Formation Gibbs energy of K 2NbF 7, K 2NbOF 5, and K 2NbO 2F 3 from electrochemical measurements

Abstract

The formation Gibbs energies of molten K 2NbF 7, K 2NbOF 5, and K 2NbO 2F 3 at the temperature of 750°C were calculated from the previously performed voltammetric and chronopotentiometric measurements in the system LiF–NaF–K 2NbF 7–Na 2O [1–3] and from the equilibrium constants of reactions leading to the formation in the melts of the individual oxyfluoroniobates. For the formation Gibbs energy of K 2NbF 7 the average value Δ f G 0(K 2NbF 7)=−(1340±25) kJ mol −1 was obtained. For K 2NbOF 5 the formation Gibbs energy is Δ f G 0(K 2NbOF 5)=−(1526±16) kJ mol −1. This value may, however, not be exactly correct since the product of the electrochemical reduction was not pure niobium, but the solid solution of oxygen in niobium. For the formation Gibbs energy of K 2NbO 2F 3 the value Δ f G 0(K 2NbO 2F 3)=−(1100±100) kJ mol −1 was calculated. This value may be charged by a substantial error as well due to the probably incorrect value of the formation Gibbs energy of K 2NbOF 5.