Ferroelastic Phase Transition in TlH₂PO₄

Abstract

The dielectric constant along the b axis, ε b , of TlH 2 PO 4 was measured and its domain structures were observed. It is found that TlH 2 PO 4 is ferroelastic with the spontaneous shear strain x 5S =2.3 ×10 -2 at room temperature and undergoes a ferroelastic phase transition at 84°C ( T 1 ), above which x 5S vanishes. ε b shows a small anomaly at T 1 . There are two kinds of domain structures at room termperature; one is the domain structure with the (100) and/or (001) domain walls which disappears above T 1 and the other is the structure with the (201) wall which has no relation with the 84°C transition.