Raman spectra of apatites: La 10− x Si 6− y (Al,Fe) y O 26±δ

Raman spectra of eight polycrystalline apatites of the general formula La 10− x Si 6− y M′ y O 26 ± δ with M′ = Al or Fe were obtained at 300 K. Raman spectra of La 10 Si 4 Fe 2 O 26 and La 9.83 Si 4.5 Al 1.5 O 26 were investigated in the range 80–1000 K and 80–623 K, respectively. Tentative assignments of bands to stretching and bending modes of SiO 4 tetrahedra and to M'O vibrations are proposed. Except for the two new bands, which appear around 700 cm −1 when Al is replaced by Fe, only some band broadenings and relative intensity changes are observed as a function of the rate of O5 or La vacancies. Most of the bands soften and broaden continuously when raising the temperature. This is an indication that the Al‐ and Fe‐substituted apatites do not undergo any structural change up to 1000 K. Above 1000 K, the broad and weak shoulder observed at 850 cm −1 for La 10 Si 4 Fe 2 O 26 is replaced by a strong band at 868 cm −1 , suggesting that SiO 4 tetrahedra undergo a structural modification. All compounds show the same residual band broadening at 80 K. This suggests that there is a small rate of static disorder preferentially related to the solubility of Al and Fe in the Si sublattice rather than to other defects. Moreover, the observation of FeO modes indicates that the dynamics of the solid solution obeys the so‐called two‐mode behavior . The occurrence of FeO stretching vibrations 150 cm −1 lower than for those of SiO suggests that the coordination number of iron could be larger than 4, particularly for the Fe 4+ species. Copyright © 2006 John Wiley & Sons, Ltd.