The Crystal Structure of Magnesium Platinocyanide Heptahydrate

Abstract

Positions of the Mg and Pt atoms in crystals of MgPt${\mathrm{}\left(\mathrm{CN}\right)\mathrm{}}_4$·7${\mathrm{H}}_2$O. These have been definitely determined by means of x-ray oscillating-crystal photographs and Laue photographs, using the theory of space-groups. Because the other atoms are too light in comparison with the metal atoms, especially Pt, their positions could not be determined. The Pt atoms are located at 0 0 0 and ½ ½ ½, the Mg atoms at 0 0 ½ and ½ ½ 0, in a tetragonal unit of structure 14.6A×14.6A×3.13A. Two units of structure are shown in the figure. The peculiar optical properties are believed to be associated with the unusual arrangement of the heavier atoms in widely spaced rows parallel to the tetragonal axis. In these rows Mg atoms alternate with Pt atoms, and the distance between any two adjacent atom-centers is 1.57A. The shortest distance between rows, however, is 10.3A, 6.6 times the distance between atoms in the same row. The atomic radii of Mg and Pt as determined by Bragg from other crystal data do not agree with the observed distance between these atoms, the calculated value being 2.7A, the observed distance 1.57A. The observed distance, however, is consistent with that calculated by the method of Davey, who assumes that the radius of an ionized atom differs much from the radius of the same atom un-ionized, and that the radii of ${\mathrm{Cs}}^{+}$ and ${\mathrm{I}}^{-}$ are substantially equal in crystals of CsI.