Elastic constants of KMnF3 as functions of temperature and pressure

We have measured the single-crystal elastic constants of KMnF3 and their first pressure derivatives from 200 to 430 K at a frequency of 20 MHz by means of the ultrasonic-pulse-superposition method. Both sets of data show cusplike anomalies above the improper Oh-D4h ferroelastic transition at 186 K that extend up to 350 K. Above 350 K the temperature dependence is linear, making it possible to determine the bare harmonic and anharmonic constants by means of linear extrapolation to T=0. At room temperature the second pressure derivatives of the elastic constants were also measured. Analysis of the data on the basis of a model with Coulomb and short-range central force interactions indicates anomalously large fourth derivatives of the Mn-F and F-F pair potentials. A calculation of the temperature dependence of the elastic Grüneisen parameter is consistent with models for the acoustic anomaly according to which coupling between strains and order-parameter fluctuations is limited to frequencies less than the relaxation rate of the soft R25 mode.