Expanding MAX phases: Discovery of a double-A-layer Ti₂Bi₂C with rhombohedral symmetry

The ongoing search for new exotic M_n+1AX_n (MAX) phases, including the double-A-layer class, continues to expand their structural and chemical diversity. In this study, we report the discovery of a 221 double-A-layer MAX phase, Ti₂Bi₂C, synthesized via reactive synthesis in a sealed quartz ampule. First-principles calculations based on density functional theory (DFT) predict the stability of this phase. Ti₂Bi₂C is the first known MAX phase to adopt a rhombohedral crystal structure (space group R-3m), marking a significant addition to the structural diversity of MAX phases. The structure and composition of Ti₂Bi₂C were confirmed through X-ray diffraction (XRD) and high-resolution scanning transmission electron microscopy (STEM). Furthermore, a dominant orientation relationship of (102)Bi//(001)Ti₂Bi₂C and [010]Bi//[010]Ti₂Bi₂C was identified between Ti₂Bi₂C and Bi. The successful synthesis of Ti₂Bi₂C not only expands the MAX-phase family but also provides valuable insights into the potential for new, complex structures within this class of materials.