Alkali scandium arsenates. I. The framework structures of KSc(HAsO4)2 and RbScAs2O7

The crystal structures of hydro­thermally synthesized potassium scandium hydrogen arsenate(V), KSc(HAsO₄)₂, (I), and rubidium scandium diarsenate(V), RbScAs₂O₇, (II), were determined from single‐crystal X‐ray diffraction data collected at room temperature. Compound (I) represents a new microporous structure type, designated MCV‐3, which is characterized by a three‐dimensional framework of corner‐sharing alternating ScO₆ octahedra and HAsO₄ tetrahedra. Intersecting tunnels parallel to 101] and 110] host eight‐coordinate K atoms. There is one hydrogen bond of medium strength O?O = 2.7153 (18) Å]. Compound (II) is the first reported diarsenate with a KAlP₂O₇‐type structure and is isotypic with at least 27 A^IM^III diphosphates. The average Sc—O bond lengths in (I) and (II) are 2.09 (2) and 2.09 (3) Å, respectively. The K and Sc atoms in (I) lie on an inversion centre and a twofold axis, respectively. All atoms in (II) are in general positions.