Pure solid solutions (\mathrm{Ca}{3} \mathrm{Sn}{2+x} \mathrm{Si}(\mathrm{Ge}){1-x} \mathrm{Ga}{2} \mathrm{O}{12}) with the garnet structure have been evidenced in the composition range (0 \leq x \leq 0.95). This implies that a part of the (\mathrm{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 (\mathrm{SnO}{4}) tetrahedra is observed in the IR spectrum, probably because of a strong mixing between (\mathrm{SnO}{4}) and (\mathrm{GaO}{4}) vibrations. In contrast, the totally symmetric stretching mode (\nu_{1}) of the (\mathrm{SnO}{4}) tetrahedra is easily observed near (775 \mathrm{~cm}^{-1}) in the Raman spectrum, where it is well separated from the corresponding mode of the (\mathrm{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. (B) 1993 Academic Press, Inc.