Phase stability in the Ginzburg-Devonshire model

The thermodynamic potential in the form traditional for the theory of ferroelectric phase transitions in barium titanate has been considered. It has been proved that the potential admits the existence of a monoclinic phase. The physical reasons for its absence in $BaTiO_3$ have been found out. The conditions under which the polarization jump on the equilibrium surface of polar phases in phase transitions is negligibly small have been determined.