Simultaneous Occurrence of Sn 4+ on the Three Cationic Sites of the Garnet Structure: The Solid Solutions Ca xSn xGa 8-2 xO 12 (2.5 < x P;lt; 3.0)

Abstract

The first example of the simultaneous occurrence of the same cation, namely Sn 4+, on the three cationic sites of the garnet structure is reported; the single phase solid solution Ca x Sn x Ga 8-2 x O 12, resulting from the double substitution 2Ga 3+ → Sn 4+ + Ca 2+ exists within the composition range 2.65 ≤ x ≤ 2.95, i.e., between the limiting compositions Ca 2.65Sn 2.65Ga 2.70O 12 and Ca 2.95Sn 2.95Ga 2.10O 12. Such compositions imply that the Sn 4+ cations are distributed over the three cationic sites of the garnet structure, e.g. for x = 2.65 (Ca 2.65Sn 0.35) VIII(Sn 2) VI(Ga 2.70Sn 0.30) IV. This has been definitely confirmed by X-ray powder diffraction structure calculations. In the Raman spectrum, the totally symmetric ( v 1) stretches of SnO 4 and GaO 4 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.