The crystal structure of tin tetraiodide

Abstract

The crystal structure of tin tetraiodide was determined by Dickinson (1923). The unit cell is cubic with a o --12.26 /~ (Dickinson's a 0 = 12.23 _~); there are eight molecules per unit cell. The space group is ~ P a 3 . The structure, as described by Dickinson, can be visualized in terms of small deviations from an 'ideal' structure consisting of tin atoms occupying one-eighth of the tetrahedral interstices in a cubic close packing of iodine atoms. In the 'ideal' structure the tin atoms and one of the iodine atoms of each SnI~ tetrahedron would be located on one of the threefold cube axes; the other three iodine atoms of the tetrahedron would lie in a plane perpendicular to this axis. Assuming that the regular tetrahedral coordination of the SnI~ group is maintained, three shifts from the 'ideal' may be considered: Ca) The tetrahedron may be translated along the threefold axis; (b) the tetrahedron may be contracted or expanded; (c) the iodine atoms in general positions may be rotated about the threefold axis. Dickinson permitted all three types of deviations from the 'ideal' structure in addition to possible distortion of the tetrahedron. In terms of the unit-cell edge the magnitudes of these shifts were 0.004 for type (a), 0-001 for contraction of type (b), and 3 ° for type (c). His determination was based on analysis of Laue diagrams and spectral photographs, and the parameters (Table 1) were obtained by a systematic trial-and-error procedure.