Abstract
Optical and X-ray examinations of CsHSeO 4 were made above and below the phase transition point ( T c ). CsHSeO 4 undergoes an improper ferroelastic phase transition at 128°C and has spontaneous strains e 0 13 =2.7×10 -2 and ( e 0 22 - e 0 11 )/2=7.9×10 -2 at room temperature. The crystal is tetragonal above T c with the unit cell parameters a t = b t =4.18 Å and c t =7.20 Å at 130°C, and exhibits a super-lattice structure with the monoclinic unit cell parameters a m ≃2 a t and b m ≃2 b t below T c . It is shown that if we assume 4/ m m m as the point group above T c , all the domain structures observed in CsHSeO 4 below T c , which disappear above T c , can be deduced from the strain-compatibility relations for domain boundaries by taking account of the symmetry change from 4/ m m m to 2/ m associated with the phase transition.
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