Synthesis, Characterization, and Theoretical Studies of (Pb₄O)Pb₂B₆O₁₄: A New Lead(II) Borate with Isolated Oxygen-Centered Pb₄O Tetrahedra and Large Second Harmonic Generation Response

A new lead­(II) borate, (Pb₄O)­Pb₂B₆O₁₄, has been synthesized by low-temperature hydrothermal method. It crystallizes in the noncentrosymmetric polar space group P1 and structurally features a three-dimensional framework composed of paralleled spiral B₆O₁₄ chains connected by the Pb cations and isolated oxygen-centered Pb₄O tetrahedra. It exhibits a phase-matching powder second harmonic generation (SHG) effect of about 3.5 times that of KH₂PO₄ (KDP) at the 1064 nm fundamental wavelength. It also has optical band gap of 3.3 eV and high thermal stability up to 480 °C, which make it a promising candidate for nonlinear optical (NLO) material. In addition, electronic structure and NLO property are studied by the first-principles method, and the calculated NLO coefficients are consistent with the experimental observations. The important contributions of Pb₄O tetrahedra and the borate groups to the SHG response have been identified by the SHG-density method.