Title: GAUSSIAN FLUCTUATIONS OF POLARIZATION IN THE REGION OF PHASE TRANSITIONS IN DMAGAS-DMAALS FERROELECTRICS IN THE FRAMEWORK OF THE FOUR-STATE MODEL
Authors: I.V.Stasyuk, O.V.Velychko (Institute for Condensed Matter Physics of the National Academy of Sciences of Ukraine, 1 Svientsitskii Str., 79011 Lviv, Ukraine)

A description of thermodynamics of the DMAGaS-DMAAlS family ferroelectrics improved by taking into account the Gaussian fluctuations of polarization is presented. Fluctuations become important in the vicinity of the second order (or the first order close to the second order) phase transitions which is the case of the mentioned crystals. The more elaborated theory, adapting the Onyszkiewicz approach to the purpose of the four-state model, provides equations for components of polarization, their fluctuations and free energy in a simple form with a clear physical sense. The results obtained by numerical calculations demonstrate that in the considered system at the sufficiently long-ranged interaction the Gaussian fluctuations lead to the slight decrease of temperatures of phase transitions from paraphase to ferrophase and in the region of the triple point they are of the same order of magnitude for both first and second order phase transitions.

Key words: ferroelectrics, DMAGaS, DMAAlS, microscopic model, Gaussian fluctuations
PACS: 77.84.-s, 64.60.Cn