Acoustic Phonon Softening and Spontaneous Strains in KD3(SeO3)2

Ultrasonic velocities at 5 and 10 MHz and spontaneous strains of KD 3 (SeO 3 ) 2 were measured and the results were compared with those for KH 3 (SeO 3 ) 2 . Elastic constants were obtained from the velocity data. The phase transition of KD 3 (SeO 3 ) 2 accompanies an instability of the soft s 44 mode alone, as in the case of KH 3 (SeO 3 ) 2 . Deuteration yields no significant isotope effect in the Curie-Weiss constant C s , although it gives large isotope effect in the transition temperature T c .