Structure and electrical properties of sodium tungsten bronzes thin films

Abstract

Highly oriented thin films of Na_xWO₃ (x=0.25-0.7) are grown on sapphire (1 1 0 2 ) and SrTiO₃ (1 0 0) substrates by pulsed laser deposition. The colour of the films changes from blue through red to bronze yellow on varying the sodium content. The films are composed of particles less than 100 nm size. As in bulk Na_xWO₃, the resistivity is found to decrease on increasing the sodium content, and at around x=0.25, a non-metal to metal transition is clearly observed. The conductivity measurement of Na_0.25WO₃ shows a crossover from temperature-activated Arrhenius behaviour at high temperatures to a variable range hopping mechanism near 70 K. The metallic samples show linear dependence of resistivity above 50 K. XPS studies confirm the presence of sodium in +1 and tungsten in +5 and +6 oxidation states.