Ferroelastic Phase Transition in (NH₄)₃H(SeO₄)₂

Abstract

Optical observation of twin structures, X-ray examinations and the measurement of the dielectric constant of (NH₄)₃H(SeO₄)₂ were carried out in reference to the phase transition at 29°C (T_c). It was found that (NH₄)₃H(SeO₄)₂ undergoes the improper ferro-elastic phase transition at T_c. The crystal is determined to be monoclinic with the space group A2⁄a below T_c and trigonal with the space group R\bar3m above T_c. The crystal has spontaneous strain (e₂₂−e₁₁)⁄2=−3.0×10⁻³ and e₁₃=3.3×10⁻³ at room temperature. It was clarified by both observation of domains and symmetry consideration that the crystal exhibits twin structures with two types of domain walls and three kinds of domain orientations at room temperature. The behaviour of the dielectric constant observed near T_c is well explained by the Landau theory.