Lowering Ternary Oxide Synthesis Temperatures by Solid-State Cometathesis Reactions

Low- temperature synthesis routes are necessary for synthesizing many metastable solid-state materials. Here, we identify a cooperative effect that starting materials have in lowering temperatures in solid-state metathesis reactions by studying the formation of yttrium manganese oxide. Previous studies have shown that YMnO3 can be synthesized by ternary metathesis with an alkali halide being produced as a secondary product. In this contribution, we show that by using alkaline earth metals instead of alkali metals, the polymorph selectivity of the reaction is changed, as orthorhombic YMnO3 forms at lower temperatures than the hexagonal polymorph. Reactions were studied using ex post facto synchrotron X-ray diffraction. These experiments reveal that reactions using alkaline earth manganese oxides as a starting material require high temperatures to progress. Reaction temperatures can be lowered from 700 to 550 degrees C while maintaining phase selectivity by reacting both MgMn2O4 and CaMn2O4 with YOCl in a cooperative cometathesis reaction. The nascent halide salts appear to improve the reaction kinetics. Since the onset temperature for YMn( )3 formation falls 50 degrees C below the MgCl2-CaCl2 liquidus, the enhanced reactivity is consistent with the surface melting of a nasscent salt byproduct at the interfaces. Cometathesis routes have similar phase selectivity and temperature reduction in reactions that form TbMnO3, ErMnO3, and DyMnO3. Cometathesis lowers reaction temperatures while preserving the reaction selectivity of the end members, making it a valuable approach for synthesizing metastable targets.

Automated summary

The study identified a cooperative effect in solid-state metathesis reactions that involves using alkaline earth metals instead of alkali metals to lower the formation temperature of orthorhombic YMnO3. Experiments showed that by reacting MgMn2O4 and CaMn2O4 with YOCl in a cooperative cometathesis reaction, reaction temperatures can be reduced to 550°C while maintaining phase selectivity. This approach also showed potential in synthesizing other metastable targets such as TbMnO3, ErMnO3, and DyMnO3 with reduced temperatures and preserved reaction selectivity.