Thermal conductivity and heat capacity of solid phases of NH4Cl under pressure

Abstract

The thermal conductivity, lambda , and the heat capacity per unit volume, rho Cp, have been measured for solid phases I, II and III of NH4Cl using the transient hot-wire method. Results were obtained in the temperature range 140-520K, and at pressures up to 2.0 GPa. At the II-III transition, measurements as a function of temperature, to a reduced temperature of order 10-2, showed that lambda was continuous and non-divergent, but that (d lambda /dT)p changed abruptly. The expected maximum in rho Cp was detected, but reliable values of the critical exponent could not be obtained due to experimental limitations. The II-III transition was first order near zero pressure, and probably up to about 0.2 GPa. No direct evidence for a first-order transition was detectable at higher pressures. For phase I, the temperature dependence of lambda was similar to that of plastic crystal phases in other substances. The general form of lambda for NH4Cl was compared to that of NH4F, and found to show significant similarities.