SOLUBILITY OF LANTHANUM IN STRONTIUM-TITANATE IN OXYGEN-RICH ATMOSPHERES

Perovskite (ABO(3))-type lanthanum substituted strontium titanate cera mics (lanthanum content x), which had been sintered in pure oxygen at 1400 degrees C, were investigated from x = 0 up to x = 0.6 by light op tical and scanning electron microscopic means in conjunction with X-ra y analysis, X-ray diffraction, and pyconometry in order to determine t he mechanism being responsible for the compensation of the electronic excess charge resulting from the ''donor'' lanthanum. A pure strontium vacancy compensation mechanism was observed for lanthanum contents up to x = 0.3. Above x = 0.4 titanium vacancies occur additionally but t heir concentration remains negligible compared to the predominating st rontium vacancies. No indication of a solubility limit of lanthanum at x = 0.4, as stated in former works was observed. At x = 0.5 and 0.6 t he lattice structure was found to be slightly distorted, tetragonally and orthorhombically, respectively. The lattice parameter obeys Vegard 's law up to the end member La(2/3)square(1/3)TiO(3) (square: vacant s ite). These results were completely confirmed by pycnometry data.