Thermodynamic properties of niobium oxides

Abstract

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 NbO_2.422 from the elements can be expressed as: Δ_fG^o_NbO±547/J·mol^-1=-414 986+86.861(T/KΔ_fG^o_NbO2±548/J·mol^-1=-779 864+164.438(T/K)Δ_fG^o_NbO2.422±547/J·mol^-1=-911 045+197.932(T/K) 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 NbO_2.422(Nb₁₂O₂₉). 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 Nb₂O₅ 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 Nb₂O_5-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 Nb₂O_5-x with x = 0.036.