Phase Transition in Ammonium Sulfate

-ferrielectric nature-

Abstract

Ferrielectric behaviour in ammonium sulfate has been studied by means of two methods. One of them is measurements of the temperature dependence of X-ray Bragg reflection intensities, and a qualitative interpretation of them was made. It is suggested that there are two kinds of independent order parameters. The other is crystal structure analyses of solid solutions (NH₄) _{2 (1-x)} R_2xSO₄, R=K, Rb, Cs. Occupation parameters of the ammonium ions corresponding to the two kinds of the crystallographically independent cation sites were successfully obtained, which can explain the spontaneous polarizations of the solid solutions by using an expanded Kittel's antif erroelectric free energy. The improper f erroelectric-like free energy function previously proposed seems unnecessary.