Knudsen effusion mass spectrometric approach to the thermodynamics of Na2O-Nb2O5 system

► We measure activity of Na2O and Nb2O5 in two-phase regions of Na2O-Nb2O5 system. ► We determine Gibbs energy of formation and enthalpy of formation of five phases of Na2O-Nb2O5 system. ► We find that NaNb3O8 is thermodynamically stable against decomposition also below 950 K.