Thiosilicate der selten-erd-elemente: IV. Die quasi-isostrukturellen Verbindungen

The four lanthanoid(III) thiosilicates NaSm ₃S ₃[SiS ₄] (1), CuCe ₃S ₃[SiS ₄] (2), Ag _0,63Ce ₃S _2,63Cl _0,37[SiS ₄] (3) and Sm ₃S ₂Cl[SiS ₄] (4) crystallize all hexagonally in the non-centrosymmetric space group P6 ₃ (no. 173) showing basically the same structure. Besides an eightfold square-antiprismatic sulfide coordination sphere about the M ³⁺ cations these compounds display discrete tetrahedral thiosilicate units [SiS ₄] ^4- which are trigonally distorted. All tops of the remaining pyramids point into the same direction along the c axis and therefore the structure needs to be non-centrosymmetric. However, the main feature of these crystals are channels along [001], built up by non-silicon bonded sulfide anions and providing trigonal planar (CN = 3) and trigonal antiprismatic (CN = 6) voids. In the sodium derivative (1) the Na ⁺ cations reside almost in the center of the latter ones exhibiting a trigonally distorted octahedral surrounding (CN = 6) while the Cu ⁺ cations of the respective thiosilicate (2) are situated in the middle of an (S ^2-J ₃ triangle (CN = 3), just 7 pm above its plane. The same is true for the silver compound (3) with the exception of the Ag ⁺ cations sitting about 53 pm above the sulfide triangle and showing a marked under-occupation along with a very large U ₃₃ displacement parameter. These facts lead to the assumption that (3) could be a promising material for ionic Ag ⁺ conduction. In order to maintain electroneutrality the silver derivative has to contain a certain amount of Cl ^- anions (CeCl ₃ was used as fluxing agent) substituting S ^2- which was doubtlessly confirmed by WDX analyses. In the absence of any monovalent cation (A ⁺) the analogous structure is realized in (4) displaying an empty channel along [001] and a complete substitution of one sulfide by one chloride anion per formula unit, also confirmed by electron-beam microanalysis.