Acoustic phonon mode softening in InCd alloy single crystals

To examine the effect of alloying In with Cd on the lattice stability, the elastic constants of single crystals of the tetragonal In 3.42 at.% Cd alloy have been determined. The shear modulus $\text{1}\text{2}\text{(C}{}_{11}\text{? C}{}_{12}\text{)}$ is even less in the alloy than in In and decreases rapidly as temperature rises. It is concluded that the 110], q6$\text{1}\text{1}\text{0}$] acoustic phonon mode is soft near the zone centre and softens even further as the phase boundary demarking the incipient diffusionless phase transition is approached.