Anomalous effective charges and far-IR optical absorption ofAl2Ru from first principles

Abstract

For the orthorhombic intermetallic semiconductor ${\mathrm{Al}}_2$Ru, the band structure, valence charge density, zone-center optical-phonon frequencies, and Born effective-charge and electronic dielectric tensors are calculated using variational density-functional perturbation theory with ab initio pseudopotentials and a plane-wave basis set. Good agreement is obtained with recent measurements on polycrystalline samples, which showed anomalously strong far-IR absorption by optical phonons, while analysis of the valence charge density shows that the static ionic charges of Al and Ru are negligible. Hybridization is proposed as the single origin both of the semiconducting gap and the anomalous Born effective charges. Analogous behavior is expected in related compounds such as NiSnZr, PbTe, skutterudites, and Al-transition-metal quasicrystals.