Pressure dependence of elastic constants and of shear mode instability forNb3Sn

Abstract

The pressure dependence of the three single-crystal elastic constants of ${\mathrm{Nb}}_3$Sn has been measured by means of the ultrasonic pulse superposition method from about 13 to 300 K. The first pressure derivative of the soft shear modulus $\frac{1}{2}(c_{11}-c_{12})$ decreases almost monotonically with decreasing temperature from + 1.4 at 300 K to - 1 above the structural transformation temperature at 45 K. The pressure coefficient of the critical temperature corresponding to vanishing shear modulus agrees approximately with the pressure coefficient of the structural transformation temperature, which is of opposite sign to that in ${\mathrm{V}}_3$Si. Above 100 K the Grüneisen parameter calculated from the elastic data in the anisotropic continuum approximation agrees well with the thermal Grüneisen parameter, but below 80 K discrepancies occur which are attributed to precursor effects of the transformation. The temperature variation of the pressure coefficient of the soft shear modulus cannot be explained on the basis of the Labbé-Friedel model even if pressure-induced interband charge transfer is included.