High-pressure ultrasonic investigation of the order-disorder transition in ND₄Cl

The adiabatic elastic constant c₁₁ has been measured in single-crystal ND₄Cl at high pressures. Data were obtained in the range 220-300 K and 0-7 kbar, with special emphasis on the vicinity of the order-disorder transition line. From a comparison with previous measurements, it can be seen that the temperature dependence of C₁₁ in the disordered phase is quite sensitive to deuteration and to pressure while the temperature dependence of the heat capacity is not. This difference in the behavior of the thermal and mechanical properties can not be understood in terms of the compressible Ising model.