Thermodynamic study of NH4Br and ND4Br crystals under constant high-pressures.

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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 transition was discussed as due to the spatial fluctuation of domains of the γ and δ phases. An argument was put forward that the observed δ-γ phase transition was probably superheated. An additional anomalous heat-capacity peak was observed just below the triple point pressure in between the temperatures of the δ-γ and γ-β phase transitions, and explained in terms of the effects of the superheating in the δ-γ phase transition and of the coexistence of the δ and γ phases domains in the crystal.
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Thermodynamic study of NH4Br and ND4Br crystals under constant high-pressures.

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