Nonstoichiometry in vanadium pentoxide by coulometric titration
The equilibrium oxygen partial pressure over nonstoichiometric V2O5−x and the related two-phase mixture has been measured by coulometric titration using yttria-stabilized zirconia (YSZ) as the solid electrolyte at temperatures between 880 and 936 K. The nonstoichiometry (x) is observed up to 0.055 at 920 K and is related to the oxygen partial pressure (Po2) as x∞P −14o2 in the region of x < 0.017. This fact suggests the formation of the defect associate between an oxygen vacancy and a tetravalent vanadium ion. The equilibrium oxygen partial pressure over the coexisting mixture of V2O5 and V4O9 and the standard free energy change of the reaction, 4V2O5→2V4O9+O2 at temperatures between 880 and 936 K are determined to be log(Po2/atm)=8.67 − 12.2×103 T−1 and ΔG°/J mol−=2.33×105−1.65×102T, respectively.