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.