Predictive Targeted Synthesis

Despite rapid progress in the computational design of novel functional materials, the materials discovery pipeline often remains bottlenecked by the difficulty of synthesizing predicted compounds in the laboratory. To realize the vision of accelerated materials design, a more quantitative and predictive theory of materials synthesis is urgently needed. From a theoretical perspective, three important guiding questions for predictive synthesis are: 1) Which compounds designed in silico can be synthesized? 2) For a computationally-designed material, which materials synthesis method—e.g.  solid-state, hydrothermal, vapor deposition, etc.—is best to synthesize it? 3) Within the parameter space of that synthesis method, what synthesis ‘recipe’ can lead to a phase-pure synthesis of the predicted compound?

Developing a predictive theory of materials synthesis requires a more quantitative and mechanistic understanding of metastable phases, which are ubiquitous and form reliably under specific synthesis conditions, but which are difficult to anticipate within existing thermodynamic frameworks. Mechanistic understanding of which metastable phases form, and the experimental conditions where they are preferenced, would allow one to more rationally navigate the thermodynamic and kinetic energy landscape of materials formation: circumventing undesired metastable phases that obstruct the formation of a desired equilibrium phase, or directing processing conditions towards the synthesis of specific metastable polymorphs with superior properties.

By carefully considering the local thermodynamic conditions where materials nucleate can help us anticipate which stable or metastable phases may form during synthesis. We work closely with experimental chemists to observe materials synthesis reactions in situ at synchrotron beamlines, and concurrently develop theoretical models to rationalize our observations. Guided by these insights, solid-state chemists can more rationally navigate the thermodynamic and kinetic energy landscape towards the targeted synthesis of desired material phases.

A key takeaway from our work is that there may be many unanticipated forms of thermodynamic work operative at the local microscopic (or nanoscopic) scale where materials nucleate and grow. These thermodynamic effects can include surface energies, chemical heterogeneities that arise from diffusion limitations, local electric fields, etc. Although these thermodynamic influences are often implicitly operative during materials synthesis, they are not always anticipated or accounted for. By carefully relating these local thermodynamic influences to experimental synthesis parameters, we can successfully predict the formation and persistence of non-equilibrium phases, enabling the rational design of synthesis pathways towards or away from these metastable intermediates.

Metastability and Non-Equilibrium Thermodynamics

We integrate fundamental theory and data-driven tools to develop new quantitative theories of thermodynamics and kinetics to understand the formation of metastable materials during synthesis. To develop new thermodynamic phase diagrams, we are building computational tools to examine phase stability under additional forms of thermodynamic work—such as elastic, surface, electromagnetic or electrochemical work, which can grow the free-energy expression of materials into higher (≥3) dimensions. These new phase diagrams often expose bulk metastable phases that are stabilized under unusual but relevant alternative forms of thermodynamic work.

Other forms of metastable materials include non-equilibrium reaction byproducts—which appear ubiquitously during materials formation, but are difficult to anticipate within existing theoretical frameworks. We are building new kinetic theories to unify classical thermodynamics, nucleation, diffusion, and crystal growth theories, to enable the prediction of Temperature-Time-Transformation (TTT) diagrams for solid-state ceramic synthesis. These TTT diagrams aim to capture crucial features in the kinetic evolution of ceramic powder precursors; including reaction sequence, reaction onset temperature, phase decomposition, liquid formation, and metastable intermediates. Guided by these TTT diagrams, a solid-state chemist can more rationally navigate through the thermodynamic and kinetic energy landscape towards the phase-pure synthesis of target materials.

Relevant Publications

Miura, Akira, C. J. Bartel, Wenhao Sun*, et al. "Observing and Modeling the Sequential Pairwise Reactions that Drive Solid‐State Ceramic Synthesis." Advanced Materials (2021): 2100312.

Miura, Akira,* Wenhao Sun* et al. "Selective metathesis synthesis of MgCr 2 S 4 by control of thermodynamic driving forces." Materials Horizons 7.5 (2020): 1310-1316.

Bianchini, Matteo, et al. "The interplay between thermodynamics and kinetics in the solid-state synthesis of layered oxides." Nature Materials 19.10 (2020): 1088-1095.

Sun, Wenhao, et al. "Non-equilibrium crystallization pathways of manganese oxides in aqueous solution." Nature Communications 10.1 (2019): 1-9.

Chen, Bor-Rong, Wenhao Sun, et al. "Understanding crystallization pathways leading to manganese oxide polymorph formation." Nature communications 9.1 (2018): 1-9.

Sun, Wenhao, et al. "Nucleation of metastable aragonite CaCO3 in seawater." Proceedings of the National Academy of Sciences 112.11 (2015): 3199-3204.

Metastability

Sun, Wenhao, et al. "The thermodynamic scale of inorganic crystalline metastability." Science Advances 2.11 (2016): e1600225.

Powell-Palm, Matthew J., Boris Rubinsky, and Wenhao Sun. "Freezing water at constant volume and under confinement." Communications Physics 3.1 (2020): 1-8.

Sun, Wenhao, and Matthew J. Powell-Palm. "Generalized Gibbs' Phase Rule." arXiv preprint arXiv:2105.01337 (2021).

Kitchaev, Daniil A., et al. "Thermodynamics of phase selection in MnO2 framework structures through alkali intercalation and hydration." Journal of the American Chemical Society 139.7 (2017): 2672-2681.