Soft acoustic mode in thallium arsenic Sulphide Tl3AsS42

Ultrasonic measurements of the slow shear mode velocity in Tl₃AsS₄ have been made as a function of hydrostatic pressure up to 2.6 GPa at room temperature. The velocity is a decreasing function of pressure over the entire pressure range. Two pressure-induced phase transitions are observed, one (at 2.1 GPa) characterized by a discontinuous drop in velocity, and the other (at 2.6 GPa) by a continuous softening of the velocity to zero. The results are discussed in terms of the Miller-Axe theory of internal-strain coupling to a soft optic mode. Comparison is made with results on Tl₃PSe₄, where only one transition is observed.