Thermal expansion, crystal structure, XPS and NEXAFS spectra of Fe-doped bismuth tantalate pyrochlore

The thermal behavior, crystal structure and charge state of ions in the composition of a new pyrochlore Bi 1 . 6 Fe 0 . 8 Ta 1 . 6 O 7.6 (refined formula Bi 1 . 50 Fe 0 . 60 Ta 1 . 40 O 6.77 ) synthesized via standard ceramic method are studied. The disordered pyrochlore structural parameters were obtained by the Rietveld method (sp. gr. Fd-3m:2 (227), 10.49689(3) Å, Z = 8). Iron (III) ions occupy octahedral positions of tantalum (V). The sample is characterized by a loose microstructure formed by partially fused grains with a longitudinal size of 0.2–2 μm. Using the high-temperature x-ray diffractometry method it was found that Fe-doped bismuth tantalate pyrochlore is thermally stable up to a temperature of ∼1140 °C. The unit cell parameter a and coefficient of thermal expansion (TEC) uniformly weakly increase from 10.4796 Å to 10.5603 Å and from 4.5 × 10 −6 °C −1 to 9.2 × 10 −6 °C −1 , respectively, in the temperature range of 30 ÷ 1050 °C. This behavior is typical for frame structures of the pyrochlore type. The pyrochlore thermal behavior in the low-temperature region (−170 ÷ 100 °C) is characterized by high growth of TEC from 1.3 × 10 −6 °C −1 (−170 °C) to 4.6 × 10 −6 °C −1 (100 °C) with respect to a weak increase in a from 10.48627 (−70 °C) to 10.49444 Å(100 °C). According to the data of near edge X-ray absorption fine structure and X-ray photoelectron spectroscopy, the ion charge states were determined as Bi (III), Ta(V), Fe(III).