Mn doped BiNbO^sub 4^ ceramics: Thermal stability, phase transitions, magnetic properties, NEXAFS and ESR spectroscopy

Thermal stability, magnetic properties, ESR (Electron Spin Resonance) and NEXAFS (Near-Edge X-ray Absorption Fine Structure) spectra of the orthorhombic and triclinic modifications of solid solutions BiNb1-xMnxO4-δ have been studied. X-ray diffraction determined the reversibility of the phase transition from the high-temperature triclinic modification to the orthorhombic modification at 950 °C in the BiNb1-xMnxO4-δ solid solutions. The ESR spectra of the triclinic BiNb1-xMnxO4-δ revealed the sextet structure of Mn(II) ions with 8.4 mT splitting and some features at g = 3.80 and 1.47, and a broad diffuse band with g ~ 2.2 having a sextet with 8–9 mT splitting and g = 2.0 against its background. The measurements of magnetic susceptibility allowed us to calculate the parameters of exchange interactions in dimers and the distribution of manganese atoms (II), (III) and (IV) in dependence on concentrations of the solid solutions in the orthorhombic and triclinic modifications. It was established that in the manganese-doped solid solutions of triclinic modification, the degree of aggregation and the nuclearity of the clusters were higher than in the solid solutions of orthorhombic modification. The clusters were presented in the forms of dimers and trimers of Mn(II), Mn(III) and Mn(IV) atoms with different valence atoms with antiferro- and ferromagnetic types of exchange. The solid solutions BiNb1-xMnxO4-δ as well as manganese oxides MnO, Mn2O3 and MnO2 were studied by the NEXAFS spectroscopy in order to determine the degrees of oxidation of manganese atoms. The DSC (Differential Scanning Calorimetry) curves of the samples BiNb1-xMnxO4-δ revealed the endothermic effects associated with the phase transition from the orthorhombic modification to the triclinic one at 1050 °C (x = 0.03) and 1048 °C (x = 0.06). The melting peaks were observed at 1231 °C (x = 0.03) and 1220 °C (x = 0.06), respectively. The differences in the magnetic characteristics and in the results of ESR and NEXAFs spectroscopy of the BiNb1-xMnxO4-δ of orthorhombic and triclinic modifications were due to the relative content of Mn(II), Mn(III), Mn(IV) ions and the degree of distortion of their local surroundings, composition and nature of exchange interactions in the clusters of manganese atoms.