Fe4+ content and ordering of anion vacancies in partially reduced AFexTi1-xO3-y (A = Ca, Sr; x≤ 0.6) perovskites.: An 57Fe Mossbauer spectroscopy study

CaFexTi1-xO3-y (0.05 less than or equal to x less than or equal to 0.60) and SrFexTi1-xO3-y (0.20 less than or equal to x less than or equal to 0.60) perovskites were prepared by solid state reaction and subjected to different heat treatments in different atmospheres. Mossbauer spectroscopy of the CaFexTi1-xO3-y could detect Fe4+ in all the samples, even in those heated at 1000 degreesC in Ar atmosphere for 10 hours. Fe3+ coordinated by six, five and four anions were also identified and the estimated hyperfine parameters were consistent with those previously reported for the CaFexTi1-xO3-x/2 oxides where all the Fe cations were present as Fe3+. The Fe4+/Fe3+ ratios estimated from Mossbauer data are in agreement with coulometric titration data. The absence of tetracoordinated Fe3+ in SrFexTi1-xO3-y suggests that in contrast to CaFexTi1-xO3-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 proper-ties of these materials.