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The lanthanoid chalcogenide  ortho -oxosilicates M 4.667 Ch[SiO 4 ] 3  (M = Nd, Sm; Ch = O, S) with  apatite -type crystal structure have been synthesized by the reaction of either M 2 O 3  and SiO 2  or M 2 O 3 , M, S and SiO 2  using CsCl as flux in evacuated silica tubes at 850 °C for 7 d . The title compounds crystallize hexagonally in the space group  P 6 3  /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 4.667 O[SiO 4 ] 3 , which form a pentagonal bipyramid. Considering M 4.667 S[SiO 4 ] 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 2 a  (CN = 3), the sulfur represents the centre of a trigonal antiprism at the Wyckoff position 2 b  (CN = 6).

M 4,667 Ch[SiO 4 ] 3 (M = Nd, Sm; Ch = O, S): structural comparison of two apatite -type lanthanoid chalcogenide ortho -oxosilicates