Positive and Negative Contribution from Lead-Oxygen Groups and Halogen Atoms to Birefringence: A First Principles...

Oxyhalides, containing oxygen and halogen atoms and combining the advantages of oxides and halides in geometry and optical response, have great potential in optical materials. In this study, the electronic structures and the optical properties of the Pb₃O₂X₂ (X = Cl, Br, I) compounds have been investigated using the first principles method. The results show that these compounds have birefringence at 0.076, 0.078, and 0.059 @ 1064 nm, respectively. And, the asymmetric stereochemical active lone pair electrons were found around lead atoms, which were confirmed by the projected density of states, the electronic localization functions, and the crystal orbitals. The contribution of atoms and polyhedra to birefringence was further evaluated using the Born effective charge. The results show that halogen atoms give negative contribution, and lead-oxygen polyhedra give positive contribution. The spin-orbit coupling effect is also investigated, and the downshift of the conduction band and variation in the valence band are found after relevant spin-orbit coupling (SOC), which leads to a reduction in the band gap and birefringence.