Abstract
Carbon-vacancy-bearing Nb4AlC3-x has the best high-temperature mechanical robustness among MAX phases. The existing form of the vacancies has been long overlooked. Recently, the vacancies in Nb4AlC3-x have been identified to be ordered, establishing an ordered compound Nb12Al3C8. Here, the spatial distribution of the ordered vacancies and their influences on bonding characteristics and elastic properties are unraveled by thoroughly comparing Nb12Al3C8 and vacancy-free Nb4AlC3. In Nb12Al3C8, the carbon vacancies break only relatively weak Nb-C bonds and form ordered equilateral triangular carbon-vacancy networks (OETCVNs) to maximize the bond strengthening effect. The networks slightly shift partial and total density of states toward the Fermi energy level, and bring about a feature of de-metallization. Meanwhile, the presence of OETCVNs results in the softening of elastic modulus, decreasing of the anisotropy of Young's modulus, yet increasing that of shear modulus. These results shed lights on the carbon-vacancy ordering behavior of MAX phases, and provide an opportunity to tailor their electronic structures and elastic properties through defect engineering.