Pergamon Solid State Communications, Vol. 96, No. 1, pp. 5-9, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1098(95)00386-X OO38-1098/95 $9.50+.00 PHENOMENOLOGICAL LANDAU THEORY OF THE [NIL&SO4 PARA-FERROELECTRIC PHASE TRANSITION D. De Sousa Meneses Cenue de Recherche sur la Physique des Hautes Temp&atures, C.N.R.S., 45071 Orl&urs Cedex 2, France zyxwvutsrqponm (Received 11 April 1995; accepted 1 June 1995 by G. Bastard) A phenomenological Landau theory has been developed to describe the behavior of ammonium sulfate at its pamelectric-ferroelecttic phase transition: order parameter, spontaneous polarization, dielectric constants, spontaneous strains and elastic compliances. The peculiar behavior of the spontaneous polarization is well described by introducing the terms $Ps and $Pr in the expansion of the elastic Gibbs function. A comparison between the temperature dependence of the Landau order parameter and experimental results shows that the microscopic origin of the order parameter can be assigned to the difference of the occupation probabilities of the two orientations of the whole group (NH4);-(SO4)*--(NH4)s’. Keywords: A. ferroelectrics, D. phase transitions Among ferroelectric materials, ammonium sulfate is a rather peculiar case. Besides the very small values of the dielectric constant anomaly and the Curie-Weiss constant, its spontaneous polarization exhibits an atypical temperature dependence since it decreases upon cooling and changes of sign 140 K below Tc.‘-~ The disorder induced by the ammonium cations in the high temperature phase is another interesting feature which takes an important part in the mechanism of the transition. The low temperature ordered phase results from the critical slowing down of a relaxation component of Bt. symmetry which is attributed to reorientations of the whole group (NH.&+- (S04)*--(NH.&+ between two positions.& In order to shed new light on the features described above and to check the assumption that the difference between the occupation probabilities of the two positions’ constitutes a possible candidate to the microscopic origin of the order parameter,4 we decided to compare theoretical predictions of the temperature dependence of the order parameter and experimental results. All predictions of the physical quantities presented in the following are zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA obtained in the framework of the phenomenological Landau theory of phase transitions. Such calculations were already developed,’ but without explanation of the specific behavior of the polarization, and with only rough agreement between experimental and calculated data. 1. PHENOMENOLOCICAL THEORY Group-theoretical considerations show that the order parameter which describes the transition from the 0:: (Pnam) space group of the paraelectric phase to the Cl” (Pna2,) space group of the ferroelectric phase is one- dimensional and of Br. symmetry. Moreover, the presence of a third order term of the order parameter in the expansion of the elastic Gibbs function is forbidden by the fact that the point group of the low synmtry phase is a subgroup of index two of the point group of the high symmetry phase. The latter statements and the fust order character of the phase transition lead to the following form for the elastic Gibbs function G: G=G, +Gp+Ge+GnP+G,,e+Gpe (1) <