Han Uk Lee | Searcharxiv

arXiv2026

Oxygen stoichiometry directs rutile-anatase phase selection through kinetic control of nucleation

Synthesis of a target polymorph remains more empirical than predictive because crystallization often selects the most accessible nucleation pathway rather than the thermodynamically most stable phase. Here, we show that oxygen stoichiometry converts this empirical synthesis variable into a kinetic control parameter for anatase-rutile selection in TiO$_{2-x}$. Enhanced-sampling simulations reveal that oxygen content alters the nucleation-barrier landscape, switching the relative accessibility of anatase and rutile, even while rutile remains thermodynamically favored. Molecular dynamics simulations show the presence of a diffuse intermediate shell around the nucleus, where oxygen deficiency alters Ti-O coordination and connectivity and drives shell-local motif evolution from anatase-like toward rutile-like environments. A coupled-flux model that integrates barrier competition with shell-mediated attachment/exchange yields a relative nucleation-rate map consistent with reported oxygen-dependent synthesis trends. These results establish stoichiometry-controlled intermediate-shell motif evolution as a kinetic origin of polymorph selection and provide a framework for predicting target phases in composition-coupled crystallization.

cond-mat.mtrl-sci↗

arXiv2026

Precursor-Dependent Energetics as a Predictive Principle for Polymorph Selection in Thin Films

Vapor deposition allows for the synthesis of metastable polymorphs with unique properties, yet polymorph selection remains largely empirical due to the lack of predictive guidelines bridging thermodynamics, kinetics, and synthesis conditions. Here, we show that precursor chemistry can control metastable polymorph selection by modulating the reaction driving force governing nucleation. By integrating first-principles reaction energetics and substrate interactions into classical nucleation theory, we establish a quantitative framework that connects precursor-dependent reaction energetics to polymorph accessibility during vapor deposition. Using Ga2O3 as a model system, we demonstrate that highly reactive precursors with large reaction driving forces kinetically stabilize the metastable {\alpha} phase, whereas low-driving-force precursors permit thermodynamic relaxation to the stable \b{eta} phase. Furthermore, precursor flow rates amplify supersaturation, expanding the kinetic window for stabilizing the elusive \k{appa} phase. The predictive capability of this approach is further validated in the TiO2 system, where precursor-dependent reaction energetics correctly capture the competitive nucleation between rutile and anatase. These results establish precursor chemistry as a tunable chemical lever for controlling nucleation kinetics and provide a predictive design principle for metastable polymorph synthesis in vapor deposition.

cond-mat.mtrl-sci↗