TETRAHEDRAL POLYCATIONS [IN-5](7-PRODUCT OF THE ALLOY NAIN() IN NA2 3IN5O15 (=NA24IN5O15) AN UNUSUAL OXIDATION)

The already published, red-transparent, air sensitive, cubic compound Na24In5O15 (a = 1107.7 pm, Z = 2, <I(4)over bar>3m) is an intermediate oxidation product of the intermetallic phase NaIn. It is reinvestigat ed with respect to the question whether it contains the tetrahedral po lycation [InIn4](6+) or [InIn4](7+). In the latter case the chemical c omposition would be Na23In5O15 instead of Na24In5O15 and the polycatio n would be in accordance with the Zintl-Klemm concept. Na24In5O15 is p repared in single phased samples for the first time and investigated b y means of the Rietveld refinement technique of X-ray powder data. Bas ed on the results of the Rietveld refinements and new single crystal d ata in comparison to the published values we note anomalies in the occ upancies of the Na positions and a pronounced anisotropy in the therma l parameters of one oxygen position. These investigations together wit h results obtained by analytical scanning electron microscopy, Extende d Huckel and MAPLE calculations give strong evidence for the assumptio n of [InIn4](7+) instead of [InIn4](6+) and the associated correct com position Na23In5O15. The problems of a reliable characterization of th is unique compound are discussed.