The soft acoustic phonon mode and its relation to the martensitic transformation in In-Tl alloys

Phonon dispersion curves for indium and f.c.t. and f.c.c. indium-thallium alloys have been calculated using the optimised model potential theory. The presence of a soft acoustic mode along [110], polarised [1$\text{1}$0] has been established. The closer the alloy composition is to the phase boundary, the softer this mode becomes: $\text{1}\text{2}\text{(C}{}_{11}\text{–C}{}_{12}\text{)}$ tends towards zero, in agreement with experimental ultrasonic velocity data.