The isotypic family of the diarsenates MM′As2O7 (M = Sr, Ba; M′ = Cd, Hg)

The diarsenates MM'As2O7 (M = Sr, Ba; M' = Cd, Hg) were prepared under hydrothermal conditions (similar to 200 degrees C, autogenous pressure), starting from As2O5 and the corresponding metal oxides or precursor compounds thereof in aqueous solutions. Structure analyses on the basis of single crystal X-ray data revealed the four structures to be isotypic. They are the first diarsenates to crystallize in the triclinic BaZnP2O7 structure type (space group P (1) over bar, Z = 2, a approximate to 5.8 angstrom, b approximate to 7.3 angstrom, c approximate to 7.6 angstrom, alpha approximate to 101 degrees, beta approximate to 91 degrees, gamma approximate to 98 degrees). All related MM'As2O7 diarsenates reported so far (M = Sr, Ba, Pb; M' = Mg, Co, Cu, Zn) crystallize in the monoclinic alpha-Ca2P2O7 structure type (P2(1)/n, Z = 4). Hence, the size of the divalent M' cation determines which of the two structure types is adopted.