Effect of magnesium on the phase equilibria in magnesio-thermic reduction of Nb₂O₅

Nb powders were synthesized by magnesio-thermic reduction. Nb₂O₅ is reduced by Mg via an oxygen-metal cation exchange in a mixed powder bed. Although thermodynamics favor reduction of Nb₂O₅ with Mg, the final phase distributions can be affected by the Mg content and interaction between products and reactants. Mg/Nb₂O₅ molar ratio was changed from 5 to 7.5 in order to evaluate the reaction pathway and relevant phases. Due to premature Mg depletion, samples with Mg/Nb₂O₅=5 resulted in intermediate Mg-Nb-O oxides according to the phase equilibria. A time-series comparison of Mg/Nb₂O₅=5 at 700 °C and 800 °C resulted in faster Mg-Nb-O intermediate phase formation at higher temperatures. Depending on the average oxidation state, the face-centered cubic Mg-Nb-O and hexagonal Mg₃Nb₆O₁₁ phase were identified.