Electrical transport properties in vanadium bronzes obtained by the sol-gel process

Vanadium bronzes M x V 2O 5 (M=Na, Ag, Ba) are synthesized via sol-gel process accordin to four successive steps: (a) polycondensation of vanadic acid leading to the formation of V 2O 5 gels, (b) gels drying at room-temperature giving rise to a xerogel containing 0.3/0.4 H 3O + ions per V 2O 5, (c) intercalation reaction at room-temperature by ionic exchange of H 3O + ions by M z + ions, (d) heat treatment of the exchanged xerogels leading to the formation of vanadium bronzes M x V 2O 5. These materials, obtained as thin layers, exhibit a preferred orientation depending on the exchanged ion. Silver and sodium bronzes have a β-monoclinic structure ( x≈0.33), whereas barium bronze has an α-orthorhombic structure ( x≈0.16). These bronzes contain conducting species such as polarons and bipolarons. dc conductivity measurements, made between 40 and 300 K, have shown that the bronzes conductivities are strongly anisotropic and depend on the nature of the exchanged ion. Dielectric studies (1 MHz-10 GHz), performed between 200 and 400 K, have evidenced dielectric relaxations due to electron hopping along vanadium chains and ion local hopping (Ag + and Na +).