Cr and Mg codoped bismuth tantalate pyrochlores: Thermal expansion and stability, crystal structure, electrical and optical properties, NEXAFS and XPS study

A series of Bi2Cr1-xMgxTa2O9+Delta solid solutions with the pyrochlore structure was synthesized by the method of solid-phase synthesis. Structural parameters of the disordered pyrochlore were determined by the Rietveld method (space group Fd-3m:2 (No. 227), Z = 8). As the magnesium content increases, the unit cell parameter rises continuously from 10.4596 +/- 0.0001 angstrom (x = 0.3) to 10.5463 +/- 0.0001 (x = 1.0) according to the Vegard rule. The grain size varies in the range of 0.5-2 mu m, regardless of the magnesium/chromium(III) ratio. Chromium ions increase the range of thermal stability of ceramics up to 1110 degrees C.The thermal expansion coefficient in the range of 30-1110 degrees C increases continuously from 4.1 to 8.6 x 10-6 degrees C- 1 (for x = 0.5). The oxidation states of ions were determined by X-ray spectroscopy (XPS and NEXAFS). It was shown that bismuth and magnesium ions in the pyrochlore are in the Bi(+3), Mg(+2) charge states, and for chromium the states Cr(+3), Cr(+6) are most probable. Based on the presence of an energy shift in the region of Ta4f, Ta5p and Ta5d absorption edges by Delta E-0.7 eV and-1 eV, respectively, it was concluded that the oxidation degree of the Ta(+5-delta) ion decreased. The band gap for the samples in the case of direct allowed electronic transitions is in the range of 2.14-2.29 eV. At RT and in the frequency range (102-106 Hz), the dielectric permittivity of the samples is practically constant and it decreases with increasing amount of chromium from 20 (x = 0.7) to 16 (x = 0.3) at 1 MHz.In the high-frequency region (>= 105 Hz), the dielectric loss tangent for all samples is independent on frequency and takes low values of-0.004 (1 MHz). The activation energy of conductivity for the samples varies in the range of 1.39-1.48 eV. The ability to adsorb water was found for chromium pyrochlores.