High-pressure ultrasonic investigation of order-disorder phenomena in NH4Br

Abstract

The adiabatic elastic constants $C_{11}$, $C_{44}$, $C^{\prime }\equiv \frac{(C_{11}-C_{12})}{2}$, and the [100] longitudinal attenuation have been measured in single-crystal N${\mathrm{H}}_4$Br. The $C_{44}$ data establish that there is a multicritical point along the cubic order-disorder transition line at 3.2 kbar and 215.8 K. The $C_{11}$ and $C^{\prime }$ data indicate a close analogy between the behavior of N${\mathrm{H}}_4$Cl at 1 atm and the behavior of N${\mathrm{H}}_4$Br near its disorder-cubic-order-tetragonal-order triple point. The attenuation data strengthen this analogy and show a strong pressure dependence as $p$ varies from 1 atm (tetragonal ordering) to the triplet-point pressure to 3.25 kbar (cubic ordering).