Enhanced Born Charges in III-VII, IV-VII2, and V-VII3 Compounds

We report electronic-structure and lattice dynamics calculations on selected III-VII, IV-VII2, and V-VII3 compounds. The common characteristic of these largely ionic compounds is that their outmost cation-s states are fully occupied and thus the conduction-band states are derived from the more spatially extended cation-p states, resulting in significant cross-band-gap hybridization, which enhances Born effective charges substantially. The large Born charges cause large splitting between longitudinal and transverse optic phonon modes and large static dielectric constants resulting mostly from the lattice contribution. This can lead to effective screening of defects and impurities that would otherwise be strong carrier traps and recombination centers and may therefore have positive effects on the carrier transport properties in radiation detectors based on these soft-lattice halides.