Synthetic approach of ternary magnesium niobate (Mg-Nb-O) compounds

Magnesium based niobium oxides (Mg-Nb-O) were prepared by solid-state reactions owing to understand the function of transition metal oxides as promoters/catalysts for practical application. Magnesium niobate (Mg₃Nb₆O₁₁) was synthesized for the first time in nearly pure form reported in this context. MgNb₂O₆ and Mg₄Nb₂O₉ were prepared in oxidizing conditions; on the contrary, Mg₃Nb₆O₁₁ preferred reducing environment. Stoichiometric mixtures of the precursor materials MgO, Nb₂O₅ and/or metallic Nb were annealed for the syntheses which revealed the effect of temperature on phase formation, reaction kinetics and heat of reaction. The products were examined by ex-situ, in-situ X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Crystallographic parameters of various binary and ternary compounds (Mg/Nb/O) formed in different calcination conditions, were extracted by Rietveld method. In-situ experiment results in single step reaction for the MgNb₂O₆ synthesis and the heat of formation of the solid-state reaction obtained to be minimum (93 kJ/mol). In contrast, the formation of Mg₄Nb₂O₉ and Mg₃Nb₆O₁₁ compounds towards pure phases rather complicated due to multistep reactions and corresponding heat of formation were estimated to be 140 and 190 kJ/mol. Experimental results have been discussed based on kinetic and thermodynamic constrains.