Acta Cryst. (2007). B63, 205-215 [ doi:10.1107/S0108768106054942 ]
Abstract: Arsenates with arsenic in octahedral coordination are very rare. The present paper provides an overview of all known M+ arsenates(V) containing octahedrally coordinated arsenic (M+ = Li, Na, K, Rb, Cs, Ag) and the crystal structures (determined from single-crystal X-ray diffraction data) of the following seven new hydrothermally synthesized members belonging to six different structure types, four of which are novel: LiH2As3O9, LiH3As2O7, NaHAs2O6-type KHAs2O6, KH3As4O12 and isotypic RbH3As4O12, CsAs3O8 and NaH2As3O9-type AgH2As3O9. The main building unit of these compounds is usually an As4O14 cluster of two edge-sharing AsO6 octahedra sharing two apical corners each with two AsO4 tetrahedra. The different connectivity between these clusters defines the different structure types. The novel CsAs3O8 structure, based on a derivative of the As4O14 cluster, is the most condensed of all these M+ arsenates, with an O/As ratio of only 2.67 compared with values of 2.75-3.5 for the remaining members. This is achieved through polymerization of the cluster derivatives to infinite chains of edge-sharing AsO6 octahedra. The [4]As/[6]As ratio drops to only 0.5. All but two of the protonated title compounds show protonated AsO6 octahedra. Hydrogen bonds range from very strong to weak. An analysis of bond-length distribution and average bond lengths in AsO6 octahedra in inorganic compounds leads to an overall mean As-O distance for all known AsO6 octahedra (with R factors < 0.072) of 1.830 (2) Å.
Keywords: alkali arsenates; protonated arsenate octahedra; As-O bond length; connectivity.
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