Ba(ZnAsO (sub 4) ) (sub 2) .H (sub 2) O, a non-centrosymmetric framework structure related to feldspar

The new compound Ba(ZnAsO (sub 4) ) (sub 2) .H (sub 2) O was synthesised under low-temperature hydrothermal conditions. Although structurally related to paracelsian- and danburite-type compounds (P2 (sub 1) /a and Pnma, respectively), it represents a new, non-centrosymmetric structure type. Its crystal structure was determined from an inversion twin using single-crystal X-ray diffraction data collected at 293 K [space group P2 (sub 1) , a=5.291(1), b=10.418(2), c=8.055(2) Aa, beta =95.33(3) degrees , V=442.08(16) Aa (super 3) , Z=2]. In the crystal structure of Ba(ZnAsO (sub 4) ) (sub 2) .H (sub 2) O more or less regular ZnO (sub 4) and AsO (sub 4) tetrahedra are corner-connected to form a three-dimensional feldspar-like network. The tetrahedra are connected to four- and eight-membered tetrahedral rings, which are arranged to form channels parallel to the a-axis. The channels formed by eight-membered rings are occupied by the Ba atoms and water molecules; the latter are only bonded to the Ba atoms and responsible for the lowering of the space-group symmetry. To obtain further information on the anion groups and especially on hydrogen bonds, single-crystal Raman spectra were measured and evaluated. Furthermore, a comparison with topologically similar paracelsian- and danburite-type structures is given. Crystal chemistry and bonding in other paracelsian- and danburite-like structures are briefly reviewed.