Grüneisen parameter and thermal expansion of V3Si and V3Ge

The Grüneisen parameter and lattice thermal expansion of the A-15 compounds V₃Si and V₃Ge at room temperature are evaluated on the basis of the method due to Brugger and Fritz [1] from the third order elastic constants reported earlier [2]. The calculated values are compared with available experimental values and are found to fit satisfactorily.     

The elastic constants of V2O3 in the insulating phase

The initial slopes of the acoustic phonon dispersion curves in (V_0.98Cr_0.02)₂O₃ have been measured at room temperature in several of the high symmetry directions by inelastic neutron scattering. From these data several sound velocities and four independent elastic constants have been determined. Although the Cr doped specimen is in the insulating phase and pure V₂O₃ is metallic, these results are in good agreement with recent data (1) obtained by ultrasonic measurements on pure V₂O₃.     

High elastic modulus in b-axis-oriented single crystal V2O5

The elastic moduli of V₂O₅ are of great importance for the assessment of the buildup of strain during thin-film growth as well as for the analysis of defect generation and propagation. The usually rather small crystal dimensions make a precise experimental determination of the elastic constants highly challenging and only very little is known about the temperature dependent strain evolution. Here large V₂O₅ single crystals grown with different parameter sets are investigated by ultrasonic pulse experiments and by sampled continuous wave ultrasound spectroscopy. The elastic modulus C₂₂ is determined to be 220 GPa at room temperature. Temperature dependent investigations of the elastic behavior show that ultrasonic experiments are suitable for highly sensitive detection of oxygen-deficient phases in rapidly grown samples. In addition they indicate the presence of temperature dependent elastic instabilities in inhomogeneous samples.     

Radiation damage in the non-metal sublattices of V3Ge and V3Si single crystals

Channeling measurements on V₃Ge and V₃Si single crystals, which have been irradiated with He-ions, revealed a considerable narrowing of the angular yield curves for the B-atom sublattices. Analysing the data with a computer simulation, the best agreement with the experiment was achieved by adding a static contribution to the thermal vibration amplitudes. After irradiation with He-fluences, which cause a saturation in the degradation of the superconducting transition temperature in the V₃Ge and V₃Si films, it was concluded that the average static displacement amplitude was 0.006 nm for the Ge-atoms and 0.01 nm for the Si-atoms.     

Third and fourth order elastic constants of fluoperovskites CsCdF3, TlCdF3, RbCdF3, RbCaF3

The hydrostatic dependence of the second order elastic constants of several fluoperovskites has been measured and studied from a macroscopic point of view and through a force field analysis. The first part of this paper is devoted to the experimental determination of some combinations of the third and fourth order elastic constants. The second part analyses the pressure dependence of the second order elastic constants from a macroscopic point of view. The large values of the second order anharmonicity coefficients $\text{b}{}_{\mathrm{ij}}\text{=}\text{1}\text{2}\text{(}\text{?}{}^2\text{C}{}_{\mathrm{ij}}\text{?P}{}^2\text{)}{}_0\text{C}{}^{\mathrm{(0)}}{}_{\mathrm{ij}}$ are connected to the occurrence of a phase transition in some compounds, whereas the first order coefficients $\text{(}\text{?C}{}_{\mathrm{ij}}\text{?P}\text{)}{}_0$ have an order of magnitude which agrees with a quasiharmonic hypothesis. The third part is devoted to the microscopic interpretation of the third order elastic constants. A rigid ion model is used with either classical Born Mayer potentials or a modification of these potentials with an axial term (Cowley and Rousseau's models).     

