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

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.