统的研究.结果表明，V³⁺掺入Al₂O₃晶体后，上下配体氧平面间距离增大了0.0060 nm.从而成功地解释了Al₂O₃:V³⁺晶体的自旋哈密顿参量.在此基础上，研究了三角晶场下3d²离子自旋哈密顿参量的微观起源.研究发现，自旋三重态对自旋哈密顿参量的贡献是主要的，微小磁相互作用对自旋哈密顿参量的贡献只与自旋三重态有关.     

The electrochromic effect in polar WO3

Electrochromic absorption bands near 1.3 eV found in the low temperature phase of WO₃ single crystals are interpreted in terms of small polaron theory.     

Raman study of WO3 thin films

The influence of substrates, thermal treatment and coloration-bleaching cycles on the structure of WO₃ thin films used in electrochromic devices has been investigated by Raman microscopy. Films (2000–8000 Å) were prepared by RF sputtering from a metallic tungsten target at a constant pressure (5 × 10^?3Torr) of pure oxygen or a mixture of Ar20% O₂. They are amorphous, transparent and electrochromic. Thermal treatment at 360°C produces crystallization. Modifications of the WO₃ framework are also induced by coloration-bleaching cycles.     

Intrinsic polyatomic defects in Sc2(WO4)3

The intrinsic formation of polyatomic defects in Sc₂(WO₄)₃-type structures is studied by Mott Littleton calculations and Molecular Dynamics simulations. Defects involving the WO₄²⁻ tetrahedron are found to be energetically favorable when compared to isolated W and O defects. WO₄²⁻ Frenkel and (2Sc³⁺, 3WO₄²⁻) Schottky defects exhibit formation energies of 1.23 eV and 1.97 eV, respectively and therefore may occur as intrinsic defects in Sc₂(WO₄)₃ at elevated temperatures. WO₄²⁻ vacancy and interstitial migration processes have been simulated by classical Molecular Dynamics simulations. The interstitial defect exhibits a nearly 10 times higher mobility (with a migration energy of 0.68 eV), than the vacancy mechanism (with a slightly higher migration energy of 0.74 eV) and thus should dominate the overall ionic conduction. Still both models reproduce the experimental activation energy (0.67 eV) nearly within experimental uncertainty.     

Preparation of WO3 nanoparticles and application to NO2 sensor

WO₃ nanoparticles were prepared by evaporating tungsten filament under a low pressure of oxygen gas, namely, by a gas evaporation method. The crystal structure, morphology, and NO₂ gas sensing properties of WO₃ nanoparticles deposited under various oxygen pressures and annealed at different temperatures were investigated. The particles obtained were identified as monoclinic WO₃. The particle size increased with increasing oxygen pressure and with increasing annealing temperature. The sensitivity increased with decreasing particle size, irrespective of the oxygen pressure during deposition and annealing temperature. The highest sensitivity of 4700 to NO₂ at 1 ppm observed in this study was measured at a relatively low operating temperature of 50 °C; this sensitivity was observed for a sensor made of particles as small as 36 nm.     

Structural and optical properties of WO3-ZnO-PbO-B2O3 glasses

Glass samples of compositions 20PbO-80B₂O₃ and xWO₃—(20−x) ZnO-20PbO-60B₂O₃ with x varying from 0% to 10% mole fraction are prepared by the melt quench technique. Decrease in the band gap from 2.86 to 2.16 eV for ZnO-PbO-B₂O₃ glasses with an increase in the WO₃ content has been observed and discussed. The FTIR spectral studies have pointed out the conversion of structural units of BO₃ to BO₄ and WO₄ to WO₆ with the presence of W-O-W vibration of tungsten and incorporation of ZnO₄ structural units of zinc in these glasses. The increase in density from 2.75 to 4.03 gcm⁻³ for ZnO-PbO-B₂O₃ glasses is observed with an increase in WO₃ content. Due to the formation of WO₆, WO₄ and BO₄ units, changes in the atomic structure with WO₃ composition are observed and discussed.     

D3d distortion in CsUF6

A slight D_3d distortion on the octahedral UF₆^? complex splits the Γ^'₈ and Γ₈ octahedral levels into four doublets. These doublets and other electronic levels of the CsUF₆ ground configuration have been identified according to their vibronic side-bands. In the calculation of the crystal field Hamiltonian, the amount of D_3d distortion on the UF₆^? octahedron is treated as the parameter, Δ. The interactions of the 5f electron of the UF₆^? complex contains five parameters: ξ_5f, B₂⁰, B₄⁰, B₆⁰ and Δ. The calculated energy spectrum and the g-tensor of the ground state show good agreement with the experimental measurements.     

WO3表面的氧缺陷

用不同能量的Ar⁺和H⁺轰击WO₃表面,观察到W_4f峰的位移和加宽。通过谱分解处理得到相应于W_4f电子的W⁶⁺,W⁴⁺和W⁰3个不同的双峰。用表面产生氧缺陷的机理解释了还原过程。UPS谱显示出氧缺陷的存在增加了靠近费密能级处的态密度。H₂O的吸附结果说明WO₃表面的活性与W^{5+< 关键词：}     

Hydrogen on WO3(001)

Ultraviolet photoemission measurements of the (001) surface of WO₃ show that after exposure to atomic hydrogen new states appear in the energy gap region at 2.3 and 0.9 eV below the Fermi energy. A peak at 2.2 eV was also produced by hydrogen ion bombardment. To investigate whether these surface effects were due to the formation of hydrogen tungsten bronze H_xWO₃, comparison was made with spectra from the electronically similar sodium bronze. The form of the spectra were different both in the valence band and energy gap regions indicating that hydrogen adsorption or loss of oxygen from the surface rather than formation of H_xWO₃ is the major effect.     

The low temperature heat capacity and electrical resistivity of ReO3

Low temperature heat capacity and electrical resistivity measurements are reported for ReO₃. The heat capacity data give an acoustical mode Debye temperature θ = 327 K, and an electrronic density of states parameter γ = 2.83 mJ/mole-K². The observed temperature dependence of the resistivity is consistent with the existence of electron scattering both from acoustic mode phonons and from optical mode phonons of characteristic temperature θ_E = 1080 K. The above measurements are used to evaluate the electron-phonon interaction parameter λ = 0.24.     

Stiffness matrix and debye temperature of ReO3 from ultrasonic measurements

We have measured the propagation velocities of bulk acoustic waves in the simple cubic transition-metal oxide ReO₃ by ultrasonic pulse propagation. The elastic stiffness constants at 300 K are: C₁₁ = (47.9 ± 1.4) × 10¹¹ dyne/cm²; C₄₄ = (6.1 ± 0.2) × 10¹¹ dyne/cm²; C₁₂ = (?0.7 ± 2.8) × 10¹¹ dyne/cm². These elastic constants indicate a crystal with highly anisotropic shear propagation. The Debye temperature of the compound from these measurements is 528 K. This value is somewhat higher than previous results from specific heat and resistivity determinations.