LITHIUM MVO5 (M=NB,TA) BRONZES

Mixed oxides NbVO5 and TaVO5 have been synthesized by a sol-gel procedure. Both compounds are isostructural with the so-called "monophosphate tungsten bronzes" (MPTB, (PO2)4(WO3)2m, with m = 2). In such structures single MO4 tetrahedra (M = P, V), share corners with M'O6 octahedra (M' = W, Ta, Nb) building pentagonal tunnels along the [010] direction. In this work, we have studied lithium insertion redox reactions carried out by treatments with LiI in acetonitrile in MVO5 (M = Nb, Ta) hosts. Lithium insertion appears as a topotactic and reversible process. The intercalated compounds constitute a new family of Li bronzes showing LixMVO5 compositions with 0 < x < 0.3, which have been characterized by X-ray diffraction, conventional chemical analysis, and X-ray photoelectron spectroscopy. Complex impedance spectroscopy reveals the ionic character of the electrical conductivity in LixMVO5. Typical values of the specific conductivity are in the 10(-6)-10(-8) OMEGA-1 cm-1 range at 573 K, with activation energies on the order of 0.5 eV.