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.     

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.     

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.     

The sputtering of high Tc A15 Nb3Si and V3Ge

We report some initial results on the preparation of A15 Nb₃Si and V₃Ge using a getter sputtering technique. Under sufficiently clean conditions we observe an increase in the superconducting transition temperature. DC onsets in excess of 14 and 11 K have been observed for Nb₃Si and V₃Ge respectively. In each case a positive identification of the A15 phase has been made.     

Electron density distribution in V3Ge

The electron distribution map in V₃Ge superconducting alloy of A15 family was obtained from accurate X-ray diffraction data. This map was shown having no peaks corresponding to the covalent interaction in the chains of vanadium atoms. Maxima in the map were shown to correspond to the p-electrons of germanium in the field of octahedral symmetry.     

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.     

Densities of phonon and electron states in V3Si and Cr3Si crystals

The regularization method has been used to solve the non-correct problem of finding out the phonon density of states from the temperature dependence of phonon heat capacity for two single crystals with A15 structure (the superconductor V₃Si and the non-superconductor Cr₃Si. The solution found agrees with the results of neutrongraphical and tunnel study. The electron density of states obtained in a narrow energy range near the Fermi level as the regularized solution of the reverse problem (from the temperature of the electron spin susceptibility) has at the Fermi energy the sharp maximum for V₃Si, but the flat minimum for Cr₃Si.     

Singularities in V3Si point-contact spectra

The V₃Si point-contact has been studied. The low-frequency mode of electron-phonon interaction has been observed by eV ～ 16 meV unknown previously. The “dirty” point-contacts exhibited the effects of the negative contribution to the dynamic electric resistance which might be the manifestation of the electron-electron correlations.     

Third order elastic constants of NaCl

A brief review is given of the meaning of the third order elastic constants and of the general significance of the anharmonic parameters for mechanical and thermodynamic properties. The measurements of the third order elastic constants of NaCl and KCl by Chang and of NaCl by Swartz are described. In each case the changes in ultrasonic velocities were observed as a function of applied uniaxial stress. More recently a program in beam mixing has been initiated by Dunham to study the third order constants of NaCl through the conversion efficiency of the T+T →L interaction.     

Phonon density of states of V3Si

The phonon density of states of V₃Si at 300°K has been calculated from a microscopic model. It exhibits three pronounced peaks, in qualitative agreement with experimental results of Schweiss obtained from inelastic neutron scattering. By reducing the effective charges of both ions the agreement can be improved, but at the expense of agreement for the mode softening.     

Elastic constants of Cr3Si

Elastic constants of Cr₃Si have been measured on single crystals oriented along [110] and [100] as a function of temperature between 4.2 and 300 K. Their magnitudes at 4.2 K are in unit of 10¹¹ Nm^-2C₄₄ = 1.32C' = (C₁₁?C₁₂)/2 = 1.58B = 1.99 There is no peculiar behaviour as a function of temperature.     

Third order elastic constants of rubidium chloride

The third order elastic constants of RbCl, determined by measurements of the static stress dependence of ultrasonic waves, are found to be (in units of 10¹²$\text{dynes}\text{cm}{}^2$)

$\text{C}{}_{111}\text{= ?6.71 ± 0.1 C}{}_{123}\text{= 0.05 ± 0.07}$

$\text{C}{}_{112}\text{=?0.18 ± 0.04 C}{}_{144}\text{= 0.11 ± 0.02}$

$\text{C}{}_{166}\text{=?0.17 ± 0.01 C}{}_{456}\text{= 0.4 ± 0.01}$

. The calculation of third order constants using a rigid ion Born model is briefly discussed, and results are compared to the measurements. The comparison qualitatively supports the model, but no quantitative evaluation of the repulsive interaction is possible.     

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.     

Pressure dependence of positron lifetimes in V3Si

We have measured positron lifetimes in V₃Si at hydrostatic pressures of 0 to 20 kb, at room temperature. The lifetimes were independent of pressure, within the experimental uncertainty of ± 2 ps; the results of eight measurements all were between 129 and 133 ps.     

Specific heat of a transforming V3Si crystal

The specific heat of a transforming V₃Si crystal in the normal, mixed, and superconducting states has been measured from 6 to 30K in zero and 52.3 kG magnetic fields. An analysis has been carried out in a self-consistent way based on the second-order phase transition from the normal to the superconducting state in zero magnetic field. Various physical parameters characterizing the superconducting and normal states are derived from the thermodynamics and the BCS weak coupling theory. The most important parameter obtained in this analysis is 2Δ(0)/kT_c = 3.46, which indicates a weak coupling in our V₃Si sample.     

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.     

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).     

Heat capacity measurements and phase transitions in V3Si

The superconducting transition temperatures of both cubic and tetragonal phases of V₃Si have been observed unambigously for the first time, by heat capacity measurements based on steady state calorimetry technique. They are separated by about 1°K. In pressed powder compacts of V₃Si, another superconducting phase due to residual stress is found to occur with a transition temperature near 8°K.     

The paramagnetic susceptability of neutron irradiated V3Ge single crystals

The paramagnetic susceptibility χ of V₃Ge single crystals was measured in the temperature range from 10 K to 350 K after irradiation with neutron fluences of $\text{(0.3?20) × 10}{}^{18}\text{n}\text{cm}{}^2\text{(E>1}\text{MeV}\text{)}$. Unirradiated crystals show a flat maximum at T=65±6 K, which disappears at high neutron doses. The temperature dependence and the absolute values of χ are continuously reduced by increasing neutron doses. This behaviour can be understood by the models of high density of states (DOS) proposed for A15 materials confirming the assumption that a sharp peak in the electronic density of states exists which is lowered and broadened by irradiation induced defects.