Elastic Constants of Ammonium Bromide. II. High-Pressure Ultrasonic Investigation of the Phase Transitions

The abiabatic elastic constants of single‐crystal ammonium bromide have been measured at 20 MHz as functions of temperature and pressure in the region from 180°–240°K and from 0–6 kbar. A new high‐pressure ordered phase, denoted as $O_{\mathrm{II}}$ , has been discovered in this temperature range. The acoustic properties of the new $O_{\mathrm{II}}$ phase were investigated with emphasis on the regions of the first‐order phase transition from the $O_{\mathrm{II}}$ phase to the ordered tetragonal phase and the lambda transition from the $O_{\mathrm{II}}$ phase to the disordered cubic phase. The region of the lambda transition from the ordered tetragonal phase to the disordered cubic phase was also studied. A detailed comparison is made with the behavior of ammonium chloride near its order–disorder phase transition.