Nonclassical Critical Behaviour and Low Temperature Anomalies in Ferroelectric DMACA as Described by Many-Sublattice Model

A model based on the crystallographic data on dimethylammonium nonachlorodiantimonate (DMACA) is constructed. Gradual orientational ordering of three inequivalent sublattices of the dimethylammonium cations is shown to be at the origin of the anomalous behaviour of the spontaneous polarisation in this material. A quantitative comparison with the experimental data reveals a temperature dependence of a coupling between two sublattices and a strong deformability of the underlying structure.