M + M 3+ 2As(HAsO4)6 (M + M 3+ = TlGa, CsGa, CsAl): three new metal arsenates containing AsO6 octahedra

The crystal structures of hydrothermally synthesized (T = 493 K, 7 d) thallium(I) digallium arsenic(V) hexakis[hydrogenarsenate(V)], TlGa2As(HAsO4)6, caesium digallium arsenic(V) hexakis[hydrogenarsenate(V)], CsGa2As(HAsO4)6, and caesium dialuminium arsenic(V) hexakis[hydrogenarsenate(V)], CsAl2As(HAsO4)6, were solved by single-crystal X-ray diffraction. The three compounds are isotypic and adopt the structure type of RbAl2As(HAsO4)6 (R\overline{3}c), which itself represents a modification of the RbFe(HPO4)2 structure type and consists of a tetrahedral–octahedral framework in which the slightly disordered M + cations are located in channels. The three new compounds contain AsO6 octahedra assuming the topological role of M 3+O6 octahedra. The As—O bond lengths are among the shortest As—O bond lengths known so far in AsO6 octahedra.

Crystal structure of BaMn2(AsO4)2containing discrete [Mn4O18]28−units

In our attempt to search for mixed alkaline-earth and transition metal arsenates, the title compound, barium dimanganese(II) bis(arsenate), has been synthesized by employing a high-temperature RbCl flux. The crystal structure of BaMn2(AsO4)2is made up of MnO6octahedra and AsO4tetrahedra assembled by sharing corners and edges into infinite slabs with composition [Mn2(AsO4)2]2−that extend parallel to theabplane. The barium cations reside between parallel slabs maintaining the interslab connectivity through coordination to eight oxygen anions. The layered anionic framework comprises weakly interacting [Mn4O18]28−tetrameric units. In each tetramer, the manganese(II) cations are in a planar arrangement related by a center of inversion. Within the slabs, the tetrameric units are separated from each other by 6.614 (2) Å (Mn...Mn distances). The title compound has isostructural analogues amongst synthetic SrM2(XO4)2compounds withM= Ni, Co, andX= As, P.

Stability constants for metal arsenates

Environmental context. The behaviour of arsenic compounds is controlled by their solubility, which in turn controls both the forms and mobility of arsenic in the environment. Current knowledge on arsenic chemistry may be distorted because information available on the solubility of arsenic compounds usually does not include possible formation of metal–arsenate complexes. Our results show the formation of stable metal–arsenate complexes that have not been considered before, and this new data can be used to further examine the effect of these complexes on controlling the fate and transport of arsenic in the environment. Abstract. The formation of solid metal arsenates could conceivably reduce the concentrations of arsenate and metal ions in natural and contaminated aqueous ecosystems, and possibly in human body fluids. In this study, solid metal arsenates were dissolved isothermally in solutions with different molar concentrations of arsenic acid. The saturated solutions were analysed and the results processed to derive the solubility products (Ksp) for solid phases and association constants (K) for metal arsenate ion-pairs. Ion chromatography was used to confirm the presence of ion-pairs, some of which had never before been considered. Association constants were determined for the following ion-pairs: FeHAsO4+ (log K = 4.88), CoHAsO40 (log K = 1.50), ZnHAsO40 (log K = 3.28), SrH2AsO4+ (log K = 1.72), and Ag2H2AsO4+ (log K = 4.50). The following metal ions apparently do not form stable complexes with HAsO42–: Cd2+, Cr3+, Cu2+, Mg2+, Mn2+, Ni2+, Pb2+, and Sn2+. Standard state solubility products (Ksp°) were redetermined for the following compounds: Ag3AsO4, Cd3(AsO4)2, Co3(AsO4)2, CrAsO4, Cu3(AsO4)2, FeAsO4, Mg3(AsO4)2, MnHAsO4, NiHAsO4, PbHAsO4, Sn3(AsO4)2, Sr3(AsO4)2, Zn3(AsO4)2·Zn3(AsO4)2·8H2O (koettigite), Cu2Al7(AsO4)4(OH)13·12H2O (ceruleite), and Pb2CuAsO4CrO4OH (fornacite). Our results show the formation of ion-pairs for some metal arsenates and indicate that previous studies have overestimated the solubilities of many arsenates.

Die neuen Oxoarsenate(III) AAsO2 (A = Na, K, Rb) und Cs3As5O9. Darstellung, Kristallstrukturen und Raman-Spektren / The New Oxoarsenates(III) AAsO2 (A = Na, K, Rb) and Cs3As5O9. Synthesis, Crystal Structures and Raman Spectra

The new alkaline metal arsenates(III) were synthesized at a temperature of 500 °C via reaction of stoichiometric mixtures of the elemental alkali metals A and As2O3. In the crystal structures of the four title compounds, which have been determined by single crystal x-ray diffraction, the As(III) atoms are in ψ-tetrahedral coordination by oxygen exclusively. In NaAsO2 (orthorhombic, space group Pbcm, a = 1429.6(9), b = 677.3(3), c = 509.1(2) pm, Z = 8) and the compounds AAsO2 (A = K/Rb, orthorhombic, space group Pbcm, a = 715.1(2)/729.7(5), b =748.0(1)/775.2(5), c = 539.20(17)/541.1(3) pm, Z = 4) the AsO3 ψ-tetrahedra are condensed to form zig-zag chains [AsOO2/2]−. In the Cs phase Cs3As5O9 with a lower alkaline metal content (trigonal, space group P31m, a = 845.5(3), c = 602.6(2) pm, Z = 1) the two crystallographically independent ψ-tetrahedra AsO3/2 and AsOO2/2 are connected in a 2:3 ratio to give polar sheets [As5O9]3−.