A structure for ttCrO~, has been found in which one ehromium(III) ion lies in 1 (a) 0, 0, 0 and two oxygen ions in 2 (c) x, x, x; x, x, x of space group Rgrn. The single parameter x has been determined from powder data by triM-and-error to be 0.405±0"003, with a residual factor R of 0.06 for resolved reflections not seriously affected by preferred orientation. The structure consists of distorted (CrO6) -9 octahedra, each sharing six of its edges with six surrounding eoplanar octahedra to form continuous sheets in which the anions are close packed, lying parallel to the prominent {111 } cleavage. This and certain other MX 2 structures containing sheets of linked octahedra differ from one another essentially only in the manner of superposition of successive sheets. In this structure, the sheets are stacked so that oxygen ions of adjacent sheets are exactly superposed, hence the anions in the structure as a whole are only partly close packed. One hydrogen ion lies between each pair of superposed oxygen ions, this O-H-O distance being 2-55± 0.08 A, in accord with infra-red absorption data. Symmetry indicates symmetrical hydrogen bonds, and in this respect the HCrO 2 structure appears unique among known isoformular structures.
Formula Weights per Cell. 12. Formula Weight. 270.20. Density. 2.84 grams per cc. (calculated; weight of unit atomic weight 1.6602 X 10-24 gram); 2.73 and 2.77 [measured pycnometrically (Z)].
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