Properties of the perovskites, SrMn_1-xFe_xO_3-δ (x = 1/3, 1/2, 2/3)

The SrMn_1-xFe_xO_3-δ (x = 1/3, 1/2, 2/3) phases have been prepared and are shown by powder X-ray and neutron (for x = 1/2) diffraction to adopt an ideal cubic perovskite structure with a disordered distribution of transition-metal cations over the six-coordinate B-site. Due to synthesis in air, the phases are oxygen deficient and formally contain both Fe³⁺ and Fe⁴⁺. Magnetic susceptibility data show an antiferromagnetic transition at 180 and 140 K for x = 1/3 and 1/2, respectively and a spin-glass transition at 5, 25, 45 K for x = 1/3, 1/2 and 2/3, respectively. The magnetic properties are explained in terms of super-exchange interactions between Mn⁴⁺, Fe^{(4 + δ)+} and Fe^{(3 + ε) +}. The XAS results for the Mn-sites in these compounds indicate small Mn-valence changes, however, the Mn-pre-edge spectra indicate increased localization of the Mn-e^g orbitals with Fe substitution. The Mössbauer results show the distinct two-site Fe^{(3 + ε)} +/Fe^{(4 + δ)} + disproportionation in the Mn- substituted materials with strong covalency effects at both sites. This disproportionation is a very concrete reflection of a localization of the Fe-d states due to the Mn-substitution.