The model of phase transitions in sodium trihydrogen selenite, NaH3(SeO3)2

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Abstract
Methods developed in the theory of KDP-type ferroelectrics are applied to the STS-type crystals. The double-minima potential for hydrogen at the OH-bond is assumed and two locations (states) of proton are described by the spin model. Using the ferroelectric-phase structure data some conclusions on parameters of the model spin hamiltonian may be reached. Also the entropy of transition is found to be in good agreement with the experiment. The structure of β-phase, existing in STS in a certain temperature range, is predicted. The thermodynamics of transitions is investigated on the basis of the suggested hamiltonian, using the cluster approximation.In spite of the existence of many unknown constants, a number of qualitative results follows naturally from the suggested model. They include the existence of the intermediate p-phase, the peculiar change of polarization in the transition region, the influence of electric fields etc.