Ba1.09Pb0.91Be2(BO3)2F2: The First Pb-Containing Beryllium Borate Fluoride with Trigonal Prismatic PbO6 and 2D [Be3B3O6F3]infinity Layers

Ba1.09Pb0.91Be2(BO3)2F2 (BPBBF), a previously unreported lead-containing beryllium borate fluoride, has been successfully grown through a high-temperature flux method. Its structure is solved by single-crystal X-ray diffraction (SC-XRD), and it is optically characterized via infrared, Raman, UV-vis-IR transmission, and polarizing spectra as well. SC-XRD data suggests that it can be indexed by a trigonal unit cell (space group P3m1) with lattice parameters a = 4.7478(6) A, c = 8.3856(12) A, Z = 1, and V = 163.70(5) A. This material could be considered as a derivative of the Sr2Be2B2O7 (SBBO) structural motif. It consists of 2D [Be3B3O6F3]infinity layers in the crystallographic ab plane, with divalent Ba2+ or Pb2+ cations serving as spacers among the adjacent layers. Ba and Pb were found to adopt a disordered arrangement in the trigonal prismatic coordination within the BPBBF structural lattice, which is evidenced by both structural refinements against SC-XRD data and energy dispersive spectroscopy. The UV absorption edge (279.1 nm) and birefringence (Delta n = 0.054(a 546.1 nm) of BPBBF are confirmed by UV-vis-IR transmission and polarizing spectra, respectively. The discovery of this previous unreported SBBO-type material, BPBBF, along with other reported analogues such as BaMBe2(BO3)2F2 (M = Ca, Mg, and Cd), provide a prodigious example for tuning the bandgap, birefringence, and short UV absorption edge via simple chemical substitution.