Thermodynamic Study of NH4Br and ND4Br Crystals under Constant High-pressures

Heat capacities and volumes of NH4Br and ND4Br crystals were measured in the temperature range from 100 to 260 K at several constant pressures up to 110 MPa with a special attention paid to the behaviors around the point of ND4Br at 66 MPa and 202 K. The experimental results at an atmospheric pressure were in good agreement with the previously reported ones. The δ-γ and δ-β phase transitions were accompanied with expansion of the volume while the γ-β phase transition with contraction. Detailed phase diagram obtained for ND4Br disclosed that there was no maximum in any branch of the transition lines. The triple point was represented essentially as the point of intersection of the three straight lines. There was no pressure dependence of the total entropy of transition from the δ to β phase. The entropy values can be explained by the mechanism of the order-disorder change in the orientations of NH4+ tetrahedra. The existence of the heat capacity anomaly in the high temperature side of the δ-γ phase transiti...