Pb2B3O5.5(OH)2 and [Pb3(B3O7)](NO3): Facile Syntheses of New Lead(II) Borates by Simply Changing the pH Values of the Reaction Systems

Using lead metaborate as starting material, by only adjusting pH values of the reaction systems, a series of lead(II) borates were obtained in high yields. The new polar material, namely, Pb2B3O5.5(OH)(2) (1), crystallizes in the noncentrosymmetric space group Pnn2 of the orthorhombic system. Its structure features a novel three-dimensional (3D) anionic network with large 14 member rings (MRs) tunnels composed of unique one-dimensional (1D) chains and dimeric B2O7 fragments, both of which are built up from solely BO4 tetrahedra, and the Pb2+ cations are located at the above 14-MRs tunnels. The synergistic effect of the stereoactive lone-pairs on Pb2+ cations in 1 produces a strong SHG response of similar to 3X KDP (KH2PO4) which is type I phase-matchable. The first example of lead(II) borate nitrate, namely, [Pb-3(B3O7)](NO3) (2), crystallizes in space group Pnma, and its structures features a 3D lead(II) borate cationic network structure in which (B3O7)(5-) anions are bridged by lead(II) cations, the nitrate anions are isolated, and located at the small voids of the cationic network. Thermal stability and optical properties as well as theoretical calculations based on density functional theory (DFT) methods were also performed.