Heat Capacities and Phase Transitions of the (NH 4 ) x Rb 1- x H 2 PO 4 System

Heat capacities of the solid solution system (NH 4 ) x Rb 1- x H 2 PO 4 with x =0, 0.70, 0.74, 0.79 and 0.89, were measured with adiabatic calorimeters. The antiferroelectric ordering occurs in two steps for x =0.79 and 0.89. No distinct anomaly due to phase transition was observed in the heat capacities of x =0.70 and 0.74. However, slight excess heat capacites were found to exist over the normal lattice contribution which was calculated by assuming the additivity of those of both neat crystals. The excess heat capacity can be correlated with the anomalous variation of the lattice constants and the dielectric permittivities of the corresponding system. These effects can be considered to reflect the development of local ordering in their proton arrangements.