Understanding crystallization pathways leading to manganese oxide polymorph formation

Abstract

Hydrothermal synthesis is challenging in metal oxide systems with diverse polymorphism, as reaction products are often sensitive to subtle variations in synthesis parameters. This sensitivity is rooted in the non-equilibrium nature of low-temperature crystallization, where competition between different metastable phases can lead to complex multistage crystallization pathways. Here, we propose an ab initio framework to predict how particle size and solution composition influence polymorph stability during nucleation and growth. We validate this framework using in situ X-ray scattering, by monitoring how the hydrothermal synthesis of MnO2 proceeds through different crystallization pathways under varying solution potassium ion concentrations ([K+] = 0, 0.2, and 0.33 M). We find that our computed size-dependent phase diagrams qualitatively capture which metastable polymorphs appear, the order of their appearance, and their relative lifetimes. Our combined computational and experimental approach offers a rational and systematic paradigm for the aqueous synthesis of target metal oxides.

Non-Equilibrium Crystallization Pathways of Manganese Oxides in Aqueous Solution

“…However, precipitation of manganese oxides often proceeds by Ostwald's 'Rule of Stages' 14 , where a variety of metastable manganese oxides and oxyhydroxides nucleate and grow prior to the formation of the equilibrium phase 15,16 . These non-equilibrium crystallization pathways can occur both with 17,18 , and without 19 impurity ions in solution. Further complicating the matter, small variations in precursor choice and solution redox conditions can change which metastable phases are observed, as well as their lifetimes, even when the final equilibrium product remains unchanged 20 .…”

“…The Pourbaix potential can be further extended to include the thermodynamic effect of intercalating aqueous impurity ions, as shown in ref. 13,18 for intercalating Li + , K + , Na + , Mg 2+ , and Ca 2+ into various MnO 2 polymorphic frameworks, by performing another Legendre Transformation on the bulk Gibbs free energy, replacing G in Eq. (12)…”

Strategy to control magnetic coercivity by elucidating crystallization pathway-dependent microstructural evolution of magnetite mesocrystals

“…We propose that the crystallization pathways of Fe 3 O 4 can be selected by varying the chemical content. Recent experimental and theoretical studies indicate that the solution chemistry affects the free-energy landscape that directly governs polymorph selection 28,36,37 . The freeenergy landscape is determined by the ratio of surface to bulk energy, which is a function of solution chemistry.…”

Phase formation of manganese oxide thin films using pulsed laser deposition

“…Recent advances in density functional theory (DFT) have enabled the calculation of the thermodynamic ground state structures, and even some non-equilibrium crystallization pathways for a few manganese oxides over a range of proton (pH) and alkali concentration 6,8 . In turn, this ab-initio framework has allowed for predictions of how particle size and solution composition influence polymorph stability in MnO 2 grown from alkali containing solution 9 . But it is not yet clear how these results will translate to other synthesis techniques, specifically vacuum based methods, where the absence of hydration or alkali could drive the phase formation through other pathways.…”