Thermodynamic properties of three oxides of niobium have been measured using solid state electrochemical cells incorporating yttria-doped thoria (YDT) as the electrolyte in the temperature range T = (1000 to 1300) K. The standard Gibbs energies of formation of NbO, NbO2, and NbO2.422 from the elements can be expressed as: △ f G NbO o ± 547 / J · mol − 1 = − 414 986 + 86 .861 ( T / normalK ) △ f G NbO 2 o ± 548 / J · mol − 1 = − 779 864 + 164 .438 ( T / normalK ) △ f G NbO 2.422 o ± 775 / J · mol − 1 = − 911 045 + 197 .932 ( T / normalK ) The results are discussed in comparison with thermodynamic data reported in the literature. The new results refine data for NbO and NbO2 presented in standard data compilations. There are no data in thermodynamic compilations for NbO2.422 (Nb12O29). In the absence of the heat capacity and enthalpy of formation measurements, only the Gibbs energy of formation of NbO2.422 can be assessed. The free energy of formation of stoichiometric Nb2O5 is evaluated on the basis of measurements on NbO2.422 and information available in the literature on phase boundary compositions and isothermal variation of nonstoichiometric parameter with oxygen potential for Nb2O5−x . The results suggest a minor revision of data for Nb2O5. A minimum in the Gibbs energy of mixing for the system Nb−O occurs in the nonstoichiometric domain of Nb2O5−x with x = 0.036.

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