Sm4S3[Si2O7] and NaSm9S2[SiO4](6): Two sulfide silicates with trivalent samarium

The sulfide silicates Sm4S3[Si2O7] and NaSm9S2[SiO4](6) are obtained as lig ht yellow transparent crystals by the reaction of Sm, Sm2O3, S, and SiO2 wi th fluxing SmCl3 or NaCl, respectively, in suitable molar ratios in fused e vacuated silica tubes (850 degrees C, 7 d). Tetragonal crystals of Sm4S3[Si 2O7] (I4(1)/amd; Z = 8; a = 1186.4(1); c= 1387.0(2) pm) with ecliptically c onformed [Si2O7](6-)-groups of corner sharing [SiO4]-tetrahedra are formed. These double tetrahedra as well the sulfide anions (S2-) coordinate two cr ystallographically independent metal cations. They provide coordination num bers of 8 + 1 (5 S2- and 3 + 1 O2-) for Sm1 and 9 (3 S2- and 6 O2-) for Sm2 . NaSm9S2[SiO4](6) crystallizes hexagonally (P6(3)/m; Z = 1; a = 975.32(9); c = 676.46(7) pm) in a modified bromapatite-type structure. The coordinati on spheres about the two crystallographically different Sm3+ cations are bu ilt up by oxygen atoms of the orthosilicate units ([SiO4](4-)) and sulfide anions (S2-). As a result, Sm1 and Sm2 have coordination numbers of 9 and 8 , respectively. Na+ and (Sm1)(3+) occupy the position 4f in a molar ratio o f 1:3 whereas the lower coordinated (Sm2)(3+) occupies the 6h position.