Abstract
The crystal structure of braithwaiteite, NaCu2+ 5(Sb5+Ti4+)O2(AsO4)4(AsO3OH)2(H2O)8, triclinic, Z=1, P 1, a=7.0308(4), b=9.8823(5), c=10.6754(6)Å, α=106.973(1), β=104.274(1), γ=93.839(1)°, V=679.76(11)Å3, was solved by direct methods and refined to an R 1 index of 0.025 for 3584 observed reflections. A prominent motif in the braithwaiteite structure is a chain of corner-sharing (Sb,TiO6) octahedra that is decorated with (AsO4) tetrahedra to form a [(SbTi)(AsO4)4O2] chain that extends along the a -direction and defines the cell dimension in this direction at ∼7Å. These chains are cross-linked in the b -direction by chains of edge-sharing (CuO6) octahedra that are decorated by arsenate tetrahedra to form a [Cu2(AsO4OH)2O4] chain; these two chains link to form a sheet parallel to (001). These sheets stack along the c -direction and are linked by (CuO6) and (NaO6) octahedra, together with an extensive network of hydrogen bonds. The [M(TO4)2Φ4] chain (M=octahedrally coordinated metal, T=tetrahedrally coordinated metal, Φ=unspecified simple anion;≡[(SbTi)(AsO4)4O2] chain) is a common motif in secondary oxysalt minerals, occurring in sulfates, phosphates, arsenates, vanadates and silicates.
Published Version
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