Dielectric, Thermal and Optical Studies of Phase Transitions in TlH₂AsO₄ Single Crystal

Abstract

Dielectric, specific heat and X-ray measurement in the single crystals of TlH₂AsO₄ were carried out in a temperature range between 80, K and 400, K, and its domain structures were observed. It was found that TlH₂AsO₄ is ferroelastic below 391, K (T_I) with the spontaneous strain x_5S=4.2× 10^-2 at 293, K. TlH₂AsO₄ has two distinct domain structures at room temperature; one is the domain structure with the (100) and/or (001) domain boundaries which disappears above T_I and the other is the structure with the (201) domain boundary which has no relation with the phase transition. The crystal belongs to a monoclinic system with the space group P2₁/a in the room temperature phase and an orthorhombic system with the space group Pcan above T_I. It is suggested that the ferroelastic phase transition at T_I may be caused by an instability of acoustic B_2g mode.