Raman active phonon modes and heat capacities of Ti2AlC and Cr2AlC ceramics:: first-principles and experimental investigations -: art. no. 101902

Raman active phonon frequencies and corresponding vibrational eigenmodes are reported for two M2AlC ceramics, Ti2AlC and Cr2AlC, by means of first-principles calculations. The theoretical modes are approved by the experimental Raman spectrum for Ti2AlC. Compared with that of the Ti3SiC2 counterpart, the number of Raman active modes,is definitely determined by the chemical formula of transition-metal carbide layers; for example, TiC0.67 in Ti3SiC2 and TiC0.5 in M2AlC. All active modes originate from stretching and bending of Al-M-C covalent bond chains, whereas the vibration of M-C bond localized inside the M6C Octahedra is not Raman active. The differences in vibration characteristics between Ti2AlC and Cr2AlC, are discussed in terms of atomic force constants. By including the phonon and electron contributions, trends in heat capacities as a function of temperature were illustrated and compared. (c) 2005 American Institute of Physics.