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A new monoclinic polymorph of Ba(AsO₃OH) was synthesized under hydrothermal conditions. It represents a previously unknown structure type. Its crystal structure was determined from a racemic twin using single-crystal X-ray diffraction data collected at 120 and 293 K [space group P2₁, a=7.2149(14)/7.2370(2), b=7.7028(15)/7.7133(2), c=21.7385(43)/21.8079(5)Å, β=95.95(3)/96.073(1)°, V=1201.6(4)/1210.51(5) Å, Z=12]. The crystal structure of P2₁-Ba(AsO₃OH) has a layered character and is built up of four types of regularly alternating layers parallel to (0 0 1). Every AsO₃OH tetrahedron is chelating to two Ba atoms and bridged by another two Ba atoms. Each OH group acts as hydrogen bond donor toward the oxygen atoms positioned in the same or adjacent layers. Although the H atoms could not be located, no ambiguities are present in the hydrogen-bonding scheme. Single-crystal vibrational spectroscopy (FTIR and Raman) was used to describe the vibrational behavior of the hydrogen bond system; particularly the spectroscopic manifestation of the very short and short hydrogen bonds (2.462(7)2.575(7) Å). In order to complement spectroscopic data on protonated orthoarsenates, infrared spectra of triclinic F1̄-Sr(AsO ₃OH) and the orthorhombic variety of Pbca-Ba(AsO₃OH) were recorded and discussed. Furthermore, structural features of other alkaline earth hydrogen arsenates are discussed. © 2010 ElsevierInc. All rights reserved.