Abstract
Optical observation of twin structures, the measurement of lattice deformation and X-ray examinations of K 3 H(SeO 4 ) 2 were carried out in a temperature range between 25°C and 130°C. It was found that K 3 H(SeO 4 ) 2 undergoes the improper ferroelastic phase transition at 117°C ( T c ) and has spontaneous strains e 13 =-5.1×10 -3 and ( e 22 - e 11 )/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\bar3 m above T c . It was clarified by both observation and symmetry consideration that K 3 H(SeO 4 ) 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) g 2 .
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