Anisotropy of acoustic and thermal expansion properties of TlInSe₂ crystals

Based on experimental studies of acoustic wave velocities in TlInSe₂ crystals, all components of their elastic stiffness and compliance tensors are determined. TlInSe₂ has three acoustic axes parallel to the principal crystallographic directions. The lowest velocities of acoustic eigenwaves are used to estimate the acousto-optic figure of merit. We analyse deviations from purely longitudinal and transverse polarization states, as well as the obliquity of acoustic energy flow. Temperature dependences of strains along the three principal crystallographic directions and of acoustic wave velocities v₁₁, v₂₂ and v₃₃ are measured. Anomalous behaviour of these parameters occur at T₁ $C_{11}=C_{33}$245 K and T₂ $C_{12}=C_{13}$145 K. It is shown that the symmetry of TlInSe₂ becomes lower than tetragonal below the temperature point T₁.