TWINNING AND PSEUDO-SYMMETRY IN BARIUM SUCCINATE. Leonore Wiehl, Mineralogisches Institut, Universitfit Bonn, Poppelsdorfer Schloß, D-53115 Bonn, Germany

The crystal structure of barium succinate, BaC₄H₄0₄, shows a symmetry very close to space group I4₁/amd. The small deviations of the actual atomic arrangement from this high symmetry gives rise to multiple twinning, which leads to the tetragonal pseudo-symmetry observed in the crystal morphology. The deviations are associated with the low symmetry of the succinate molecule O₂ - CH₂ - CH₂ - O₂, which is incompatible with the site symmetry of 42m occupied in I4₁/amd. In quantity, however, these deviations of some carbon and oxygen positions from the "ideal" values are so small, that the crystal structure could be refined to R-values of about 6% in a lot of different space groups, for example Fddd, Pnma, C222₁, P4₁, C2/c or P2₁, all being subgroups of I4_l/amd which share the same connectivity network of Ba²⁺-ions and succinate chains along the pseudo-four-fold axis. The true space group C2 could only be found by optical experiments in conjunction with group-theoretical considerations. The twin law is represented by the "lost" symmetry elements, especially the 4-fold axis.

Twinned crystals of BaC₄H₄O₄ were grown from aqueous solution. They show tetragonal symmetry with faces {112} and {100} , as first described by Haushofer [1]. The crystal structure was not investigated until now, in contrast to many other succinates [2]. A single crystal was prepared by cutting a twin under the polarizing microscope. X-ray-diffraction intensities (full sphere up to 2ø = 65deg., Mo K[[alpha]]) were measured in tetragonal geometry (a = 7.601(1) Å, c = 10.293(1) Å) on a 4-circle diffractometer (AFC6R, RIGAKU/MSC). The deviation from tetragonal geometry is of the order of 0.2deg. as detected by optical means as misfit angle between faces of a twinned crystal.