Surface Space-Charge Layers in Barium Titanate

Above the Curie temperature, pyroelectric currents can be produced in single crystals of barium titanate even though there is no electric field applied. The polarization that remains at these temperatures is ascribed to space-charge fields in the crystal. From studies of the wave forms of the pyroelectric current signals it is concluded, tentatively, that space-charge layers of up to 10^-5 cm in thickness reside at the crystal surface and that these space charges also produce a field through the interior of the crystal. Further evidence for space-charge fields is provided by the occurrence of an associated photovoltaic effect and by asymmetrical hysteresis loops. The space-charge fields vary considerably in magnitude from crystal to crystal. In general, they can be modified by suitable heat treatment but return to their original condition when fields are applied above the Curie point. The space charge fields apparently influence the direction in which the domains polarize when the crystal is cooled through the Curie point. They will also affect capacity measurements above the transition and will influence the actual temperature of the transition. It is quite possible that the fields play an important role in the process of domain nucleation.