SOME SIMPLE THERMODYNAMIC APPROACHES TO SUPERIONIC DISORDER IN FLUORITE-TYPE CRYSTALS - APPLICATION TO SRCL2 AND K2S

Some simple phenomenological models of superionic disorder (both those allowing and not allowing for the interaction between defects) have b een analysed using potassium sulphide and strontium chloride as exampl es, with reference to the quantitative description of the gradual tran sition of AB(2) fluorite (antifluorite) type compounds to the highly-c onducting state. The approach as a whole is based on the use of the ex cess values of enthalpy (H-E), entropy (S-E) and free energy (G(E)). T heir temperature dependence in the region of the diffuse transition ha s been determined based on available experimental data for the high-te mperature heat content of these compounds. It is established that the change in defect concentration vs temperature in the superionic region is characterized by deviations from an Arrhenius-type dependence, and is consistent with the analogous behaviour of ionic conduction. At th e melting points the degree of disorder is still limited, being about 10-20% for SrCl2 and 20-40% for K2S of the maximum possible value. The excess entropy S-E includes a significant fraction due to a vibration al component. However, neither of the models considered describes exac tly all the peculiarities of the superionic transition in fluorites.