Charge Compensation Mechanisms in La-Doped BaTiO₃

The mechanism of doping BaTiO3 with La has been investigated by a combination of X-ray diffraction, electron probe microanalysis, scanning and transmission electron microscopy and impedance measurements. Phase diagram results confirm that the principal doping mechanism involves ionic compensation through the creation of titanium vacancies. All samples heated in oxygen at 1350-1400 degreesC are electrical insulators, consistent with an ionic compensation mechanism. Samples heated in air or atmospheres of low oxygen partial pressure, at similar temperatures, lose a small amount of oxygen and this gives rise to a second, electronic compensation mechanism in addition to the main, ionic compensation mechanism; as a result, samples are dark-coloured and semiconducting. The change from insulating to semiconducting behaviour is reversible, by changing the atmosphere on heating at 1350-1400 degreesC. We find no evidence for any changes in cationic composition of the BaTiO3 solid solutions arising from changes in oxygen content.