SIMULTANEOUS OCCURRENCE OF SN4+ ON THE 3 CATIONIC SITES OF THE GARNET STRUCTURE - THE SOLID-SOLUTIONS CAXSNXGA8-2XO12 (2.5-LESS-THAN-X-LESS-THAN-3.0)

The first example of the simultaneous occurrence of the same cation, namely Sn4+, on the three cationic sites of the garnet structure is reported; the single phase solid solution CaxSnx Ga8-2xO12, resulting from the double substitution 2Ga(3+) --> Sn4+ + Ca2+ exists within the composition range 2.65 less than or equal to x less than or equal to 2.95, i.e., between the limiting compositions Ca2.65Sn2.65Ga2.70O12 and Ca2.95Sn2.95Ga2.10O12. Such compositions imply that the Sn4+ cations are distributed over the three cationic sites of the garnet structure, e.g. for x = 2.65 (Ca2.65S0.35)(VIII)(Sn-2)(VI)((GaSn0.30)-Sn-2.70)(IV). This has been definitely confirmed by X-ray powder diffraction structure calculations. In the Raman spectrum, the totally symmetric (nu(1)) stretches of SnO4 and GaO4 tetrahedra are observed near 780 and 750 cm(-1), respectively, whereas in the infrared spectrum, the corresponding antisymmetric vibrations coalesce in a broad, asymmetric absorption. (C) 1995 Academic Press, Inc.