The elastic behaviour of the ternary zincblende structure semiconductor CuGe2P3

A single crystal has been grown of CuGe₂P₃, a ternary semiconductor with a zincblende structure in which the copper and germanium atoms are randomly distributed on the cation sites. The second order elastic constants measured by the ultrasonic pulse echo overlap technique (C₁₁ = 13.66, C₁₂ = 6.17, C₄₄ = 6.66 and bulk modulus B = 8.67 in units of 10¹⁰Nm^?2 at 291 K) are closely similar to those of GaP and conform well to Keyes' correlation for zincblende structure crystals. The hydrostatic pressure derivatives of the second order elastic constants ($\text{?C}{}_{11}\text{?P}\text{= 4.40}$, $\text{?C}{}_{12}\text{?P}\text{= 3.9}$, $\text{?C}{}_{44}\text{?P}\text{= 0.93}$ and $\text{?B}\text{?P}\text{= 4.07}$) and the third order elastic constants (C₁₁₁ = ? 8.45, C₁₁₂ = ? 3.49, C₁₂₃ = ? 1.13, C₁₄₄ = ? 2.82, C₁₅₅ = ? 1.44 and C₄₅₆ = ? 0.69 in units of 10¹¹Nm^?2) also resemble those of GaP: even the anharmonicity of the long wavelength acoustic modes of this ternary semiconductor resembles that of its binary “parent” compound. The positive signs of the hydrostatic pressure derivatives and the negative signs of the third order elastic constants show that the acoustic modes at the long wavelength limit stiffen under pressure, an effect which is quantified here by computation of the mode Grüneisen parameters, which are all positive. The harmonic and anharmonic force constants, obtained in the valence force field model, fit well into the pattern shown by related zincblende structure compounds: the ratio ($\text{β}\text{α}$) for bond bending (β) to stretching (α) force constants is greater than 4:1; the dominating anharmonic force constant is γ: most of the anharmonicity is associated with nearest neighbour atoms. Analysis of the temperature dependence of the elastic constants on the basis of a simple anharmonic model again emphasises the similarity between the elastic behaviour of CuGe₂P₃ and GaP. The thermal expansion of CuGe₂P₃ varies almost linearly with temperature between 291 K and 1000 K, the linear coefficient of thermal expansion α being 8.21 ± 0.02 × 10^?6 °C^?1. The similar lattice parameters and elastic behaviour of CuGe₂P₃ and GaP indicate that semiconducting devices made of epitaxial deposits of CuGe₂P₃ on a GaP substrate should prove to be almost strain-free.     

Lattice parameter anomalies in V2O3 at high temperature

The rhombohedral lattice parameters of pure V₂O₃ show anomalous behaviour in the high temperature region. The change in the rate of expansion of the rhombohedral angle α_R occurs at 533 K where the lattice constant a_R reaches a minimum. The sharp dip at 533 K in the a_R curve can be correlated with the peak observed in recent small angle scattering of neutrons and might be due to some dynamic magneto elastic effect.     

Insulator-metal transition in V3O5

The results of a study of the insulator-metal phase transition in V₃O₅ occuring at T = 450 K are presented. Conductivity, thermopower, magnetic, structural and optical data are reported. On a basis of the data obtained we conclude that the high-temperature phase is a poor metal with strong electron correlations and localized magnetic moments. The metal-non-metal transition is presumably driven by the spatial ordering of V³⁺ and V⁴⁺ ions.     

Formation of V2O3 nanocrystals by thermal reduction of V2O5 thin films

We report the formation of homogeneous and stable V₂O₃ nanocrystals, directly from V₂O₅ thin films, at 600 °C, as observed by using in situ electron microscopy experiments. Thermally-induced reduction of V₂O₅ thin films in vacuum is remarkably different when compared to reduction of V₂O₅ single crystals and results in the formation of nanophase V₂O₃. Thermally grown V₂O₃ nanocrystals exhibit hexagon or square shape and are stable at higher temperature as well as room temperature. The formation of stable nanocrystals through the reduction process in a non-chemical environment (vacuum) could provide a basis for understanding the complex processes of vanadium oxide phase transitions and for controlling the chemical processes to produce oxide nanocrystals.     

The electronic structure of hypothetical Nb3Si and V3In

Self-consistent LMTO (Linear Muffin Tin Orbital) band calculations have been performed for Nb₃Si and V₃In. Calculated pressures show agreement with the results from existing Nb₃X and V₃X compounds, when the lattice parameters are 5.125 and 5.025 Å (± 0.025 Å) respectively. At these lattice dimensions, the superconducting transition temperatures are predicted to be approximately 18 K for Nb₃Si and 31 K for V₃In.     

First order phase transition in V3Si

Thermal expansion measurements on a single crystal of V₃Si for the first time give evidence that the low temperature structural transition is of first order. The associated relative volume change is of the order of 10^-5.     

碳热还原法制备V2O3-C双层包覆磷酸铁锂及其电化学性能

通过V₂O₅的碳热还原反应制备了具有优异倍率性能和循环稳定性的V₂O₃-C双层包覆的磷酸铁锂正极材料. 粉末X射线衍射、元素分析、高分辨投射电镜和拉曼光谱研究表明V2O3相与碳层共包覆于磷酸铁锂颗粒表面. 在V₂O₅的碳热还原反应后,碳含量明显降低,但石墨化程度未发生明显改变. 电化学测试结果表明少量V₂O₃显著改善了磷酸铁锂正极材料的倍率性能和高温循环性能,包含1%氧化钒的复合正极材料在0.2 C放电容量为167 mAh/g,5 C时放电容量为129 mAh/g,并且循环稳定性优异;在55 oC和1 C时放电容量为151 mAh/g,循环100次后无明显容量衰减.     

Electronic charge density of V3Si

The electronic charge density of V₃Si has been calculated using a self-consistent version of the linear-augmented-plane-wave (LAPW) method. The results exhibit a substantially nonspherical charge distribution at and beyond the V 3d-shell maximum, directional-bonding along the V-atom chains, and a net charge transfer from Si to V. While the two latter features are in qualitative agreement with the empirically-derived charge densities of Staudenmann et al., significant differences exist between the theoretical and empirical results.     

