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

The noncentrosymmetric lead borate Pb-6[B4O7(OH)(2)](3) has been synthesized by hydrothermal method with a yield of similar to 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 pi-conjugated system make main contribution to the nonlinear optical (NLO) effect according to the SHG-density method.