Thermodynamic study of nonstoichiometric tungsten trioxide WO3?x by EMF measurements at high temperature

The values of ΔG(O₂), ΔH(O₂), and ΔS(O₂) have been determined from electrochemical cell measurements, within the whole homogeneity range of WO_3?x, between 700 and 900°C. The samples have been previously prepared by equilibration of WO₃ pellets with COCO₂ mixtures and their composition has been determined by thermogravimetry. A single phase has been found between WO₃ and WO_2.9760. The results may be understood by considering a structure involving point defects, singly ionized oxygen vacancies V^·_O between WO₃ and WO_2.9880. For larger departure from stoichiometry, the variations of ΔH(O₂) and ΔS(O₂) suggest the formation of more complex defects. The enthalpy of formation of V^·_O has been calculated: 78 kcal · mole^?1.