Dynamic Acoustic Anomalies in KSCN Near the Improper Ferroelastic Phase Transition

Abstract

The tensor of the elastic stiffness constants Cij of potassium thiocyanate (KSCN) has been measured in the temperature range 290 K to 420 K, including the order-disorder phase transition at 415 K, by the ultrasonic pulse echo overlap method at frequencies f = 20 MHz. Using the measured values for Cij the isothermal elastic constants Cij T were calculated from the data on specific heat Cp and the thermal expansion coefficients αi. It turns out that there is a huge difference between the ultrasonic and the isothermal elastic constants in the low symmetry phase. The ultrasonic elastic constants exhibit only small positive anomalies which are proportional to the square of the order parameter, while the isothermal ones tend to diminish on approaching Tc . Measurements of the elastic constants of KSCN at ultralow frequencies (f = 0.1 Hz–50 Hz) show two dispersion regimes. One is due to the isothermal-adiabatic crossover (between 0.1 Hz and 50 Hz) and the second (between 50 Hz and 20 MHz) results from the order parameter dynamics.