Oxygen Vacancies as an Entropy Lever to Raise the Melting Point of Oxide Ceramics

The pursuit of oxidation-resistant materials for temperatures exceeding 2500°C is fundamentally bottlenecked by the melting points of oxide ceramics. While maximizing cohesive enthalpy has been the traditional route, this approach has reached its thermodynamic limit. Herein, we report a paradigm shift by deliberately engineering entropy—the long-overlooked variable in melting-point control. Using the model system Lu_xHf_1-xO_2-0.5x, we demonstrate that oxygen vacancies act as a potent “entropy lever.” Their introduction creates disorder on the anion sublattice, which selectively amplifies the solid's configurational entropy without equivalently raising the liquid's entropy, while simultaneously driving the solid's local bonding environments closer to those of the liquid and reducing their vibrational entropy difference. This synergistic effect drastically lowers the entropy of fusion, resulting in a record-high melting point of ∼2912°C at an optimal composition (x ≈ 0.4). This work establishes entropy-mediated design, realized through defect engineering, as a transformative strategy to break the long-standing melting-point ceiling in ultra-high-temperature oxide ceramics.