Crystal and magnetic structures in Perovskite-related La _1-xCa_xFeO_3-δ(x=0.2, 0.33)

Using solgel synthesis, single phase perovskite-related compounds in the family La1_-xCa^xFeO_3-δ have been formed for x=0.2 and x=0.33, but not for x=0.5. The x=0.2 and x=0.33 compounds are isostructural with LaFeO³ (Pnma). The magnetic structure of La ^0.8 Ca^0.2FeO_3-δ has been studied through Mössbauer spectroscopy and neutron powder diffraction. La ^0.8 Ca ^0.2FeO_3-δ is a G-type antiferromagnet with a magnetic moment magnitude of 3.0±0.2^μB at room temperature. The reduction in the magnitude of the antiferromagnetic moment compared to that published for LaFeO³ is explained by the measurement being taken at room temperature rather than 4 K and by the presence of Fe⁴ ions which have weaker exchange interactions than Fe³, causing a strong reduction in T_N. Room temperature Mössbauer shows a broad magnetic hyperfine field distribution on the Fe sites in both La^0.8 Ca^0.2FeO_3-δ and La^0.67 Ca ^0.33FeO_3-δ. On cooling, disproportionation of Fe1⁴ into Fe³ and Fe⁵ is apparent, and the resulting Fe⁵ sextet measured at low temperature gives a reliable measure of the Fe⁴ fraction. This in turn shows that creation of high-oxidation-state Fe is the dominant charge balance mechanism on doping Ca² into the Ln³ site indicating a disordered distribution of Fe³ and Fe⁴. The lack of broadening of the Fe ⁵ sextet suggests that there may be ordering in the distribution of Fe⁵.