Lattice parameter model and oxidation mechanism of substoichiometric zirconium carbide ceramics with intrinsic oxygen

Zirconium carbide is a versatile ultra-high temperature ceramic with a broad tunable substoichiometry range. However, many reported compositions exhibit unintended oxygen incorporation, which leads to scattering in lattice parameters. This complicates the understanding of synergistic effects of carbon vacancies and oxygen on crystal structure, and highlights the scarcity of oxidation studies across varying substoichiometries. Here, we synthesize substoichiometric zirconium carbide ceramics and, to our knowledge, develop the first empirical model to capture vacancy–vacancy interactions and trace oxygen occupation. We further present a systematic study of their oxidation kinetics over a wide range of carbon deficiencies, revealing critical roles of carbon and vacancies in governing oxidation pathways. In substoichiometric zirconium carbides, the parabolic rate constant exhibits a non-monotonic dependence on carbon stoichiometry, highlighting the complex defect-mediated nature in zirconium carbide oxidation. These results establish a defect-based framework for understanding the structural evolution and oxidation mechanisms of carbon-deficient ultra-high temperature carbides.