Thermodynamic Properties of Niobium Oxides

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, NbO 2, and NbO 2.422 from the elements can be expressed as: The results are discussed in comparison with thermodynamic data reported in the literature. The new results refine data for NbO and NbO 2 presented in standard data compilations. There are no data in thermodynamic compilations for NbO 2.422 (Nb 12 O 29 ). In the absence of the heat capacity and enthalpy of formation measurements, only the Gibbs energy of formation of NbO 2.422 can be assessed. The free energy of formation of stoichiometric Nb 2 O 5 is evaluated on the basis of measurements on NbO 2.422 and information available in the literature on phase boundary compositions and isothermal variation of nonstoichiometric parameter with oxygen potential for Nb 2 O 5− x . The results suggest a minor revision of data for Nb 2 O 5 . A minimum in the Gibbs energy of mixing for the system Nb−O occurs in the nonstoichiometric domain of Nb 2 O 5− x with x = 0.036.