Reduction of the titanium niobium oxides. I. TiNb2O7 and Ti2Nb10O29

Reduction of the titanium-niobium oxides follows a common pattern. TiO₂ is eliminated, to form a new phase richer in titanium than the original compound, and Nb(iv) replaces Ti(iv) in the original block structure, which is thereby enriched in niobium. With TiNb₂O₇, the second phase is a TiO₂NbO₂ solid solution, with the rutile structure, initially with a high titanium content, in equilibrium with a solid solution of composition Me₃O₇, isostructural with TiNb₂O₇. At log p_O2 (atm) about ?9.0 this reaches the limiting composition Ti_0.72Nb_2.28O₇, in equilibrium with Ti_0.56Nb_0.44O₂. The Me₃O₇ block structure then transforms into the Me₁₂O₂₉ block structure (Ti₂Nb₁₀O₂₉Nb₁₂O₂₉ solid solution), which rapidly increases in niobium content as reduction continues. Reduction of Ti₂Nb₁₀O₂₉ at oxygen fugacities above log p_O2 (atm) = ?9.0 forms the Me₃O₇ phase as the titanium-rich phase. At log p_O2 = ?9.0, and a composition about Ti_1.6Nb_10.4O₂₉, the rutile solid solution takes over as second phase. The niobium/titanium ratio in both phases rises as reduction proceeds, and the last vestiges of the Me₁₂O₂₉ phase, in equilibrium with the final product, Ti_0.17Nb_0.67O₂, are almost denuded of titanium.