Polymorphism of newly discovered Ti4GaC3: A first-principles study

The newly discovered MAX phase, Ti4GaC3, can exist in one of three polymorphs, alpha, beta and gamma, all with the space group P63/mmc. The Ti and Ga (underlined) atomic arrangements are, respectively, ABABACBCBC, ACACACACAC and ABCBACBABC. Herein first-principles calculations are used to investigate the phase stabilities, electronic structures, elastic properties and compressibilities of the three polymorphs. Since the alpha- to gamma-phase transition only involves shuffling of the A-atoms, it occurs much more easily than those to beta-Ti4GaC3 despite the fact that the latter is thermodynamically less stable than gamma-Ti4GaC3. For alpha-Ti4GaC3, the total density of states, TDOS, around the Fermi energy, E-f, lies in a local minimum; for the two other polymorphs, the TDOS is near a local minimum. The electrons occupy all the bonding states for alpha-Ti4GaC3, but the bonding states are partially occupied for both beta- and gamma-Ti4GaC3. Both bond stiffness and bond angle play an important role in the compressibility. In general, with increasing pressure, all the bonds become shorter, and the rate of increase in bond stiffness also increases. The bulk moduli of the alpha-, beta- and gamma-polymorphs were calculated to be 178, 174 and 169 GPa, respectively. The corresponding theoretical densities are 5.14, 5.12 and 5.11 g cm(-3). (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.