Structure of a new bulk Ti 5Al 2C 3 MAX phase produced by the topotactic transformation of Ti 2AlC

Abstract

Upon annealing cold-pressed Ti 2AlC, -325 mesh powders, at 1500°C for 8h in argon, the resulting partially sintered sample contained 43(±2)wt.% of the layered ternary carbide Ti 5Al 2C 3. Herein, the X-ray powder diffraction pattern of Ti 5Al 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 P3m1) with a unit cell consisting of 3 formula units and cell parameters of a=3.064Å, c=48.23Å. The lattice parameters determined through first principles calculations agree reasonably well with the experimentally determined values. At 147.1GPa, the calculated bulk modulus falls between the bulk moduli of Ti 2AlC and Ti 3AlC 2. The transformation from Ti 2AlC to Ti 5Al 2C 3 is topotactic. © 2012 Elsevier Ltd.