Birefringence in the surface layer of cubic BaTiO₃

A birefringence pattern of a ⟨100⟩ plane of BaTiO3 is reported for temperatures less than 5 K above the thermo-dynamic transition temperature (410 K). The patterns allow mapping of the static polarization in the surface layer. The layer thickness and the polarization at the surface are critical functions of temperature. The data are qualitatively analyzed in terms of a model involving space charge and ferroelectricity. At Tc the polarization distribution is entirely governed by space charge, and upon cooling below Tc , the ferroelectric phase starts in the interior of the crystal.