Raman active phonon modes and heat capacities of Ti_2AlC and Cr_2AlC ceramics: first-principles and experimental investigations

摘要

Raman active phonon frequencies and corresponding vibrational eigenmodes are reported for two M_2AlC ceramics, Ti_2AlC and Cr_2AlC, by means of first-principles calculations. The theoretical modes are approved by the experimental Raman spectrum for Ti_2AlC. Compared with that of the Ti_2SiC_2 counterpart, the number of Raman active modes is definitely determined by the chemical formula of transition-metal carbide layers; for example, TiC_(0.67) in Ti_3SiC_2 and TiC_(0.5) in M_2AlC. 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 M_6C octahedra is not Raman active. The differences in vibration characteristics between Ti_2AlC and Cr_2AlC 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.