Alkali scandium arsenates. II. The framework structures of alpha- and beta-CsSc(HAsO4)2.

The crystal structures of hydrothermally synthesized alpha-, (I), and beta-caesium scandium bis[hydrogen arsenate(V)], (II), both CsSc(HAsO4)2, have been determined from single-crystal X-ray diffraction data collected at room temperature. The dimorphs are both characterized by a three-dimensional negatively charged framework of corner-sharing alternating ScO6 octahedra and HAsO4 tetrahedra. The charge-balancing Cs+ cations are located in a system of three intersecting tunnels in (I) and in tunnels parallel to the a axis in (II). Strong to weak hydrogen bonds reinforce both frameworks. The average Sc-O bond lengths are 2.098 and 2.094 A, respectively. Compound (I) is triclinic and isotypic with (NH4)Fe(III)(HPO4)2, alpha-A(I)V(III)(HPO4)2 (A is NH4 or Rb) and alpha-(NH4)(Al(0.64)Ga(0.36))(HPO4)2. Compound (II) is monoclinic and isotypic with (H3O)Fe(III)(HPO4)2, beta-A(I)V(III)(HPO4)2 (A is NH4 or Rb), CsIn(HPO4)2 and RbSc(HPO4)2. Both (I) and (II) represent the first arsenate examples of their structure types. The Cs and Sc atoms in (I) lie on inversion centres. In (II), all atoms are in general positions. A brief overview is presented of the six structure types shown by A(I)M(III)(HXO4)2 compounds (X is P or As).