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 and retains the rhombohedral symmetry of the LDH precursor. The crystal structure of this material was determined by Rietveld refinement of an isostructural ammonium nickel molybdate. The structure is trigonal, with hexagonal unit-cell parameters of a = 6.10767(15) Å and c = 21.6409(6) Å. 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, however, are not merely intercalated within the interlayer domain but are bonded to the layers themselves through shared Mo-O-Zn bonds. This arrangement results in the formation of a net negative charge on the host structure, leading to incorporation of ammonium ions between the layers for charge balancing. The applicability of this novel synthesis route is dependent on the composition of the original LDH, and it appears that metastability in the calcined LDH favors conversion to this phase.