Calculation of the ultrasonic frequencies of the q[110] mode for the first-order and tricritical phase transitions in NH4Cl

Abstract

Abstract We study here the ultrasonic frequencies of the q [110] mode of NH 4 Cl, which we have correlated to the volume changes for the first-order and the tricritical phase transitions in this crystal. We calculated the ultrasonic frequencies of this mode by means of our correlations for the pressures of 0, 0.5 and 0.95 kbar in the first order, and for 1.5 kbar in the tricritical phase regions in NH 4 Cl using the observed data. Our calculated frequencies are in good agreement with those observed given in the literature for the q [110] mode of NH 4 Cl at the pressures studied. This indicates that the ultrasonic frequencies can be calculated by the method given here and that the observed behaviour of NH 4 Cl can be explained satisfactory near the first-order and tricritical phase transitions in this crystal.