The statistical theory of paraelectrician-ferroelectrician phase transition in KDP crystals with H_(22) structure has been developed. It have been established the temperature dependences of free energy, thermodynamic potential, order parameters and energetic constants of interatomic interaction. The dependence of order parameter on temperature has been defined and it has been shown the conditions, when this dependence is close to transition of first kind. The temperature dependences of configuration heat capacity and dielectric susceptibility have been determined in the neighbourhood of Curie point. The Curie-Veiss law and the rule of negative two have been justified. The comparison of computer-aided design, theoretical plots with experimental data have shown their qualitative agreement.
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