K2ScSn(AsO4)3: an arsenate-containing langbeinite.

The title compound, dipotassium tri-mu-arsenato-scandium(III)tin(IV), is the first arsenate-containing langbeinite to be characterized by single-crystal methods and crystallizes in the aristotype P2(1)3 cubic symmetry for this structure type in which the K(+) ions and the octahedral scandium and tin cations lie on crystallographic threefold axes. The Sc(III) and Sn(IV) ions show a slight segregation over the two octahedral sites, with Sc/Sn populations of 0.582 (5):0.418 (5) on one site and 0.418 (5):0.582 (5) on the other. Bond-valence-sum calculations indicate that the K(+) ions are significantly underbonded in this structure and the O atoms show large anisotropic displacement parameters, as also seen in other langbeinites. The crystal studied was found to be a merohedral twin with a 0.690 (16):0.310 (16) domain ratio.