Lattice Instability of High-Transition-Temperature Superconductors. I. PolycrystallineA−15Compounds

The $A-15\mathrm{}$ ($\beta$-tungsten) structure compounds ${\mathrm{Nb}}_3$Sn, ${\mathrm{Nb}}_3$Al, ${\mathrm{Nb}}_3$Ga, ${\mathrm{V}}_3$Si, and ${\mathrm{V}}_3$Ga, which are all superconducting above 14°K, have been found to exhibit elastic softening on cooling. The anomalous temperature dependence of the elastic moduli disappears in the superconducting state. This unusual behavior does not occur for the isostructural compounds ${\mathrm{V}}_3$Ge and ${\mathrm{Nb}}_3$Ge, which have comparatively low transition temperatures ($T_c\sim 5-7°$K).