Raman study of MgAl2O4–FeAl2O4 and MgAl2O4–MgFe2O4 spinel solid solutions

Spinels oxides are minerals or synthetic materials which are nowadays attracting considerable interests because of their physical and chemical properties. Ten synthetic spinels belonging to the MgAl2O4–FeAl2O4 solid-solution and four synthetic spinels belonging to the MgAl2O4–MgFe2O4 solid-solution have been studied by Raman spec-troscopy to verify the effects of the Fe2+ and Fe3+ cations on the vibrational spectra. The Raman modes are affected by the substitution of Mg by Fe2+ into the tetrahedrally coordinated sites in the first series, MgAl2O4– FeAl2O4, and by the Fe3+ substitutions for Al into the octahedrally coordinated sites in the second series, MgAl2O4–MgFe2O4. The spinels have been synthetized by flux growth method and analysed chemically by electron microprobe and structurally by single-crystal X-ray diffraction. Raman spectra were collected with a Jobin Yvon LabRam micro-spectrometer using two different laser excitation wavelengths. A blue 473.1 nm solid-state laser was used for the samples with high Fe2+ con-tent, whereas the other samples were recorded with a red 632.8 nm He-Ne laser. While the substitution Fe2+→Mg proceeds along the MgAl2O4 – FeAl2O4 series, no change in the wavenumber of the Eg, F2g(3), and A1g modes is observed up to Fe2+ ~20%. After this value, all Raman-active modes show a shift toward lower wavenumbers in agreement with the heavier cation mass of Fe2+ (Fig. 1a). The slow and non-linear decrease in the wavenumber ob-served for these modes is also due to cation disorder and to the concomitant incorporation of low amounts of Fe3+ in the spinel structure. The presence of Fe2+ and Fe3+ in the octahedron strongly influences the vibrational spectra because of the heavier cation mass of Fe than that of Al. As a matter of fact, the F2g(3) mode exhibits a two-mode behaviour: the intensity of the peak at 670 cm-1 (typical of Fe-poor spinels) decreases almost linearly, whereas that of the peak at ~ 610 cm-1 grows with increasing Fe2+ content. Along the MgAl2O4 – MgFe2O4 series, the Raman spectra are considerably different, sug-gesting that trivalent cation strongly affect vibrational dynamics (Fig. 1b). A general decrease toward lower wavenumber is observed for all the Raman modes. Besides by the substitution Fe3+→Al, the Raman spectra are affected by cation disorder, because the magnesioferrite end-member (MgFe2O4) has an almost fully inverse structure, with divalent cation occurring in the octahedron and trivalent cations distributing almost equally between the tetrahedron and the octahedron. While increasing the Fe3+ content, the intensity of the ~720 cm-1 peak, the one as-signed to the trivalent cation occurring at the tetrahedron in the MgAl2O4 end-member, increas-es, whereas that of the 765 cm-1 peak, the one assigned to divalent cation occuring at the tetra-hedron, decreases.