Crystal structures of SrSeO3 and CaSeO3 and their respective relationships with molybdomenite- and monazite-type compounds -: an example for stereochemical equivalence of ESeO3 groups (E = lone electron pair) with tetrahedral TO4 groups

Crystals of the compounds CaSeO3 and SrSeO3 were prepared at low-hydrothermal conditions by reaction of aqueous solutions of SeO(2)with the respective earth alkaline carbonates. The crystal structures were determined by direct methods and refined from single crystal X-ray diffraction data up to sin theta/lambda = 0.995. CaSeO3: space group P2(1)/1n, Z = 4, a = 6.402(1), b = 6.791 (1), c = 6.681 (1) angstrom, beta = 102.78(l)degrees, V = 283.3(1) angstrom(3) R1 = 0.015. SrSeO3: space group P2(1),/m, Z = 2, a = 4.456(1), b = 5.478(1), c 6.574(1) A, P = 107.34(1)degrees, V = 153.2(1) angstrom(3);; R1 0.021. The crystal structures are characterised by the activity of lone pair electrons of Se-IV forming typically one-sided pyramidal SeO3 groups which are linked by the earth alkaline cations to framework structures. Bond lengths and angles of the SeO3 groups comply with crystal chemical experience. The CaO7 polyhedron in CaSeO3 has a mean Ca-0 bond length of 2.455 A. In SrSeO3 the Sr2+ cations are [7 +2] -coordinated with a mean Sr-[19]-O distance of 2.741 A, which is significantly above respective values found for comparable compounds (2.687 angstrom). Both crystal structures exhibit close relationships to mineral structure types. SrSeO3 is 'SotyPiC with molybdomenite PbSeO3 and scotlandite PbSO3, as well as with further representatives of the KClO3 structure type. The tendency towards underbonding of the large Me cations as observed in SrSeO3, seems to be inherent to this structure type and correlated with the size of the various anionic groups. CaSeO3 provides an example for the sterochemical equivalence of a pyramidal SeO3 group comprising a lone electron pair E with a tetrahedral TO4 group. It belongs to a new structure type but is 'quasi-isotypic' to the structure type of monazite CePO4 where the lone electron pair E is formally replaced by an additional oxygen ligand. This results in polyhedral rotations and in a change from the sevenfold coordination of Ca in CaSeO3 to the ninefold coordination of the large Me cations in monazite type compounds.