M2As2O7(H2O)2(M = Co or Ni): hydrous diarsenates with an intersecting tunnel structure

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Abstract
Two new hydrous diarsenates, M2As2O7(H2O)2(M = Co or Ni), have been synthesized hydrothermally and structurally characterized by single-crystal X-ray diffraction. The cobalt compound was further defined by thermal analysis and magnetic susceptibilities. They are the first examples of the hydrous form of divalent transition-metal diarsenates. Both compounds crystallize in the monoclinic space group P21/n, with a= 6.525(2), b= 14.200(5), c= 7.618(2)Å, β= 94.72(2)°, Z= 4 and R= 0.0246 for 1369 unique reflections for Co2As2O7(H2O)2 and a= 6.459(2), b= 14.033(4), c= 7.543(2)Å, β= 94.24(2)°, Z= 4 and R= 0.0272 for 952 unique reflections for Ni2As2O7(H2O)2. The two compounds are isostructural. The framework consists of infinite chains of MO6 octahedra sharing either trans or skew edges along the [10] direction. Adjacent chains are linked by As2O7 groups to form wavy sheets parallel to the (101) plane. Sheets are further connected by diarsenate anions to form a three-dimensional architecture with intersecting tunnels running along the [100] and [001] directions. Water oxygens are bonded to cobalt atoms with the hydrogen atoms directed into the tunnels. The structures are compared with those of Co2As2O7 and CaCo3(P2O7)2.

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