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Iron-containing ceramic materials based on BiNbO 4 of orthorhombic modification were obtained by solid-phase synthesis. Solid solutions α-BiNb 1−x Fe x O 4−δ were synthesized in the narrow concentration interval of х ≤ 0.03. The single-phase structure of the samples prepared was determined by X-ray diffraction and SEM-analyses. In the electron paramagnetic resonance (EPR) spectra of BiNb 1−x Fe x O 4−δ , a wide band with the center at g = 2.16-2.37 was observed, and a low-intensity signal with g-factor of about 4.3 was recorded on its low-field wing. Along with the broad spectrum component, a "noise-like" signal formed by a large number of narrow lines related to the integral signal of ferromagnetic resonance (FMR) and its fine structure was observed in the magnetic field range of 190-400 mT. Based upon measurements of magnetic susceptibility, the paramagnetic components and the effective magnetic moment values of iron atoms at different temperatures and for different concentrations of solid solutions were calculated. The isotherms of the paramagnetic component of the magnetic susceptibility of iron atoms in BiNb 1−x Fe x O 4−δ were typical of antiferromagnets. The value of the effective magnetic moment of iron atoms increased with the rise in temperature from 5.84 μB at 90 K to 6.43 μB at 320 K. This can be explained by the presence of exchange-bonded aggregates of Fe (III) atoms with antiferromagnetic and ferromagnetic types of exchange in the solid solutions. The magnetic behavior of BiNb 1−x Fe x O 4−δ was investigated by theoretical calculation of magnetic susceptibility within the framework of the model of diluted solid solutions taking into account the presence of possible types of clusters of iron atoms. The best agreement between the experimental and calculated data was obtained for the dimer parameters of J dim = −17 cm −1 and J dim = 45 cm −1 for the antiferromagnetic and ferromagnetic types of exchange.

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The manganese-doped polymorphs of bismuth orthoniobate were studied using thermal analysis, ESR, XPS and NEXAFS spectroscopy. It was shown that the phase transition from the orthorhombic (α) to the triclinic (β) modification can be reversible and it can be achieved by prolonged calcination of samples at 750°C. The DSC curves revealed no thermal effects associated with the phase transformation from the triclinic to the orthorhombic modification near 750°C. In the ESR spectra, the absorption band with g = 3.8 characterized by sextet structure disappeared during the phase transformation from the - to the -form and only the broad component with average intensity was observed at g = 2.2-2.0. According to NEXAFS spectroscopy data, the content of oxidized Mn(III) atoms increased and the content of Mn(II) decreased in the orthorhombic polymorph obtained at 750°C from the tri-clinic modification.