Solid Solutions Ca 3Sn 2+ xSi(Ge) 1- xGa 2O 12 (0 ≤ x ≤ 0.95) and Tetrahedral Coordination of Sn 4+ in the Garnet Structure

Abstract

Pure solid solutions Ca 3Sn 2+ x Si(Ge) 1-xGa 2O 12 with the garnet structure have been evidenced in the composition range 0 ≤ x ≤ 0.95. This implies that a part of the Sn 4+ cations (equal to x) is located on the tetrahedral sites, a quite unusual coordination for this cation in oxygen compounds. This has been definitely confirmed by X-ray powder diffraction structure calculations. Information about the cation-oxygen distances and the distortion of the different coordinated groups is presented. No band characteristic of the SnO 4 tetrahedra is observed in the IR spectrum, probably because of a strong mixing between SnO 4 and GaO 4 vibrations. In contrast, the totally symmetric stretching mode v 1 of the SnO 4 tetrahedra is easily observed near 775 cm -1 in the Raman spectrum, where it is well separated from the corresponding mode of the GaO 4 tetrahedra. The different vibrational behavior observed in IR and Raman spectra is discussed on the basis of the symmetry properties of the vibrations.