Ordered Al-vacancy engineering in Ti₂AlC: Multi-scale establishment of the Ti₂Al_0.667C stability limit and its role in performance tailoring

MAX phases are a group of nanolaminated ternary ceramics where the high mobility of A‑site atoms underpins many of their functional properties. However, the stability limits and governing mechanisms of A‑site vacancies remain poorly understood. This work systematically investigates the formation and stabilization of Al vacancies in Ti₂AlC combining first‑principles calculation and experimental validation. Thermodynamic entropy permits up to 44% Al vacancies before exergonic decomposition, whereas disordered configurations trigger mechanical instability at 37.5% vacancy content by violating Born-Huang elastic criteria. Crucially, lattice dynamics imposes the strictest constraint, capping the vacancy composition at Ti₂Al_0.667C, where a spatially ordered honeycomb defect pattern successfully suppresses imaginary modes. Experimentally, top-down ball milling and acid etching successfully stabilize a vacancy-dense phase with an empirical stoichiometry of Ti₂Al_0.69C. This extreme non-stoichiometry significantly enhances electromagnetic absorption ( RL _min=-51.36 dB), proving that spatial defect ordering is a fundamental paradigm for engineering advanced MAX and MXene properties.