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 Bi1.6Fe0.8Ta1.6O7.6 (refined formula Bi1.50Fe0.60Ta1.40O6.77) 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) angstrom, 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 mu m. 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 similar to 1140 degrees C. The unit cell parameter a and coefficient of thermal expansion (TEC) uniformly weakly increase from 10.4796 A to 10.5603 A and from 4.5 x 10(-6) degrees C- 1 to 9.2 x 10(-6) degrees C-1, respectively, in the temperature range of 30 divided by 1050 degrees C. This behavior is typical for frame structures of the pyrochlore type. The pyrochlore thermal behavior in the low-temperature region (-170 100 degrees C) is characterized by high growth of TEC from 1.3 x 10(-6) degrees C-1 (-170 degrees C) to 4.6 x 10(-6) degrees C-1 (100 degrees C) with respect to a weak increase in a from 10.48627 (-70 degrees C) to 10.49444 A(100 degrees 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).