Ultrasonic frequencies correlated with the volume changes of the q[1(1)over-bar-0] mode for the first-order phase transition in NH4Cl

We correlate here by means of gamma-Gruneisen relations the volume changes with the ultrasonic frequencies of the q[1 (1) over bar 0] mode of NH4Cl fo r the first-order phase transition in this crystal. The ultrasonic frequenc ies were calculated by a method, which we have developed, using the length- change data from the literature at pressures of 0, 0.2, 0.3 and 0.6 kbar in the first-order phase region of NH4Cl. Our calculated frequencies are in very good agreement with the observed dat a from the literature for the q[1 (1) over bar 0] mode at these pressures. This shows that by correlating the volume changes with the ultrasonic frequ encies, the observed behaviour of NH4Cl can be explained adequately near th e first-order phase transition in this crystalline system.