Synthesis and Characterization of New Double Tungstates Li₂M^II(WO₄)₂ (M = Co, Ni, and Cu)

Two new double tungstates, Li 2 Co(WO 4 ) 2 and Li 2 Ni(WO 4 ) 2, have been prepared by solid-state reaction and characterized. The structures of these isostructural compounds (triclinic, space group P 1̄) 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 WO 6 octahedra and MO 6 octahedra (M = Co 2+ or Ni 2+ ) as in wolframite-like phases. However, the MO 6 octahedra are arranged in columns but not connected to each other; perpendicular to these columns there are rows of edge-sharing LiO 6 octahedra which also connect the different MO 6 octahedra. The structure of a previously reported lithium−copper(II) double tungstate with the same stoichiometry, Li 2 Cu(WO 4 ) 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 °C. 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 (LiO 6 ) ∞ columns.