A theoretical study of dielectric tensor, Born effective charges, LO-TO splitting and infrared response of Al4SiC4 and Al4Si2C5

The optical dielectric functions and dielectric constants are obtained using hybrid density functional HSE06 for Al4SiC4 and Al4Si2C5 structures. The LO-TO splittings, Born effective charges and phonon eigenvectors are calculated for them by density functional perturbation theory (DFPT). The dynamic charges of Al atoms are found to be close to their nominal valence charges (+3). Meanwhile, Si and most C atoms carry very different dynamic charges to their nominal ionic charges (+4 and −4). The static dielectric constants are found to be εxx0=εyy0=8.32 (6.31) and εzz0=9.80 (7.47) for Al4SiC4 (Al4Si2C5). The mode-effective charge vectors and mode-dissociated static dielectric constants are also analyzed for each IR-active phonon mode. The infrared complex dielectric functions and reflectivity are computed in [010] and [001] directions for two structures, and they both show strong dependence on the polarization direction.