Ultrasonic frequencies calculated for the q[100] mode in the first-order, tricritical and the second-order phase transitions of NH4Cl

Abstract

This study correlates the volume changes to the ultrasonic frequencies of the q[100] mode of NH4Cl for the first-order, tricritical and the second-order phase transitions in this crystal. Using these correlations we were able to calculate the ultrasonic frequencies of this mode at the pressures of 0, 0.6 and 1.1 kbar (first-order), 1.6 kbar (tricritical) and 2.15 kbar (second-order) by means of the volume change data from the literature. Our calculated frequencies are in good agreement with the observed data for the phase region below the critical temperature T(C) for the first-order, tricritical and the second-order phase transitions in NH4Cl. Above T(C) our calculated frequencies do not agree satisfactorily for the q[100] mode and it is incapable of explaining the observed behaviour of NH4Cl. Our method of calculating the ultrasonic frequencies is presented.