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Revision of the crystal structure of kettnerite CaBi[OFCO₃]

Jií Hybler^* and Michal Duek

Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Praha 8, Czech Republic

^* Corresponding author, e-mail: hybler{at}fzu.cz

The crystal structure of kettnerite, CaBi[OFCO₃] was refined in the orthorhombic cell a = 5.3641(1), b = 5.3641(1), c = 13.5771(2) Å, space group Pbaa, Z = 4, R_all = 2.71 % for 446 reflections. Both Bi and Ca cations are [8] coordinated. The structure consists of layers parallel to (001): the Bi-O layers at z = 0, Ca-F layers at z = 1/2. Both these layers are pseudotetragonal showing a subcell parameter a 3.8 Å. The layers of CO₃ groups are located between the Bi-O and Ca-F layers. The CO₃ groups are approximately parallel to (110) or (10) planes with one O atom oriented towards the Bi-O layer while the remaining two O atoms are oriented towards the Ca-F layer. The orientations of CO₃ groups are interlaced, so that each group approximately parallel to (110) is surrounded by four nearest neighbors parallel to (10) and vice versa. This arrangement requires description of the whole structure in the proposed supercell. As a consequence weak additional superlattice reflections appear in the diffraction pattern. The CO₃ layers may connect to the other layers in two different yet geometrically equivalent ways, thus the structure can be considered as an OD structure formed by more than one kind of layers. Two of them are non-polar (the Bi-O and Ca-F layers) while the layers of CO₃ groups are polar with regularly alternating sense of polarity. The strong main reflections can be interpreted as family reflections while the superlattice ones can be attributed to polytype reflections. Their diffuse streaking along c* indicates partial stacking disorder. The described structure represents the simplest possible polytype 1O. The refinement revealed deviations from the ideal OD symmetry. The O and F atoms in the Bi-O and Ca-F layers are displaced from ideal positions in the a direction and the CO₃ groups are slightly rotated from their ideal diagonal positions. The studied crystals were merohedrally twinned via 4[001] operation (the "lost" fourfold axis).

Key-words: kettnerite, carbonate, crystal structure, OD structure, polytypism.