  The experimentally observed temperature dependence of the quartic
coefficients in the Landau-Devonshire expansion for BaTiO_3 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. The tetragonal phase in BaTiO_3 is argued to
exist essentially due to the thermal fluctuations, while the true
Landau-Devonshire expansion with temperature-independent coefficients favours
the rhombohedral ferroelectric phase. Certain conclusions concerning the
temperature dependence of the sextic Landau-Devonshire coefficients are also
made.
