Ultrasonic Attenuation near the Soft Mode Transition Point in KMnF3

Abstract

Ultrasonic attenuation measurements in KMnF 3 have been performed with 20 and 60 MHz near the \(\varGamma_{25}\) 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 \(T_{\text{c}}{\cong}186.65\)°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.