Structure of a new bulk Ti5Al2C3 MAX phase produced by the topotactic transformation of Ti2AlC

Upon annealing cold-pressed Ti2AlC, -325 mesh powders, at 1500 °C for 8 h in argon, the resulting partially sintered sample contained 43(±2) wt.% of the layered ternary carbide Ti5Al2C3. Herein, the X-ray powder diffraction pattern of Ti5Al2C3 is reported for the first time. Its structure and stoichiometry are confirmed through high-resolution transmission electron microscopy. This phase has a trigonal structure (space group P3) with a unit cell consisting of 3 formula units and cell parameters of a = 3.064 Å, c = 48.23 Å. The lattice parameters determined through first principles calculations agree reasonably well with the experimentally determined values. At 147.1 GPa, the calculated bulk modulus falls between the bulk moduli of the MAX phases Ti2AlC and Ti3AlC2. The transformation from Ti2AlC to Ti5Al2C3 is topotactic.