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

We report the room-temperature Raman spectra of polycrystalline Bi${}_{25}$InO${}_{39}$ for the first time along with the spectrum of Bi${}_{25}$FeO${}_{39}$. 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 $I$23 space group.

Figure 1

Sillenite structure. The structure is stabilized from the parent compound by an M cation in the center of the tetrahedra (i.e., Si${}^{4+}$, Ti${}^{4+}$, Fe${}^{3+}$, In${}^{3+}$). The tetrahedra (blue) at the corners and center of the unit cell are connected by BiO polyhedra that can be referred to as a Bi-O framework (Bi: yellow, O: red). There are three symmetry positions for oxygens: the O(3) oxygens form the corners of the tetrahedra while the O(1) and O(2) oxygens lie outside. More detail about the structure can be found in Refs. 18 and 19.

Figure 2

SEM images. Crystallites in the order of 10 microns were obtained for both samples using the conventional solid state method.

Figure 3

From top to bottom: Raman spectra of Bi${}_{25}$InO${}_{39}$ (red), Bi${}_{25}$FeO${}_{39}$ (black), Bi${}_{25}$GaO${}_{39}$ (green), Bi${}_{12}$TiO${}_{20}$ (magenta), Bi${}_{12}$GeO${}_{20}$ (gray), and Bi${}_{12}$SiO${}_{20}$ (blue). We synthesized and measured Bi${}_{12}$SiO${}_{20}$, Bi${}_{12}$GeO${}_{20}$, Bi${}_{12}$TiO${}_{20}$, and Bi${}_{25}$GaO${}_{39}$ for comparison. Details on the spectra of these four compounds have been presented elsewhere.[32] All spectra were scaled and shifted to ease comparison.

Figure 4

Raman spectra corrected by a Bose factor for our two samples. The obtained imaginary part of the Raman susceptibility was fitted to a sum of Lorentzian oscillators: ${\chi }^{\prime \prime }\left(\omega \right)=\left[S_j^2\omega {\gamma }_j\right]/[{({\omega }_{oj}^2-{\omega }^2)}^2+{\omega }^2{\gamma }_j^2]$. The figure shows the data (black), the fitting (red), and the individual functions (blue) from the fit. An arbitrary broad oscillator was added around 950 cm${}^{-1}$ to account for higher frequency contributions and background.

Figure 5

Frequency of the most pronounced Raman peaks for various compounds with different valences in their M cations. The big (blue) points correspond to the six samples synthesized in this study. (Si${}^{4+}$, Ge${}^{4+}$, Ti${}^{4+}$, Ga${}^{3+}$, Fe${}^{3+}$, and In${}^{3+}$). Bands from all Raman spectra found in the literature were also plotted (red).

Figure 6

Frequency of the most pronounced Raman peaks vs the ionic radius of the M cation. The bands from all samples found in the literature were plotted. The data from our samples (blue), grown by the conventional solid state method, are shown as the bigger points. (Si${}^{4+}$, Ge${}^{4+}$, Ti${}^{4+}$, Ga${}^{3+}$, Fe${}^{3+}$, and In${}^{3+}$ in increasing ionic radii). A line was fitted to our data. Notice that the majority of sillenites also follow the pattern of decreasing frequency vs increasing ionic radii.

Figure 7

Frequency of the most pronounced Raman peaks vs the effective crystal radius of the M site.

Figure 8

Frequency of the most pronounced Raman peaks vs the unit cell volume of the compound. As stated in the text, there are conflicting values for the lattice parameter of some compounds. The lattice parameter measured for our compounds are presented in this graph.

Figure 9

Width of the most pronounced peaks in the Raman spectra vs the ionic radius of its M cation. Very few papers report the width of the modes; only our six samples are shown.

Figure 10

Width of the most pronounced peaks in the Raman spectra vs the effective crystal radius of the M site.

Figure 11

Comparison for the vibrational mode of the MO${}_4$ tetrahedron. The blue points correspond to our samples grown by the conventional solid state method.