Sm2As4O9: An unusual samarium(III) oxoarsenate(III) according to Sm-4[As2O5](2)[As4O8]

Pale yellow single crystals of the new samarium(111) oxoarsenate(III) with the composition Sm4As8O18 were obtained by a typical solid-state reaction between Sm2O3 and As2O3 using CsCl and SmCl3 as fluxing agents. The compound crystallizes in the triclinic crystal system with the space group P1 (No. 2, Z = 2; a = 681.12(5)7 b = 757.59(6), c = 953.97(8) pm, alpha = 96.623(7), beta = 103.751(7), gamma = 104.400(7)degrees). The crystal structure of samarium(III) oxoarsenate(III) with the formula type Sm-4[As2O5](2)[As4O8] ( 2 x Sm2As4O9) contains two crystallographically different Sm3+ cations, where (Sm1)(3+) is coordinated by eight, but (Sm-2)(3+) by nine oxygen atoms. Two different discrete oxoarsenate(111) anions are present in the crystal structure, namely [As2O5](4-) and [As4O8](4-). The [As2O5](4-) anion is built up of two psi(1)-tetrahedra [AsO3](3-) with a common corner, whereas the [As4O8](4-) anion consists of four psi(1)-tetrahedra with ring-shaped vertex-connected [AsO3](3-) pyramids. Thus at all four crystallographically different As3+ cations stereochemically active non-binding electron pairs (lone pairs) are observed. These lone pairs direct towards the center of empty channels running parallel to [010] in the overall structure, where these empty channels being formed by the linkage of (2)(infinity){[SmO4](9-)} layers with the eclipticafly conformed [AS(2)O(5)](4-) anions and the stair-like shaped [As4O8](4-) rings via common oxygen atoms (O1 - O6, O8 and O9). The oxygen-atom type O7, however, belongs only to the cyclo-[As4O8](4-) unit as one of the two different corner-sharing oxygen atoms.