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) A]. Compound (II) is the first reported diarsenate with a KAlP2O7-type structure and is isotypic with at least 27 A(I)M(III) diphosphates. The average Sc-O bond lengths in (I) and (II) are 2.09 (2) and 2.09 (3) A, 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.