Promotion of 211 MAX Phase Formation through Bismuth A‐Site Solid Solution in Ti 2 (Al 1‐ x Bi x )C

Solid‐solution engineering is a powerful strategy for expanding the chemical diversity and functional tunability of MAX phases. In this study, we realize Ti 2 (Al 1‐ x Bi x )C MAX phase solid solutions via reactive synthesis, demonstrating that an A‐site solid solution with Bi favors the formation of the 211‐type structure over its 312‐type counterpart within the Ti–Al–Bi–C system. Synthesis in a sealed quartz ampule of solid solution Ti 2 (Al 1‐ x Bi x )C MAX phase yields a composition of Ti 2 Al 2/3 Bi 1/3 C. This composition is consistent with first‐principles density functional theory (DFT) predicting the highest stability for Al/Bi mixing in the A‐layer. Guided by these A‐site alloying stability trends, subsequent scalable reactive synthesis in a tube furnace under inert atmosphere demonstrates that incorporation of Bi produces 211‐type MAX phase over a wide synthesis temperature window (1200–1500°C). MXene synthesis from both Ti 2 AlC and Ti 2 (Al 1‐ x Bi x )C yields similar structural, optical, electrical, and chemical characteristics, indicating that partial A‐site Bi substitution (∼10% of the A‐layer) does not alter the examined properties. These findings not only expand the MAX phase family but also establish A‐site alloying as an effective route to tune relative phase stability in selected MAX phase structures to facilitate scalable processing.