Microwave spectra of dimethylsulfide-d6, (CD3)2S,ground and excited torsional states

The microwave spectra of the molecular isotope (CD₃)₂S in the ground state and the first and second excited states of methyl top torsion (internal rotation) and of CSC deformation as well as the ground-state spectra of the ¹³C and ³⁴S substituted forms have been measured. The rotational constants and centrifugal distortion and rotation-vibration interaction constants could be determined. The rotational lines in the excited torsional states (1₁, 1₂, 2₁, 2₂, 2₃) were found to be split into quartets due to the interaction between molecular rotation and methyl top internal rotation. The experimental multiplet splittings were fitted to those calculated from a rotation-internal rotation Hamiltonian in order to obtain values for the internal rotation barrier V₃ and the top-top interaction potential coefficients V₁₂ and V′₁₂. V₁₂ was too highly correlated with V₃ for a separate determination. The values following from the least-squares adjustment are discussed.     

On the high temperature transition in V2O3

Magnetic susceptibility measurements on V₂O₃ powder sample using a vibrating sample magnetometer and microbalance with special attention to the region of 500–530K are reported. The high temperature transition in this compound is discussed in the framework of two interacting Hubbard bands obtaining a good qualitative agreement with thermal variations of the properties of V₂O₃.     

Substrate effects on V2O3 thin films

We apply density functional theory and the augmented spherical wave method to analyze the electronic structure of V₂O₃ in the vicinity of an interface to Al₂O₃. The interface is modeled by a heterostructure setup of alternating vanadate and aluminate slabs. We focus on the possible modifications of the V₂O₃ electronic states in this geometry, induced by the presence of the aluminate layers. In particular, we find that the tendency of the V 3d states to localize is enhanced and may even cause a metal-insulator transition.     

Ta对C—15相V2Hf和V2(Zr0.5Hf0.5)系列声子性能的影响

本文用中子飞行时间方法对C-15相的超导材料V₂Hf,V₂Ta和V₂Hf_0.8Ta_0.2以及V₂Zr_0.5。Hf_0.5和V₂Zr_0.5Hf_0.33Ta_0.17的热中子非弹性散射谱作了测量,并计算出相对的广义声子态密度。结果与早先发表的Nb对C-15相V₂Zr和V₂(Hf_0.5Zr_0.5)系列声子性能的影响一致:声子频率随超导转变温度T_c增加而软化,随T_c减小而硬化。这表明,对于此类材料弹性软化在一定程度上对提高T_c起了作用。结果还进一步表明V₂Zr或V₂Hf与V₂(Zr_0.5Hf_0.5)之间有着质的差别,V₂Hf加Ta后,T_c增加,声子频率软化,而V₂(Zr_0.5Hf_0.5)加Ta后,T_c减小,声子频率则略有硬化。这与V₂Zr和V₂(Hf_0.5Zr_0.5)加Nb的结果是一致的。此结果可以用角动量分波表象的能带论方法分析电-声耦合相互作用得出的杂化理论来定性解释。 关键词：     

Shear elastic constant softening in (V1−xCrx)2O3: Second-order nature of the low temperature phase transition

From ultrasonic wave velocities in semiconducting, corundum-structured (V_1?xCr_x)₂O₃ with x = 0.015 and 0.03, we findthat the C₄₄ shear modulus softens drastically with decreasing temperature. Along with other elastic behavior this indicates a smooth change from rhombohedral to almost hexagonal elastic symmetry before a nearly second-order transition to the low T monoclinic phase.     

Determination of isotropic and anisotropic knight shift in V3X compounds

Methods are described to determine the isotropic as well as the anisotropic part of the Knight shift from powdered samples of several V₃X compounds (V₃Co, V_77.5Ni_22.5, V₃Rh, V₃Pd, V₃Pt and V₃Ir). Measurements of the Knight shifts of the central line as well as the satellites prove to be necessary. The obtained values for K_iso are about 0.5% while K_ax is not larger than 0.03%.The influence of the X elements on the quadrupolar interaction frequency v_Q will be discussed.     

Higher order elastic constants and non-linear stress-strain relation for V3Si

Relations for the third and fourth order elastic constants are derived from a Landau-Devonshire type free energy function for crystals with the A15 structure which undergo the structural transformation at low temperature. Application to transforming V₃Si at 21° K gives in connection with experimental data for the pressure derivatives of the elastic constants unusually large numerical values for c₁₁₁, c₁₁₂, c₁₂₃ and c₁₁₁₁. The stress-' strain relation calculated from these data is in semiquantitative agreement with the directly measured, strongly non-linear behavior.