Ionic Conductivity Enhancement of La 2 Mo 2− x W x O 9 Nanocrystalline Films Deposited on Alumina Substrates by the Sol–Gel Method

Dense, crack‐free, and uniform La 2 Mo 2− x W x O 9 ( x =0, 0.1, and 0.2) nanocrystalline films were successfully synthesized on poly‐alumina substrates via a modified sol–gel method, with inorganic salt of La(NO 3 ) 3 ·6H 2 O, (NH 4 ) 6 Mo 7 O 24 ·4H 2 O, and (NH 4 ) 6 H 2 W 12 O 24 as precursors. Pure La 2 Mo 2 O 9 phase was confirmed by X‐ray diffractometer when the annealing temperature was >500°C. The average grain size of the La 2 Mo 2− x W x O 9 films is in the range of 90–400 nm, depending upon the conditions of thermal treatment, and the thickness of films can reach 1 μm by repetitive spin‐coating. The electrical conductivity increases with decreasing grain size and reaches 0.074 S/cm at 600°C in the film with a grain size of 90 nm, which is one order of magnitude higher than that in the corresponding bulk materials. W‐doping can suppress the phase transition that occurs at 580°C in pure La 2 Mo 2 O 9 and enhance the low‐temperature ionic conductivity. Furthermore, the activation energy of conductivity in the nanocrystalline La 2 Mo 2 O 9 films decreases to about 0.6 eV in comparison with 1.0 eV in the bulk ones, which implies that the grain resistance prevails in the total resistance, when grain size reduces to nanometer domain.