Thiosilicates of the rare-earth elements:: I.: The isotypic compounds KCe[SiS4] and Eu2[SiS4]

Both isotypic thiosilicates KCe[SiS4] (a = 649.15(6), b = 656.18(6), c = 863.96(8) pm, beta = 107.531(9)degrees) and Eu-2[SiS4] (a = 651.71(6), b = 659.54(6), c = 821.93(8) pm, beta = 108.437(9)degrees) crystallize monoclinically in the space group P2(1)/m and Z = 2. By the reaction of KCl, Ce2S3 and SiS2 in the ratio 1 : 1 : 1 using a sixfold molar amount of KCl as flux in evacuated silica tubes (7 d, 850degreesC) brownish yellow, plate-shaped single crystals, resistant both to air and water are obtained. The conversion of Eu, S and SiS2 in molar ratios of 2 : 2 : I with an excess of CsCl as flux in evacuated silica tubes (7 d, 850degreesC) leads to deep red, plate-shaped single crystals, which remain air- and water-stable for a few days. The crystal structure contains isolated ortho-thio silicate units, that together with the Ce3+ or (Eu2)(2+) cations build corrugated anionic layers parallel (001) according to (2)(infinity){(Ce[SiS4])(-) and (2)(infinity)(Eu2[SiS4])(2-)}, respectively. These layers are alternatingly piled with cationic layers consisting solely of K+ or (Eul)(2+) cations. The latter show coordination numbers of eight in the shape of a bicapped trigonal prism, whereas the cations of the position Ce3+ and (Eu2)2 have a (2+1)-fold capped trigonal prismatic environment with a coordination number of 8+1. The comparison of both compounds KCe[SiS4] and Eu-2[SiS4] (equivalent to EuEu[SiS4]) demonstrates, that Eu2+ is able to substitute both K+ and Ce+ isomorphically.