Low Temperature Synthesis of Metal Oxide Bronze Phases and their Electric Properties

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(1). We have found that potassium hexagonal tungsten bronze (K-HTB) synthesized from potassium peroxo-polytungstate at low temperature (500ﾟC) shows a remarkable increase in resistivity in a humidid atmosphere at room temperature. The mechanism of the resistivith increase by incorporation of water was investigated by gravimetry, complex impedance spectroscopy, and powder XRD measurement. The powder XRD profiles indicated that superlattice structure of the K-HTB emerges by incorporation of water. A layr structure model of the K-HTB was suggested from these results.(2). An amorphous salt was precipitated by reacting barium bromide with a solution of peroxo-polytangstic acidl. Heating it in a hydrogen stream at temperature near 550ﾟC yielded a hexagonal form of barium tungsten bronze, Ba_xWO_3, in a pure aphase with x being in the range 0.19-0.24. At temperature higher than 620ﾟC,this metastable compound was transformed into the tetragonal bronze phase. An unreported lead tungsten bronze in the hexagonal form (a=7.408, c=3.786 A,for Pb_<0.29>WO_3) was synthesized similarly but at lower temperature (420ﾟC). Their crystal structures were refined by the Rietveld method. The electric properties of the compounds were also investigated between 4.2K to room temperature..