Raman spectroscopy evidence of inhomogeneous disorder in the bismuth-oxygen framework of Bi25InO39 and other sillenites

We report the room-temperature Raman spectra of polycrystalline Bi25InO39 for the first time along with the spectrum of Bi25FeO39. 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.