Ferroelastic Phase Transition in TlH₂PO₄

Abstract

The dielectric constant along the b axis, ε_b, of TlH₂PO₄ was measured and its domain structures were observed. It is found that TlH₂PO₄ 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 (T₁), above which x_5S vanishes. ε_b shows a small anomaly at T₁. There are two kinds of domain structures at room temperature; one is the domain structure with the (100) and/or (001) domain walls which disappears above T₁ and the other is the structure with the (201) wall which has no relation with the 84°C transition.