Pb₂B₃O_5.5(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, Pb₂B₃O_5.5(OH)₂ (<b>1</b>), crystallizes in the noncentrosymmetric space group <i>Pnn</i>2 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 <b>1</b> produces a strong SHG response of ∼3× KDP (KH₂PO₄) which is type I phase-matchable. The first example of lead­(II) borate nitrate, namely, [Pb₃(B₃O₇)]­(NO₃) (<b>2</b>), crystallizes in space group <i>Pnma</i>, 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