Ultrasonic study of lattice stability in indium + thallium alloys

Abstract

Ultrasonic wave velocities and attenuation have been measured by the pulse echo method in both the cubic and tetragonal forms of indium + thallium alloys in the vicinity of the martensitic phase transformation. Results lead to insight into the lattice stability of the alloys and the microscopic nature of the transition; anomalously small values of the modulus $\frac{1}{2}(C_{11}=C_{12})$ found in both crystallographic modifications, coupled with marked increases in ultrasound absorption near the transition, attest to lattice mode softening as the basis of the transformation mechanism. Detailed measurements of the components of the stiffness tensor of a 10 at.% thallium alloy are presented and compared with those of indium itself. The lower crystal stability of the alloy relative to that of indium is graphically demonstrated by sections of velocity surfaces. A sensitive indication of the impending phase change is found in the linear compressibilities: indium and its tetragonal alloys with thallium show a marked inclination to adopt a cubic structure.