Electronic, dynamical, and dielectric properties of lanthanum oxysulfide

The electronic band structure, lattice dynamics and dielectric properties of lanthanum oxysulfide (La2O2S) have been investigated within density functional theory. It is found that lanthanum oxysulfide is an indirect band-gap semiconductor. Density functional perturbation theory is employed to evaluate the phonon frequencies at the center of the Brillouin zone, the Born effective charge tensors, and the dielectric permittivity tensors. The results of a band-by-band decomposition of the Born effective charges are given and the origin of the anomalous Born effective charges and the character of the bonding are discussed. The present calculation predicts the frequencies of two IR-active modes that were not observed in experiment.