An ultrasonic study of the elastic phase transition in In-Cd Alloys

Abstract

Indium rich, In-Cd alloys undergo a f.c.c. to f.c.t. structural phase transition which the presence of a cubic invariant in the Hamiltonian renders first order. Ultrasonic wave velocity measurements have been made in single crystals of both the f.c.c. and f.c.t. phases, emphasis being placed on behaviour near the phase transition. The shear modulus ½(C ₁₁-C ₁₂) tends to very small values in both phases as the transition temperature T _c is approached: the onset of instability is manifested as softening of the acoustic phonon branch with wave vector (110), particle displacement (110) near the Brillouin zone centre. However, in contrast to its behaviour in the In-Tl alloys, ½(C ₁₁-C ₁₂) does not go to zero in the In-Cd alloys. This finding corresponds to the more recognizable first-order character of the transition in the In-Cd compared to that in In-Tl alloys. The effect of the enhanced acoustic mode softening at T _c on the wave velocity and Young's modulus surfaces is discussed. In addition to the study of the phase transition mechanism, metallurgical problems concerned with the structure and nature of monocrystalline In-Cd alloys have been examined.