Ba(ZnAsO4)2·H2O, a non-centrosymmetric framework structure related to feldspar

Abstract

The new compound Ba(ZnAsO 4 ) 2 ·H 2 O was synthesised under low-temperature hydrothermal conditions. Although structurally related to paracelsian- and danburite-type compunds ( P 2 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 P 2 1 , a = 5.291(1), b = 10.418(2), c = 8.055(2) A, β = 95.33(3) °, V = 442.08(16) A 3 , Z = 2]. In the crystal structure of Ba(ZnAsO 4 ) 2 ·H 2 O more or less regular ZnO 4 and AsO 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.