Nonlinear optical response mechanism of noncentrosymmetric lead borate Pb6[B4O7(OH)2]3 with three crystallographically independent [B4O7(OH)2]4− chains

Abstract

The noncentrosymmetric lead borate Pb₆[B₄O₇(OH)₂]₃ has been synthesized by hydrothermal method with a yield of ∼85% based on Pb. It structurally features three crystallographically independent [B₄O₇(OH)₂]⁴⁻ helical chains with B₁₂O₂₉ fundamental building blocks (FBBs), which is unique in borate system. It has a wide transparency range from UV to NIR with the shortwave cut-off edge about 260 nm. The phase-matching second harmonic generation (SHG) response at 1064 nm fundamental wave was identified by the Kurtz–Perry method. First principles calculations on the band structure and partial density of states (PDOS) show that the title compound has a direct band gap determined by the interaction between lead and oxygen atoms. Stereochemically active lone pair lead cations and the BO₃ groups with π-conjugated system make main contribution to the nonlinear optical (NLO) effect according to the SHG-density method.