Chemically stabilized δ-Bi2O3 phase: Raman scattering and X-ray diffraction studies

Thanks to its peculiar structural properties, the high temperature a-phase of Bi2O3 is considered as the best oxide ion conductor. Many efforts to stabilize this structure at room temperature have been deployed. In the present study, we have successfully stabilized the a-phase by chemically introducing tetra-Te4+ and pentavalent Ta5+ cations into the structure. A series of compounds with different percentage of Te4+/Ta5+ were obtained. Their structural and vibrational properties were investigated. From the Rietveld refinement of X Ray diffraction pattern we show that the composition x = 0.2 crystallizes in the cubic symmetry, space group Fm 3m (ITA No. 225) with a lattice parameter a = 5.49 angstrom. The reliability factors are: R-F = 2.151 % and R-Bragg=2.545 % confirm the goodness of the refinement. From the evolution of Raman bands, we confirm the existence of the solid solution features. Furthermore, comparing the spectra of a-Bi2O3 with the alpha phase, we comfortably suggest that the decrease of the number of Raman bands is a consequence of an increase in the lattice symmetry. Similarly to other fluorite compounds, we show that the structure presents oxygen defects clearly identified in the Raman spectra.