Study of the Phase Transition in KH3(SeO3)2. I. Acoustic Phonon Softening

The phase transition of KH 3 (SeO 3 ) 2 , was studied by ultrasonic velocity measurements at 5 and 10 MHz together with X-ray diffraction measurement and pyroelectric examination. It was clarified that the crystal below the transition point ( T c ) is nonferroelectric and belongs to the centrosymmetric space group P2 1 / b . All of the elastic stiffnesses c i j were determined front the velocity data. The phase transition accompanies with a large phonon softening only in the c 44 mode. It was shown that the ferroelastic behavior above and below T c can well be explained by a phenomenological theory.