Abstract
The vapor pressures over nonstoichiometric NbO 2 ± x (s) (1.972⩽2.037) were measured by mass-spectrometric method in the temperature range 1958–2326 K. The congruently vaporizing composition in the NbO 2 ± x phase was determined to be stoichiometric NbO 2.000 from the composition dependence of the total vapor pressures. The partial pressures of oxygen were calculated as a function of temperature and O/Nb composition from the partial pressures of the gaseous species NbO 2(g) and NbO(g) over nonstoichiometric NbO 2 ± x , from which the partial molar enthalpies and entropies of oxygen were calculated as a function of O/Nb composition. The composition dependence of the partial molar enthalpy and entropy obtained suggested the existence of some kind of short-range ordering in the nonstoichiometric NbO 2 ± x (s) phase. The enthalpies of formation of nonstoichiometric NbO 2 ± x (s) were also determined as a function of composition by combining the partial molar enthalpies of oxygen with the enthalpy of formation of stoichiometric NbO 2.00(s). The phase diagram around NbO 2 ± x at high temperatures was determined from the vaporization study.
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