Synthesis of metal oxides from a liquid-mixed amorphous inorganic precursor

What is it about?

A citrate-melt synthesis method of quaternary oxides of transition metals with alkaline- and rare-earth metals via liquid mixing and amorphous solid precursors was described in 2003 https://doi.org/10.1016/S0022-4596(02)00008-7, based on the liquid-mix process published in 1993 https://doi.org/10.1111/j.1151-2916.1994.tb07028.x. The point is to maintain the atomic mixing up to the inorganic precursor that is then crystallized by sintering at the temperature and oxygen partial pressure needed for the desired oxygen non-stoichiometry. The process starts by dry mixing the rare-earth oxide powder with crystalline citric acid monohydrate, adding a small amount of water at the bottom and melting the acid upon stirring until the oxide dissolves. Once the clear melt cools below 100 °C, the alkaline-earth carbonate and water is added to be dissolved upon heating a stirring. The transition metal dissolved in nitric acid is then added gradually to the citrate solution upon heating and stirring accompanied by evolution of NO2. The resulting clear solution of metals in melted citric acid is poured onto a heated porcelain bowl in drying oven at 180 °C and dried there overnight into a brown solid foam of the organic precursor. That is milled in a vibration agate-ball mill into fine powder to be subsequently incinerated over three weeks in a lid-covered crucible in a table-top crucible furnace at 390 °C into an X-ray amorphous inorganic powder—the precursor for calcination and sintering into the desired crystal. The component amounts for this synthesis: In this article, they are given for PrBaFe2O5. For Y, Ba, Cu oxides, the minimum amounts of citric acid per 1 mole metals were tested in https://doi.org/10.1111/j.1151-2916.1994.tb07028.x for three phases in this system. The minimum number of moles of citric acid was 2 per 1 mole of metals, but for a phase with high Y content, it increased to 3. The safe amount of citric acid monohydrate used since 1993 has been 4 moles of citric acid monohydrate per 1 mole of metals in the starting components. The dissolution of the transition metal in nitric acid was performed by adding 5 mL of water per gram of the metal, and then by gradually adding the concentrated nitric acid of total volume estimated as 10 mL per gram of the metal to be dissolved upon warming and stirring. The amount of citric acid was then increased by 2 moles per 1 mole of the metal dissolved in the nitric acid.

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Why is it important?

Synthesis of functional oxides of several metals and desired oxygen non-stoichiometry is facilitated by having its reaction components mixed at an atomic scale. Right choice of temperature and oxygen-gas partial pressure under the sintering will make the amorphously mixed precursor powder to sinter into the crystalline phase stable at those conditions and limit or avoid impurities.