Dielectric properties, Mössbauer study, ESR spectra of Bi2FeTa2O9.5 with pyrochlore structure

•Bi2FeTa2O9.5 were synthesized by the solid-phase reaction method.•In the Bi2FeTa2O9.5 ESR spectra a dipolar-broadened line (g⁓2.05) from Fe3+ ion clusters are observed.•The Mössbauer spectrum of Bi2FeTa2O9.5 demonstrates a slightly asymmetric doublet.•The doublet is reproduced by a superposition of a doublet (IS = 0.365 mm/s, QS = 0.604 mm/s) and a singlet IS=0.175 mm/s.•The permittivity and the dielectric loss tangent at 20 °C of Bi2FeTa2O9.5 were determined and are 57 and 0.003 (1 MHz). The pyrochlore with the composition Bi2FeTa2O9.5 was obtained by solid-phase synthesis. The electronic state and the nature of the local environment of iron atoms were studied by electron spin resonance (ESR) and Mössbauer spectroscopy. In the Bi2FeTa2O9.5 ESR spectra a dipolar-broadened line with g ∼ 2.05 from Fe3+ ion clusters are observed. This line is the superposition of components with widths of about 150 and 400 mT. On the low-field wing of the band, there is a feature with g ∼ 4.3 relating to trace amounts of isolated Fe3+ ions in an octahedral position with a strong rhombic distortion. The Mössbauer spectrum of the Bi2FeTa2O9.5 compound demonstrates a slightly asymmetric doublet. The form of the doublet is reproduced by a superposition of a doublet and a singlet. About 93% of the spectral area of the paramagnetic part of the spectrum falls on the Fe(III)-doublet with IS = 0.365 ± 0.0020 mm/s, QS = 0.604 ± 0.034 mm/s, correlated with Fe3+ ions in regular axial octahedral positions. The remaining part is represented by a Fe(III)-singlet with IS = 0.176 ± 0.0288 mm/s from Fe3+ ions in a symmetric environment. The electrical properties of the compound were studied at temperatures of 20 – 450 °C in the frequency range of 25 Hz – 1 MHz. It was found that synthesized ceramics exhibit dielectric properties. The permittivity and the dielectric loss tangent at 20 °C were determined and are 57 and 0.003 (1 MHz), respectively.