Structure of a new bulk Ti_5Al_2C_3 MAX phase produced by the topotactic transformation of Ti_2AlC

摘要

Upon annealing cold-pressed Ti_2AlC, -325 mesh powders, at 1500 °C for 8 hin argon, the resulting partially sintered sample contained 43 (±2) wt.% of the layered ternary carbide Ti_5 Al_2C_3. Herein, the X-ray powder diffraction pattern of Ti_5 Al_2C_3 is reported for the first time and its structure and stoichiometry are confirmed through high-resolution transmission electron microscopy. This phase has a trigonal structure (space group P3ml) with a unit cell consisting of 3 formula units and cell parameters of a = 3.064 A, c = 48.23 A. 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 Ti_2AlC and Ti_3Al_C_2. The transformation from Ti_2AlC to Ti_5 Al_2C_3 is topotactic.