Room-temperature sol-gel synthesis of sodium hexaniobate in an immiscible hexane-water system and its conversion into NaNbO₃

Sodium hexaniobate (Na₇[HNb₆O₁₉] ·15H₂O) was synthesized by a one-pot sol-gel method at room temperature by using an immiscible, biphasic solution. An organic solution was prepared by dissolving niobium pentaethoxide [Nb(OC₂H ₅)₅] in ethanol, which was then added to hexane. This solution was brought into contact with an aqueous NaOH solution to form a reaction system consisting of two separate phases. A white precipitate was formed within 12 h at the bottom of the aqueous solution. It exhibited well-defined rod-like morphology on a micrometer scale. A comparative study was performed on the basis of a single-phase solution by mixing Nb(OC ₂H₅)₅ in ethanol and the aqueous NaOH solution. The results indicated that the product formed by way of the single-phase solution was rather different in morphology from that obtained in the biphasic system. In both cases, Na₇[HNb₆O₁₉] ·15H₂O could be converted to NaNbO₃ by heating at 600 °C. Mechanisms underlying the formation of Na₇[HNb ₆O₁₉]·15H₂O and its conversion to NaNbO₃ are discussed on the basis of the experimental results.