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 t he q[100] mode of NH4Cl for the first-order, tricritical and the second-ord er 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 (sec ond-order) by means of the volume change data from the literature. Our calc ulated frequencies are in good agreement with the observed data for the pha se region below the critical temperature T-C for the first-order, tricritic al and the second-order phase transitions in NH4Cl. Above T-C our calculate d frequencies do not agree satisfactorily for the q[100] mode and it is inc apable of explaining the observed behaviour of NH4Cl. Our method of calcula ting the ultrasonic frequencies is presented. (C) 2001 Elsevier Science B.V . All rights reserved.