Ultrasonic attenuation measurements in KMnF 3 have been performed with 20 and 60 MHz near the \(₂₅\) 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 \(Tc{}186.65\)°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.
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Furukawa et al. (1970) studied this question.