Ferroelastic Phase Transition in K₃H(SeO₄)₂

Abstract

Optical observation of twin structures, the measurement of lattice deformation and X-ray examinations of K₃H(SeO₄)₂ were carried out in a temperature range between 25°C and 130°C. It was found that K₃H(SeO₄)₂ undergoes the improper ferroelastic phase transition at 117°C (T_c) and has spontaneous strains e₁₃=−5.1×10⁻³ and (e₂₂−e₁₁)⁄2=4.8×10⁻³ at room temperature. The crystal is monoclinic with the space group A2/a below T_c and trigonal with the space group R\bar3m above T_c. It was clarified by both observation and symmetry consideration that K₃H(SeO₄)₂ below T_c indicates two types of domain boundaries and three kinds of orientations (or domains) which appear as a result of the phase transition. It is suggested that the phase transition may be caused by a soft mode with k=(1⁄2)g₂.