Synthesis and characterization of new double tungstates Li2MII(WO4)(2) (M = Co, Ni, and Cu)

Two new double tungstates, Li2Co(WO4)(2) and Li2Ni(WO4)(2), have been prepa red by solid-state reaction and characterized. The structures of these isos tructural compounds (triclinic, space group P (1) over bar) have been deter mined from X-ray and neutron diffraction data and found to be built up of a lternating layers of zigzag rows of edge-sharing WO6 octahedra and MO6 octa hedra (M = Co2+ or Ni2+) as in wolframite-like phases. However, the MO6 oct ahedra are arranged in columns but not connected to each other; perpendicul ar to these columns there are rows of edge-sharing LiO6 octahedra which als o connect the different MO6 Octahedra. The structure of a previously report ed lithium-copper(II) double tungstate with the same stoichiometry, Li2Cu(W O4)(2), has also been determined and found to be similar to that proposed f or these new double tungstates. The three compounds melt incongruently at t emperatures between 750 and 770 degreesC. Conductivity measurements reveale d that these compounds are not good ionic conductors probably because of th e full occupancy of lithium positions which hinders the motion of the ions along the (LiO6)(infinity) columns.