MICROSCOPIC MODEL OF SEQUENCE OF SUPERIONIC-FERROELASTIC PHASE-TRANSITIONS IN (NH4)(3)H(SEO4)(2) CRYSTAL

The microscopic approach based on symmetry analysis and the proton ord ering model is developed for the description of phase transitions from superionic phase II to ferroelastic phases III and IV in (NH4)(3)H(Se O4)(2) crystal. The phase diagram determining the conditions of realiz ation of proton orderings observed in phases III and IV is constructed . It is shown that the main features of phase transitions (including t heir order) can be described within the mean field approximation allow ing for the long-range proton interactions; the short-range proton cor relations considered in cluster approximation do not change qualitativ ely the phase diagram but can lead to a change in transition temperatu re and favour the transformation of the II-III transition order from a continuous one to the discontinuous. Investigations of thermodynamica l functions are performed numerically. The obtained results are compar ed with conclusions which follow from phenomenological Landau expansio n approach as well as the experimental data.