Reaction Selectivity in Cometathesis: Yttrium Manganese Oxides

Synthesis of metastable materials by control of reaction pathways is facilitated by low-temperature routes. Cometathesis reactions have recently been shown to lower reaction temperatures when compared to single-ion metathesis reactions. Here, we share the discovery of how and why different precursor combinations radically change the reaction pathway and selectively yield different product polymorphs. By studying reactions of the general form, xA_yMnO₂ + (1 − x) A′_zMnO₂ + YOCl → YMnO₃ + xA_yCl + (1 − x)A′_zCl (A and A′ = Li, Na, Mg, and Ca, y and z = 1/2 or 1, and 0 ≤ x ≤ 1) using ex post facto synchrotron X-ray diffraction, we determine reaction onset temperatures and reaction intermediates for various combinations of A and A′. These observations highlight the importance of the nascent halide salt product in determining the reaction onset temperature, which is lower for all studied cometathesis reactions than the reaction temperatures of the constituent single-ion metathesis reactions. In addition, the spectating alkali and alkaline earth species determine the accessible intermediates, which steers the reaction pathways toward different product phases. While each of the studied cations has a unique reaction pathway, polymorph-selective synthesis is only achieved with a mixture of alkali or alkaline earth cations. Specifically, cation combinations of Li and Na produce phase-pure products of the hexagonal polymorph of YMnO₃ and mixtures of Li and Mg or Mg and Ca produce orthorhombic YMnO₃. Altogether, this study highlights how chemical potentials at reacting interfaces and the propensity to form defective structures dictates the reaction pathway such that one can target metastable materials by controlling the reaction pathways.