Fe⁴⁺content and ordering of anion vacancies in partially reduced AFexTi1-xO3-y(A = Ca, Sr;x⩽0.6) perovskites. An⁵⁷Fe Mössbauer spectroscopy study

CaFe x Ti 1- x O 3- y (0.05⩽ x ⩽0.60) and SrFe x Ti 1- x O 3- y (0.20⩽ x ⩽0.60) perovskites were prepared by solid state reaction and subjected to different heat treatments in different atmospheres. Mössbauer spectroscopy of the CaFe x Ti 1- x O 3- y could detect Fe 4+ in all the samples, even in those heated at 1000 °C in Ar atmosphere for 10 hours. Fe 3+ coordinated by six, five and four anions were also identified and the estimated hyperfine parameters were consistent with those previously reported for the CaFe x Ti 1- x O 3- x /2 oxides where all the Fe cations were present as Fe 3+ . The Fe 4+ /Fe 3+ ratios estimated from Mössbauer data are in agreement with coulometric titration data. The absence of tetracoordinated Fe 3+ in SrFe x Ti 1- x O 3- y suggests that in contrast to CaFe x Ti 1- x O 3- y no ordering of anion vacancies takes place even for x = 0.60 and explains the differences in the Fe concentration dependence of the electric transport properties of these materials.