(387d) Thermodynamic Modeling and in Situ Characterization to Understand Solid-State Synthesis Pathways

Solid-state synthesis is the bedrock of inorganic materials chemistry and an integral component of materials design. However, this approach is typically a “black box”, where the only observation is what formed from the precursor materials after reaction at a predetermined set of conditions. The opaque nature of this process precludes an understanding of how the final phases formed from the precursors (e.g., through the formation of intermediate phases). In order to rationally synthesize new inorganic materials, it is critical to not only understand but predict what intermediates form and how they influence the reaction towards the synthesized phase(s).

In this work, we study the synthesis of the classic high-temperature superconductor, YBa₂Cu₃O_7-x (YBCO). This material has been synthesized many thousands of times, including in countless undergraduate laboratories for the demonstration of superconductivity upon cooling with liquid nitrogen. Despite the prominence of this synthesis, little is actually known about how or why the chosen precursors form YBCO. Using in situ synchrotron X-ray diffraction (XRD) and transmission electron microscopy (TEM), we are able to show the evolution of phases during YBCO synthesis for the first time. Importantly, at each step of the synthesis pathway, we rationalize the phase evolution within a thermodynamic framework built upon density functional theory (DFT) calculations and a machine-learned descriptor for compound thermochemistry. Ultimately, we present a general framework for understanding pathways in solid-state synthesis reactions that can be utilized for the future rational synthesis of new compounds.