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 prepared by solid-state reaction and characterized. The structures of these isostructural compounds (triclinic, space group P (1) over bar) have been determined from X-ray and neutron diffraction data and found to be built up of alternating layers of zigzag rows of edge-sharing WO6 octahedra and MO6 octahedra (M = Co2+ or Ni2+) as in wolframite-like phases. However, the MO6 octahedra are arranged in columns but not connected to each other; perpendicular to these columns there are rows of edge-sharing LiO6 octahedra which also connect the different MO6 Octahedra. The structure of a previously reported lithium-copper(II) double tungstate with the same stoichiometry, Li2Cu(WO4)(2), has also been determined and found to be similar to that proposed for these new double tungstates. The three compounds melt incongruently at temperatures between 750 and 770 degreesC. Conductivity measurements revealed that these compounds are not good ionic conductors probably because of the full occupancy of lithium positions which hinders the motion of the ions along the (LiO6)(infinity) columns.