First-Principles Study of Electronic and Vibrational Properties of BaHfN2
Abstract
The transition metal nitride BaHfN2, which consists of weakly bonded neutral slabs of closed shell ions, has structural and chemical similarities to other layered nitrides which have impressive superconducting Tc when electron doped: AxHfNCl, AxZrNCl, AxTiNCl, with Tc= 25.5, 15.2 and 16.5 K, respectively for appropriate donor (A) concentrations x. These similarities suggest the possibility of BaHfN2 being another relatively high Tc nitride upon doping, with effects of structure and the role of specific transition metal ions yet to be understood. We report first-principles electronic structure calculations for stoichiometric BaHfN2 using density functional theory with plane-wave basis sets and separable dual-space Gaussian pseudopotentials. An indirect band gap of 0.8 eV was obtained and the lowest conduction band is primarily of Hf 5dxy character, similar to β-ZrNCl and α-TiNCl. The two N sites, one in the Hf layer and another one in the Ba layer, were found to have very anisotropic Born effective charges (BEC):deviations from the formal charge (-3) are opposite for the two sites, and opposite for the two orientations (in-plane, out of plane). LO-TO splittings and comparison of BECs and dielectric constant tensors to those of related compounds are discussed, and the effect of electron doping on the zone-center phonons is reported.