Low-temperature synthesis routes are necessary for selectively 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 °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 YMnO3 formation falls 50 °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.

In the synthesis of complex oxides, solid-state metathesis provides low-temperature reactions where product selectivity can be achieved through simple changes in precursor composition. The influence of precursor structure, however, is less understood in solid-state synthesis. Here we present the ternary metathesis reaction (LiMnO₂ + YOCl → YMnO₃ + LiCl) to target two yttrium manganese oxide products, hexagonal and orthorhombic YMnO₃, when starting from three different LiMnO₂ precursors. Using temperature-dependent synchrotron X-ray and neutron diffraction, we identify the relevant intermediates and temperature regimes of reactions along the pathway to YMnO₃. Manganese-containing intermediates undergo a charge disproportionation into a reduced Mn(II,III) tetragonal spinel and oxidized Mn(III,IV) cubic spinel, which lead to hexagonal and orthorhombic YMnO₃, respectively. Density functional theory calculations confirm that the presence of Mn(IV) caused by a small concentration of cation vacancies (∼2.2%) in YMnO₃ stabilizes the orthorhombic polymorph over the hexagonal. Reactions over the course of 2 weeks yield o-YMnO₃ as the majority product at temperatures below 600 °C, which supports an equilibration of cation defects over time. Controlling the composition and structure of these defect-accommodating intermediates provides new strategies for selective synthesis of complex oxides at low temperatures.