Effect of oxygen-deficiency on electrical transport properties of tungsten trioxide crystals

• Published in: Journal of solid state chemistry 1970, Vol.2 (1), p.109-133
• Main Authors: Berak, James M., Sienko, M.J.
• Format: Article
• Language: English
• ISSN: 0022-4596; 1095-726X
• DOI: 10.1016/0022-4596(70)90040-X

Single crystals of various substoichiometric tungsten trioxide crystals have been prepared by vapor phase transport and annealed under varying conditions. Electric resistivity, Hall voltage and Seebeck coefficient have been investigated as functions of temperature in the range 100–500°K and as functions of composition down to the compound WO 2.90. For stoichiometric WO 3, the results are in general agreement with previous reports. The jump in resistivity at the monoclinic-triclinic transition is due mainly to a decrease in electron mobility. The change at the triclinic-(low temperature) transition is almost entirely the result of a large decrease in the carrier concentration. Changes in thermoelectric power cannot be explained on the basis of the ordinary electronic component; a lattice contribution must be present. Removal of oxygen pushes the transitions to lower temperatures, increases the carrier concentration, decreases the Hall mobility, and changes the sign of its temperature dependence. Segregation of oxygen defects apparently occurs even for x < 0.0001 in WO 3− x , and it is believed that the basic defect unit is not the oxygen vacancy but the Anderson-Hyde disk of shear. The upper bound for this type of defect structure appears to be at x = 0.013. The increase of mobility with increasing temperature is attributed to hopping of charge carriers in place of band processes in the defect regions. Decreases in the optical phonon drag and transport energy terms are needed to fit the thermoelectric power behavior of the monoclinic phase. X-ray studies show essentially no change in the cell parameters for the reduced crystals. There is strong optical absorption in the red and near infrared associated with the oxygen deficiency, but only negligible paramagnetism. DTA studies show that the monoclinic-triclinic and triclinic-(low temperature) phase transitions are of the first-order and displacive type as in BaTiO 3; they are accompanied by transition heats of about 20 and 39 cal/mole, respectively. Measurements of resistivity and Seebeck coefficient on a W 20O 58 crystal suggest metallic-like behavior.