Landau theory of the ferrielectric phase transition with two order parameters

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Abstract
In order to explain ferrielectric behaviour, a phenomenological model given in terms of the polarisations of two non-equivalent sublattices is proposed. The free energy for a second-order phase transition is G=1/2f1P12+1/2f2P22+gP1P2+1/ 4h(P14+P24) (G>0, h>0), where f1-(T-T1)/C1 and f2=(T-T2)/C2; this is the same form as that for the antiferroelectrics given by Kittel (1951) other than for the temperature dependences of f1 and f2. The dielectric susceptibility shows a crossover from that of an antiferroelectric at high temperatures to that of a ferroelectric with decreasing temperature down to Tc. The calculated results for the small spontaneous polarisation, the reduction of the Curie-Weiss constant and the unusual E-P curves are discussed in connection with experimental data on ammonium sulphate.