Chemical synthesis and electronic conduction properties of sodium and potassium tungsten bronzes Original Research Article

NaxWO3 and KxWO3 gels have been prepared by reduction of WO3 in aqueous solution with NaBH4 or KBH4 induced by re-addition of hydrochloric acid. These tungsten bronze gels, as good precursors, were annealed at 600 or 850°C to yield crystalline product. Their compositions are Na0.56WO3 and K0.58WO3, respectively, as determined by energy dispersive analysis. X-ray diffraction patterns indicate that NaxWO3 annealed at 600°C is a metastable tetragonal phase, which transforms to cubic perovskite phase when annealed at 850°C; while KxWO3 annealed at either 600 or 850°C has a tetragonal structure. Resistivity measurement indicates that Na0.56WO3 shows a change from metallic to anomalous semiconducting behavior accompanied by phase transformation from tetragonal to cubic phase. This is in contrast to metallic behavior in cubic NaxWO3 (x>0.25) generally reported before. Both of the samples of K0.58WO3 annealed at 600 and 850°C are semiconducting.