Establishing the Temperature-Dependent Synthesis Window of β‑TaON by Coupling Predictive Thermodynamic Modeling with Experimental Validation.

Phase-pure synthesis has been a major challenge for transition-metal oxynitrides due to their sensitivity to synthesis conditions and limited understanding of their underlying thermodynamics. Beta-phase tantalum oxynitride (β-TaON), a promising candidate for (photo)-catalytic applications, is particularly difficult to reproducibly synthesize as a single-phase material at equilibrium. In this study, we developed and experimentally validated a thermodynamic model to identify optimal conditions for the single-phase synthesis of β-TaON via ammonolysis reactions. Gibbs free energies of reactant, product, and byproduct phases were predicted as a function of temperature using first-principles calculations with the quasi-harmonic approximation (QHA), as well as implemented from available thermodynamic databases. Utilizing these Gibbs energies, the calculation of phase diagrams (CALPHAD) approach was employed to develop a thermodynamic model for assessing the phase equilibria associated with the ammonolysis reactions, enabling prediction of temperature-dependent synthesis windows for β-TaON. Experimental syntheses were carried out across a range of temperatures and gas conditions, validating model predictions and iteratively refining the model accuracy through an integrated feedback loop. Co-flown gases beyond ammonia and water were also shown to be influential on β-TaON phase purity and reaction kinetics. A three-dimensional (3D) phase diagram predicted on the axes of parameters that can be controlled during practical synthesis quantitatively reveals a narrow, phase-pure synthesis window for β-TaON.