We have synthesized samples in the system CaTi 1 - x Fe x O 3 - x /2 with x = 0.05 - 0.38 at temperatures of 900 - 1320 °C under controlled conditions of oxygen fugacity. After drop quenching, samples were characterized by x-ray diffraction, transmission electron microscopy (TEM) and Mössbauer spectroscopy. Electrical conductivity measurements were made as a function of temperature, oxygen fugacity and sample composition. X-ray diffraction showed no long-range ordering of oxygen vacancies, which was confirmed by TEM measurements. Mössbauer spectroscopy showed Fe occurring in octahedral, tetrahedral and pentacoordinated sites. Analysis of area ratios indicates that a small number of Ti atoms must be located on non-octahedral sites, where the relative proportion of Ti on these sites decreases with increasing concentration of oxygen vacancies. Results are interpreted in terms of a model for short-range ordering of oxygen vacancies where clusters form chains of increasing length. The structure is dominated by oxygen vacancy monomers at low Fe concentrations, and the average number of vacancies per chain increases with increasing Fe concentration. Results are consistent with conductivity results which show high ionic conductivity at moderately low Fe concentrations.
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McCammon et al. (2000) studied this question.
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