CHIMIE DOUCE SYNTHESIS OF A LAYERED AMMONIUM ZINC MOLYBDATE

A layered ammonium zinc molybdate material was derived by a novel room -temperature chimie douce synthesis technique from a metastable mixed oxide prepared by calcination of a layered double hydroxide (LDH). The compound obtained, (NH4)HZn2(OH)(2)(MoO4)(2), is highly crystalline a nd retains the rhombohedral symmetry of the LDH precursor. The crystal structure of this material was determined by Rietveld refinement of a n isostructural ammonium nickel molybdate. The structure is trigonal, with hexagonal unit-cell parameters of a = 6.10767(15) Angstrom and c = 21.6409(6) Angstrom. The host structure consists of divalent cation octahedra which share edges to form layers perpendicular to the c axis , analogous to the LDH precursor. The tetrahedral molybdate species, h owever, are not merely intercalated within the interlayer domain but a re bonded to the layers themselves through shared MoO-Zn bonds. This a rrangement results in the formation of a net negative charge on the ho st structure, leading to incorporation of ammonium ions between the la yers for charge balancing. The applicability of this novel synthesis r oute is dependent on the composition of the original LDH, and it appea rs that metastability in the calcined LDH favors conversion to this ph ase.