Magnetic properties and NEXAFS-spectroscopy of Co-doped ferroelectric ceramic Bi5Nb3O15

The majority of bismuth-containing compounds with a layered perovskite-like structure, analogues of the so-called Aurivillius phases, are of practical and theoretical interest owing to their ferroelectric properties. Bismuth niobate Bi5Nb3O15 belongs to the group of mixed layered compounds. Its structure is characterized by the ordered alternation of fragments formed by one and two niobium-oxygen octahedra. Magnetic properties and NEXAFS of cobalt-containing solid solutions with a layered perovskite-like structure Bi5Nb3−3xCo3xO15−δ have been studied. Solid solutions of Bi5Nb3−3xСo3xO15−δ (х ≤ 0.005) can be crystallized in tetragonal syngony (sp. gr. P4 / mmm), as cobalt content increases, monoclinic distortion of the unit cell emerges at 0.005 < х ≤ 0.04. The solid solutions as well as cobalt oxides СoO, Co3O4 were studied by the NEXAFS spectroscopy. The analysis of the NEXAFS Co2p-spectra of cobalt-containing solid solutions and cobalt oxides revealed that the studied Co atoms were mainly in the +2 and +3 oxidation state. The isotherms of paramagnetic component of magnetic susceptibility of cobalt atoms in Bi5Nb3−3xCo3xO15−δ are typical for antiferromagnets. The effective magnetic moments of single cobalt atoms calculated by extrapolating concentration dependencies of [χpara(Co)] to infinite dilution of the solid solutions exceed pure-spin values and increase as the temperature increases from 6.18 μB (90 K) to 6.69 μB (320 K). The formation of exchange-bound aggregates of Сo(III) and Co(II) atoms predominantly with antiferromagnetic exchange types has been found in the solid solutions.

Magnetic susceptibility and NEXAFS spectra of Fe, Mg-codoped bismuth niobate pyrochlore

N.A. Zhuk, B.A. Makeev, S.V. Nekipelov, R.I. Korolev, A.A. Utkin, G.I. Chernykh

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K.Y. Shakhnazarov, E.I. Pryakhin, E.Y. Troshina