Charge compensation in Ca(La)TiO3 solid solutions

Different modes of charge compensation for trivalent rare earth ions substituted for Ca in perovskite formed by high-temperature sintering are discussed. Cation vacancies were deduced from positron annihilation lifetime spectroscopy (PALS) measurements to exist in Ca((1-x))LaxTiO3 solid solutions (x = 0.001- 0.2) fabricated in air at similar to1500degreesC whether or not they were further heated in reducing atmospheres. Electron paramagnetic resonance measurements have shown the presence of paramagnetic species in Ca((1-x))LaxTiO3 samples fabricated in air, but their identities are not yet clear (other than Mn2+ impurities), although their intensities are not linear in La content, so the species of interest are unlikely to be Ti3+. (139) static nuclear magnetic resonance (NMR) measurements show the La ions in Ca(1-x)La samples fabricated in air to yield very broad featureless line shapes, indicative of low site symmetry and/or interactions with paramagnetic species. However, corresponding broad resonances were also observed in vacancy-compensated samples; this line broadening observed in the compensated and uncompensated systems is attributed to low site point symmetry and short-range disorder in bond angles and bond lengths around each La3+ position. This proposition is supported by the observation of narrow (139) NMR resonances in the vacancy-compensated system Sr((1-3x/2))LaxTiO3 (for x . 0.1), where the point symmetry of the Sr site was cubic rather than orthorhombic, as encountered in the analogous Ca((1-x))LaxTiO3 system. Samples were fabricated in air with Ca((1-x))LaxTi((1-x/4))O-3 stoichiometry to promote Ti vacancies, and the solid solubility limit corresponded to 0.10 f.u. of such vacancies.