Dependence of the electrical and thermal properties of [Na 1− x (NH 4) x] 2SO 4 solid solutions on their compositions

Abstract

The temperature dependences of the dielectric constant, the d.c. resistivity and the pyroelectric current were studied, and a differential thermal analysis and a dilatometric analysis in the vicinity of the phase transition III-I was performed, for various [Na 1− x (NH 4) x ] 2SO 4 solid solution compositions. Large changesin the measured electrical and thermal parameters were found for compositions close to the morphotropic boundary x = 0.7. The marked change in the detailed character of the different properties of this phase transition in the presence of ammonium ions (NH 4 +) is the most interesting new result obtained from the present work. The general behaviour of the electrical and thermal parameters showed that, as the concentration of NH 4 + increased, the transition tended to change from first order to second order. Dilatometric analysis showed that the thermal expansion which accompanied this phase transition is limited in the presence of NH 4 + and disappeared completely for samples containing more than 70% NH 4 +. According to these electrical and thermal measurements a phase diagram for mixed crystals of sodium and ammonium sulphates is proposed. This phase diagram predicts that the phase transition is shifted backwards by about 10 °C when the mixed crystal system contains 70% NH 4 +.