M10S14O-type oxysulphides (M  La, Ce, Pr, Nd, Sm) as an “oxygen trap” in oxidation reactions of reduced lanthanide chlorides with sulphur

Abstract

Oxidation of reduced chlorides (KM₂Cl₅; M  Nd, Sm) or chloride-hydrides (MClH_0.67; M  La, Ce, Pr) of the lanthanides with sulphur in the presence of NaCl in the latter case (tantalum capsule, 850 °C, 7 days) results in the formation of binary sulphides and ternary chlorides with potassium and sodium respectively, both with trivalent cations (M³⁺), as the main products. Oxidic impurities (e.g. MOCl) react to yield M₁₀S₁₄O-type oxysulphides (M  La, Ce, Pr, Nd, Sm) in small amounts as bead-shaped single crystals of high quality (M₁₀S₁₄O, tetragonal, $\text{I4}{}_1\text{acd}$ (no. 142), Z = 8; M  La: a = 1536.51(4) pm, c = 2037.85(9) pm, R = R_w = 0.028; M  Ce: a = 1521.24(4) pm, c = 2018.43(9) pm, R = 0.014, R_w = 0.013; M  Pr: a = 1511.63(4) pm, c = 2006.27(9) pm, R = 0.028, R_w = 0.029; M  Nd: a = 1503.80(4) pm, c = 1996.46(8) pm, R = 0.015, R_w = 0.014; M  Sm: a = 1485.96(4) pm, c = 1974.04(8) pm, R = 0.016, R_w = 0.014). Their crystal structure is built up from isolated O²⁻-centred tetrahedra [OM₄]¹⁰⁺ which are surrounded by a “sea” of lanthanide sulphide according to the formulation [OM₄]S₇M₆S₇. Three crystallographically independent M³⁺ cations occur with coordination numbers of eight (M1: seven plus one S²⁻; M2: six plus one S²⁻ and one O²⁻, both bicapped trigonal prisms; M3: eight S²⁻, trigonal dodecahedron). Four different S²⁻ (coordination numbers from four plus one to six) provide for the necessary coupling of the isolated [O(M2)₄] tetrahedra to their metal sulphide vicinity.