Raman spectroscopy evidence of inhomogeneous disorder in the bismuth-oxygen framework of Bi ₂₅InO ₃₉ and other sillenites

We report the room-temperature Raman spectra of polycrystalline Bi ₂₅InO ₃₉ for the first time along with the spectrum of Bi ₂₅FeO ₃₉. Both samples were synthesized by the conventional solid state method. The spectra of these compounds are remarkably similar to each other and those of other sillenites. A comparison of these sillenites, and others reported in the literature, shows that the Bi-O modes soften for compounds with larger (ionic radii) M cations. The widths of the modes increase for the compounds with larger M cations: the increase in width is attributed to inhomogeneous disorder in deviations of the Bi-O bond lengths across different unit cells. The results show that large M cations affect the Bi-O framework around the tetrahedron. The parameters of each Raman mode were obtained by fitting the spectral lines to a Lorentzian oscillator model, and the modes were assigned to symmetry-allowed vibrations of the I23 space group.