Chemical synthesis and electronic conduction properties of sodium and potassium tungsten bronzes

NaxWO3 and KxWO3 gels have been prepared by reduction of WO3 in aqueous sol ution with NaBH4 or KBH4 induced by readdition of hydrochloric acid. These tungsten bronze gels, as good precursors, were annealed at 600 or 850 degre esC to yield crystalline product. Their compositions are Na0.56WO3 and K0.5 8WO3, respectively, as determined by energy dispersive analysis. X-ray diff raction patterns indicate that NaxWO3 annealed at 600 degreesC is a metasta ble tetragonal phase, which transforms to cubic perovskite phase when annea led at 850 degreesC; while KxWO3 annealed at either 600 or 850 degreesC has a tetragonal structure. Resistivity measurement indicates that Na0.56WO3 s hows a change from metallic to anomalous semiconducting behavior accompanie d by phase transformation from tetragonal to cubic phase. This is in contra st to metallic behavior in cubic NaxWO3 (x > 0.25) generally reported befor e. Both of the samples of K0.58WO3 annealed at 600 and 850 degreesC are sem iconducting. (C) 2000 Elsevier Science Ltd. All rights reserved.