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

We study here the ultrasonic frequencies of the q[110] mode of NH4Cl, which we have correlated to the volume changes for the first-order and the tricr itical phase transitions in this crystal. We calculated the ultrasonic freq uencies 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 ph ase regions in NH4Cl using the observed data. Our calculated frequencies are in good agreement with those observed given in the literature for the q[110] mode of Nt(4)Cl at the pressures studied. This indicates that the ultrasonic frequencies can be calculated by the met hod given here and that the observed behaviour of NH4Cl can be explained sa tisfactory near the first-order and tricritical phase transitions in this c rystal. (C) 2000 Elsevier Science B.V. All rights reserved.