Crystal structure, magnetic susceptibility and electrical conductivity of pure and NiO-doped MoO₂ and WO₂

Single crystals of MoO₂ and WO₂, pure and doped with NiO, were grown by chemical transport in Vycor ampoules using iodine as the transporting agent. From X-ray diffraction analyses it was found that all the crystals were monoclinic, and that doping up to 5% does not change the unit cell constants. The pure dioxide crystals are weakly paramagnetic χ_M < 100×10^-6 emu, but doping raises the magnetic susceptibility markedly: ∼ 2500×10^-6 emu. Resistance studies show that these materials are metallic conductors, the room temperature specific resistivity being of the order of 10^-4 - 10^-3 Ωcm, decreasing by one order of magnitude at liquid nitrogen temperature. Doping also substantially lowers the conductivity.