The non-isotypic pair CsCe[SiS4] and CsCe[SiSe4]: A structural comparison

At first glance CsCe[SiS4] and CsCe[SiSe4] appear to be isostructural. CsCe[SiS4] crystallizes orthorhombically (a = 1787.41(9) pm, b = 674.13(3) pm, c = 647.32(3) pm) and so does CsCe[SiSe4] with slightly increased axis lengths (a = 667.65(3) pm, b = 705.29(4) pm, c = 1841.03(9) pm). The thiosilicate exhibits the space group Pnma (no. 62), however, while the selenosilicate crystallizes non-centrosymmetrically in space group P212121 (no. 19). In terms of the coordination sphere about the Cs+ cations, both compounds show C.N. = 11 with pentacapped trigonal prisms and two rather long Cs+-Ch2− (Ch = S, Se) distances. In CsCe[SiS4] a coordination number of 8 + 1 for cerium arranged as a tricapped trigonal prism is apparent, but the Ce3+ cations in CsCe[SiSe4] only maintain a coordination number of 7 with a monocapped trigonal prismatic coordination sphere. The same distinction exists for the cationic environment about the isolated chalcogenosilicate tetrahedra [SiCh4]4−. In both structures Cs+ coordinates them as terminal, edge- and face-capping ligand, while the Ce3+ polyhedra share common edges and faces with them in CsCe[SiS4], but only common edges and vertices in CsCe[SiSe4]. The whole structure can be considered to consist of anionic {Ce[SiCh4]}−∞2 layers separated by Cs+ cations. The centrosymmetric thiosilicate contains a mirror plane where the Cs+, the Ce3+, and the [SiS4]4− ions are situated at, but it also comprises a rather vigorous oscillation of the (S2)2− ligand about this plane. This oscillation seems to be frozen at one of both sides of the mirror plane compared to the selenosilicate, so that this symmetry disappears and the structure finally becomes non-centrosymmetric.