Abstract

The lanthanoid chalcogenide ortho-oxosilicates M_4.667Ch[SiO₄]₃ (M = Nd, Sm; Ch = O, S) with apatite-type crystal structure have been synthesized by the reaction of either M₂O₃ and SiO₂ or M₂O₃, M, S and SiO₂ using CsCl as flux in evacuated silica tubes at 850 °C for 7d. The title compounds crystallize hexagonally in the space group P6₃/m with two formula units per unit cell. Their crystal structure contains two crystallographically independent lanthanoid cations, of which (M1) is surrounded by nine O ligands in the shape of a tricapped trigonal prism. Additionally, this site is only occupied by 81–84% to assure that the composition is electroneutral. The (M2) cations have an environment of seven O anions in the case of M_4.667O[SiO₄]₃, which form a pentagonal bipyramid. Considering M_4.667S[SiO₄]₃ this polyhedron changes, when one of the apical oxide particles is substituted by two sulfide ligands which then form a “two-legged” pentagonal bipyramid. Both the non-silicon bonded oxygen in the oxide derivative and the sulfur in the sulfide homologue are situated in a channel along [001]. While oxygen prefers a trigonal planar cationic coordination at the Wyckoff position 2a (CN = 3), the sulfur represents the centre of a trigonal antiprism at the Wyckoff position 2b (CN = 6).