Abstract
Bismuth calcium aluminate, Bi(2)Ca(6)Al(12)O(27), has been prepared as a ceramic and a single crystal. Analysis of reciprocal space using both electron and X-ray diffraction show an R-centred hexagonal unit cell: a = b = 17.3892 (4), c = 6.986 (1) Å. Additional weak reflections are observed; they require the introduction of a modulation wavevector q = 0.0453 (2)c* for indexing. The modulated structure has been solved using the superspace formalism [superspace group X3(00γ)0]. A framework of corner-sharing AlO(4) tetrahedra forms corrugated sixfold rings and uncommon triple rings. The Ca(2+) cations exhibit an eightfold coordination sphere; edge-sharing CaO(8) polyhedra form intertwinned zigzagging rows along c creating a three-dimensional net. Bi atoms are located in large hexagonal tunnels parallel to c and form Bi(2)O(3) pairs, which adopt a trigonal bipyramidal configuration. The 6s(2) lone-electron pairs (Lp) point along c, in the opposite direction to the three Bi-O strong bonds to form two BiO(3)Lp tetrahedra with a common base. Different orientations of the Bi(2)O(3)Lp(2) pairs, rotated by 60° around c, are observed. Their stacking modes in each of the hexagonal tunnels are described. The sequence of the stacking varies along c in each of the tunnels.
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