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Magnetic susceptibility and EPR of iron-containing solid solutions with layered perovskite-like structure Bi2BaNb2-2xFe2xO9-δ and Bi5Nb3-3xFe3xO15-δ have been studied. The solid solutions Bi2BaNb2-2xFe2xO9-δ as well as iron oxides FeO, Fe2O3, and Fe3O4 were studied by the NEXAFS spectroscopy. The formation of exchange-bound aggregates of Fe(III) atoms with antiferroand ferromagnetic exchange types has been found in the solid solutions. In the ESR spectra of the solid solution samples, the lines with g ≈ 2.0 and g = 4.27 having a weak shoulder at g ~ 8 were attributed to Fe(III) atoms in the octahedral field with strong rhombic distortion.

Lead-containing perovskite of the complex composition PbNaKNb2FeO9.5 (sp. gr. Pnma, a = 5.654 ± 0.003 Å, b = 7.977 ± 0.001 Å, c = 5.646 ± 0.003 Å) was synthesized for the first time by the solid-phase reaction method. The sample is characterized by a non-porous microstructure formed by randomly oriented cubic crystallites. The band gap (about 2.10 eV) of a complex perovskite for a direct allowed electronic transition was calculated from the data of the diffuse reflection spectrum. The dielectric permittivity of the sample at room temperature and frequency 106 Hz reaches 1365; the dielectric loss tangent is equal to 0.1. [ABSTRACT FROM AUTHOR]

The effect of Fe and Mg-codoping on the crystal structure, optical and dielectric properties of bismuth tantalate-based pyrochlores has been studied. Samples of Bi2MgxFe1−xTa2O9.5−Δ (x ≤ 0.7) are characterized by a porous dendrite-like microstructure. Fe,Mg-codoped bismuth tantalate pyrochlores are thermally stable up to a temperature of 1140 °C (x = 1). The Bi2Mg0.5Fe0.5Ta2O9.5−Δ thermal expansion coefficient increases uniformly and weakly from 3.6 to 9.3 × 10−6 °C−1 (30–1050 °C). The unit cell parameter of solid solutions increases uniformly from 10.5009(1) Å (x = 0.3) up to 10.5225(7) Å (x = 0.7). The structural parameters of disordered pyrochlore are determined by the Rietveld method (sp. gr. Fd 3 ¯ m:2 (227), Z = 8). According to near edge X-ray absorption fine structure and X-ray photoelectron spectroscopy data, ions in solid solutions are in the charge states Bi (+3), Mg (+2), Fe (+3), Ta (+5-δ). The Mössbauer spectrum is represented by a symmetric doublet with parameters IS = 0.365 ± 0.0020 mm/s, QS = 0.604 ± 0.034 mm/s, related to Fe3+ ions in regular axial octahedral positions. The samples exhibit the properties of dielectrics. The permittivity and the tangent of dielectric losses at 20 °C increases with the growth of iron content in the samples in the range of 28.5–30.5 and 0.001 (1 MHz). The width of the band gap of the obtained materials for direct allowed electronic transitions is in the range of 2.16(5)–2.41(5) eV. The studied samples satisfy the condition of efficient conversion of solar energy into an electrical one and are promising as catalysts and light-absorbing elements for solar panels. [ABSTRACT FROM AUTHOR]

Nickel pyrochlore with the composition Bi2NiTa2O9 (sp. gr. Fd-3m, a = 10.5238 Å) was synthesized by the solid-phase reaction method. The phase formation process for nickel-containing bismuth tantalate Bi2NiTa2O9 with pyrochlore structure was investigated. The synthesis reaction for the pyrochlore phase is described by the interaction of orthorhombic bismuth orthotantalate and nickel oxide. Synthesis proceeds mainly at temperatures above 900°C. The samples are characterized by a porous dendritic microstructure with grain size 0.5 – 1.0 μm. The calcination temperature did not significantly affect the microstructure of the samples. The calcination time of the preparations effects coalescence of grains with the formation of larger particles. The optimal conditions for obtaining pure pyrochlore are: temperature 1050°C and calcination time 15 h. [ABSTRACT FROM AUTHOR]

Phase transformations of bismuth orthoniobate BiNbO4 were studied by the methods of differential scanning calorimetry and high-temperature X-ray diffraction. The DSC curve in the heating–cooling mode of the orthorhombic samples (α-BiNbO4) showed endothermic and exothermic effects associated with the phase transition from the orthorhombic modification to the triclinic one at the temperatures of ~ 1328 K and ~ 1270 K. This phase transition passed through a high-temperature phase of γ-BiNbO4, in which reflections were indicated in the temperature range of 1323 K → 1353 K → 1263 K. The X-ray diffraction pattern of the sample quenched in air (77 K) and calcined at the temperature from this range, 1323 K, corresponded to the triclinic modification. During the heating, the triclinic modification (β-BiNbO4) was transformed into the high-temperature phase of γ-BiNbO4, which was stable up to ~ 1270 K. Then, it was transformed into the triclinic modification again. The DSC curve of the sample of triclinic BiNbO4 showed endo- and exothermic effects at temperatures of ~ 1274 K and ~ 1270 K associated with the transition into the high-temperature phase of γ-BiNbO4. The cooling of the sample leads to the phase transition from the triclinic to the orthorhombic modification of BiNbO4. Thermostating at 1223 K for 1 h and quenching in air the compact sample or dispersed powder of bismuth orthoniobate of the triclinic modification led to the formation of the orthorhombic modification. [ABSTRACT FROM AUTHOR]

Phase transformations of bismuth orthoniobate BiNbO4 were studied by the methods of differential scanning calorimetry and high-temperature X-ray diffraction. The DSC curve in the heating–cooling mode of the orthorhombic samples (α-BiNbO4) showed endothermic and exothermic effects associated with the phase transition from the orthorhombic modification to the triclinic one at the temperatures of ~ 1328 K and ~ 1270 K. This phase transition passed through a high-temperature phase of γ-BiNbO4, in which reflections were indicated in the temperature range of 1323 K → 1353 K → 1263 K. The X-ray diffraction pattern of the sample quenched in air (77 K) and calcined at the temperature from this range, 1323 K, corresponded to the triclinic modification. During the heating, the triclinic modification (β-BiNbO4) was transformed into the high-temperature phase of γ-BiNbO4, which was stable up to ~ 1270 K. Then, it was transformed into the triclinic modification again. The DSC curve of the sample of triclinic BiNbO4 showed endo- and exothermic effects at temperatures of ~ 1274 K and ~ 1270 K associated with the transition into the high-temperature phase of γ-BiNbO4. The cooling of the sample leads to the phase transition from the triclinic to the orthorhombic modification of BiNbO4. Thermostating at 1223 K for 1 h and quenching in air the compact sample or dispersed powder of bismuth orthoniobate of the triclinic modification led to the formation of the orthorhombic modification. [ABSTRACT FROM AUTHOR]