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The molecular configuration of the decaniobate ion (Nbt00~8) has been found by the determination of two crystal structures, [N(CH3)414Na2Nb~0028.8H20.½CH3OH and IN(CH3)4]6Nbz0028.6H20. The octahydrate compound is monoclinic, space group I2/m, with cell dimensions a = 12.056(7), b -20.501 (8), c --: 10.980(7) /~, fl = 90.05(4) ° (Z = 2); the hexahydrate crystallizes in P2~/n, with a = 14.534(8), b = 18.668 (6), c = 10.990 (7) /~, [J = 92.50 (4) ° (Z --2). Unique data sets of 4105/2156 reflections for the hexahydrate/octahydrate were collected by the 0-20 scan technique; both structures were determined from Patterson and Fourier syntheses and refined by least-squares methods to R = 4%. The decaniobate ion is an isolated group of ten condensed NbO 6 octahedra. In the crystal structure, these ions are electrostatically linked by tetramethylammonium ions (and sodium ions) as well as by a hydrogen-bond network. Ten Nb atoms are situated on the corners of two edge-shared octahedra with 3.274-3.481 ,/~ Nb-Nb separations; the 1-73-2.55/~ range of Nb-O separations indicates the distorted nature ofthe decaniobate ions. * Prepared for the US Energy Research and Development Administration under Contract AT(29-1)-789.amorphous solid, which, when dried, exhibits excellent ion-exchange properties.The crystals of the tetramethylammonium complex, TMN, and the analogous sodium-containing material, STMN, are the only stable crystalline compounds obtained in the preparation of the ion-exchange materials to date. A few extremely unstable intermediate hydrol-

Positional and thermal parameters for PtSi and PtGe have been refined by the full-matrix least-squares method using 123 and 131 three-dimensional intensity data, reslzectively. The compounds are isostructural with MnP, space group Pbnm, four formulae per cell, with lattice constants a=5"916 (1), b= 5.577 (1), c=3.587 (1)/~ for PtSi, and a=6"084 (2), b=5.719 (2), c=3.697 (1)/~ for PtGe. Linear thermal expansivities have been determined from the lattice constants measured as a function of temperature over the interval 100-900 °K for PtSi and 298-900 °K for PtGe. An anomaly in the expansivities for PtSi occurs at 400°K.

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