Iron and molybdenum valences in double-perovskite (Sr,Nd)₂FeMoO₆: Electron-doping effect

Double perovskite, (Sr_1-xNd_x)₂FeMoO₆, was doped with electrons through partial substitution of divalent Sr by trivalent Nd (0 ≤ x ≤ 0.2). The Fe valence and the degree of B-site order were probed by ⁵⁷Fe Mössbauer spectroscopy. Replacing Sr by Nd increased the fraction of Fe and Mo atoms occupying wrong sites, i.e. antisite disorder. It had very little effect on the Fe valence: a small but visible increase in the isomer shift was seen for the mixed-valent Fe ^II/III atoms occupying the right site indicating a slight movement towards divalency of these atoms, which was more than counterbalanced by the increase in the fraction of antisite Fe atoms with III valence state. It is therefore argued that the bulk of the electron doping is received by antisite Mo atoms, which - being surrounded by six Mo^V/VI atoms-prefer the lower IV/V valence state. Thus under Nd substitution, the charge-neutrality requirement inflicts a lattice disorder such that low - valent Mo ^IV/V can exist.