Electronic structure and optical properties of the nonlinear optical crystal Pb4O(BO3)2 by first-principles calculations

Pb4O(BO3)2 has a layered-type arrangement with optimally aligned BO3 triangles. The optical band gap is 3.317 eV obtained via the extrapolation method from the UV–vis–IR optical diffuse reflectance spectrum, consequently the absorption edge is about 374 nm. Density functional calculations using a generalized gradient approximation were utilized to investigate the electronic structures and optical properties of Pb4O(BO3)2. The calculated band structures show a direct gap of 2.608 eV, which is in agreement with the experimental optical band gap. A delocalized ππ bonding of BO3 triangles and the stereo-effect of the lone pair 6s2 of lead cations are studied in electron densities. The birefringence is about 0.039–0.061 with the wavelength larger than about 375 nm. The calculated second-order susceptibility d24=3.5 d36 (KDP) which is well consistent with the powder SHG intensity.

Graphical abstractThe density of state (DOS) show that the bottom of the valence bands is mainly derived from of the lone pair 6s2 of Pb2+, and the top of the valence band is attributed to the hybridization orbitals from B-O groups. Calculated electronic structures indicate that the BO3 group with typical delocalization π orbitals and strongly distorted lead oxygen polyhedra with highly asymmetric lobes on lead cations make a large SHG effect in Pb4O(BO3)2. Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Lone pair effect on Pb2+ and delocalization ππ orbital in BO3 group is studied. ► The combination of PbOn (n=3,4,5) and BO3 group makes Pb4O(BO3)2 a large SHG effect. ► Pb4O(BO3)2 is a direct gap material with the gap 2.608 eV by the ab initio method. ► The calculated birefringence is about 0.039–0.061 with the wavelength of about 375 nm. ► The second-order susceptibility d24=3.5d36 (KDP) in accordance with the experimental result.