Heat Capacity of Deutero-Ammonium Bromide from 17° to 300°K

The heat capacity of deutero-ammonium bromide has been measured from 17° to 300°K. The heat-capacity curve is characterized by the appearance of two maxima. These are associated with polymorphic transitions in the crystal lattice and with the ordering of the ammonium ion tetrahedra with respect to the two equilibrium positions in the unit cell. The excess entropy associated with the maxima is found to be in agreement with the order-disorder nature of the upper transition. A frequency of torsional oscillation for the ammonium ion in good agreement with infrared data is obtained from the Debye-Einstein function representing the heat capacity below 100°K.