Mössbauer study of the lattice properties of the systemV3Ga1−xSnx

The temperature dependence of the ${}_{}{}^{119}{}_{}{}^{}\mathrm{Sn}$ Mössbauer shift and the relative recoil-free fraction has been measured in the $A-15\mathrm{}$ pseudobinary compounds ${\mathrm{V}}_3{\mathrm{Ga}}_{1-x}{\mathrm{Sn}}_x$ ($x=0.9,0.5,0.1$). A two-Debye model for the phonon spectra describes the Sn vibrational motion, $〈v^2〉$ and $〈x^2〉$. For ${\mathrm{V}}_3$ ${\mathrm{Ga}}_{0.9}$ ${\mathrm{Sn}}_{0.1}$ a deviation of the thermal shift from Debye behavior is shown to be due to lattice softening; that is, phonon mode shifting is observed for the Ga-rich high-$T_c$ compound. Comparison with heat-capacity results indicates that the vibrational motion of the vanadium atom also softens at low temperature. Moreover, a consideration of the concentration dependence of the Sn and Ga phonon contribution to the mass enhancement $\lambda$ suggests that the vanadium site controls the dependence of $\lambda$, and hence $T_c$, on concentration.