The Crystal Structure and Crystal Chemistry of Mineral-like Cd5(VO4)2(OH)4, a Novel Isomorph of Arsenoclasite and Gatehouseite

The pentacadmium bis(vanadate(V)) tetrahydroxide Cd5(VO4)2(OH)4 was synthesized under hydrothermal conditions, and its crystal structure was determined with single-crystal X-ray diffraction. The investigated compound is the second known compound next to Cd(VO3)2·4H2O synthesized in the CdO–V2O5–H2O system and crystallizes isotypically to the minerals gatehouseite, Mn5(PO4)2(OH)4, and its As analog arsenoclasite, Mn5(AsO4)2(OH)4. Its symmetry is orthorhombic, with a space group of P212121 and unit cell parameters of a = 19.011(4), b = 6.0133(12), c = 9.5411(19) Å, V = 1090.7(4) Å3, and Z = 4. The structure consists of double ribbons of M(O,OH)6-octahedra (M = Cd2, Cd3, Cd4) extending along [010] interconnected by edge- and corner-shared M(O,OH)6-octahedra (M = Cd1, Cd5) and discrete, slightly distorted VO4 tetrahedra, which form double chains of coupled polyhedra [V1O4–Cd5O4(OH)2–Cd1O5(OH)–V2O4]n running along the same direction. The interesting feature is the existence of V–Cd distances (3.0934(7) and 3.1081(7) Å for V1–Cd5 and V2–Cd1, respectively), which are shorter than the sum of the van der Waals radii of 3.71 Å. The V1–V2 distances of 4.1214(9) Å are also shorter than the sum of the van der Waals radii of 4.26 Å. The O–H···O hydrogen bonds additionally link the two subunits, ribbons, and chains into a three-dimensional structure. Raman spectra confirmed the presence of the hydrogen bonds and mutually isolated VO4 groups.