Synthesis and structure of CaBiO₂Cl and SrBiO₂Cl, new distorted variants of the Sillen X1 structure

Two new layered oxyhalides, MBiO(2)Cl (M = Ca, Sr), have been synthesized and characterized. Both are derived from the ideal Sillen X1 structure type, being composed of fluorite-like [MBiO(2)](+) layers separated by single Cl- layers. However, deviations from the idealized tetragonal structure due to cation site ordering lead to two different superstructures being formed. The structure of CaBiO2Cl has been solved ab initio and refined from a combination of X-ray and neutron powder diffraction data: monoclinic, space group P2(1)/m, a = 7.7311(1) Angstrom, b = 4.1234(1) Angstrom, c = 6.3979(2) Angstrom, beta = 105.21(1)degrees (a similar to 2a(ideal), b similar to b(ideal), c similar to c(ideal)/2). The large distortion from the ideal structure arises due to the small size of the Ca2+ and consequent reduction of the cation coordination numbers from 8 (4O + 4Cl) to 7 (4O+3Cl). SrBiO2Cl is shown by Rietveld refinement of powder X-ray data to adopt the BaBiO2Cl superstructure: orthorhombic, space group Cmcm, a = 5.7109(2) Angstrom, b = 12.4081(5) Angstrom, c = 5.5888(2) Angstrom (a similar to root 2 a(ideal), b similar to c(ideal), c similar to root 2 b(ideal)). (C) 1997 Academic Press.