Ultrasonic Attenuation near the Soft Mode Transition Point in KMnF₃

抄録

Ultrasonic attenuation measurements in KMnF₃ have been performed with 20 and 60 MHz near the Γ₂₅ soft phonon mode transition point which had been believed as a second order transition. The observed attenuation constant α obeys the formula α∝ω(T⁄T_a−1)^−β for T>T_a with T_a=187.2°K and β=1.3±0,1, where ω is the frequency of sound. Measurements of anomalous heat capacity show, however, that the phase transition is not a second order but a first order with T_c≅186.65°K accompanied with a very little thermal hysteresis. There is a definite difference between the real transition point T_c and the asymptotic point T_a of the attenuation constant α given above.