Thermodynamic study of nonstoichiometric tungsten trioxide WO 3- x by EMF measurements at high temperature
The values of ∆G(O 2), ∆H(O 2), and ∆S(O 2) 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 WO 3 pellets with COCO 2 mixtures and their composition has been determined by thermogravimetry. A single phase has been found between WO 3 and WO 2.9760. The results may be understood by considering a structure involving point defects, singly ionized oxygen vacancies V·O between WO 3 and WO 2.9880. For larger departure from stoichiometry, the variations of ∆H(O 2) and ∆S(O 2) suggest the formation of more complex defects. The enthalpy of formation of V·O has been calculated: 78 kcal · mole -1.