Fluctuations and Landau-Devonshire expansion for barium titanate

The experimentally observed temperature dependence of the quartic coefficients in the Landau-Devonshire expansion for BaTiO₃ is naturally accounted for within a proper fluctuation model. It is explained, in particular, why one of the quartic coefficients varies with temperature above T _c , while the second is constant. It is argued that the tetragonal phase in BaTiO₃ exists essentially due to thermal fluctuations, while the true Landau-Devonshire expansion with temperature-independent coefficients favors the rhombohedral ferroelectric phase.