Crystal structure of the lead bromo-borate Pb 2 [B 5 O 9 ]Br from precision single-crystal X-ray diffraction data and th
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CTURE OF INORGANIC COMPOUNDS
Crystal Structure of the Lead Bromo-Borate Pb2[B5O9]Br from Precision Single-Crystal X-ray Diffraction Data and the Problem of Optical Nonlinearity of Hilgardites E. L. Belokoneva, A. G. Al-Ama, S. Yu. Stefanovich, and P. A. Plachinda Department of Crystallography and Crystal Chemistry, Faculty of Geology, Lomonosov Moscow State University, Leninskie gory, Moscow, 119992 Russia e-mail: [email protected] Received May 18, 2006
Abstract—The structural features responsible for the high nonlinear optical activity of crystals of the hilgardite-like anhydrous bromo-borate Pb2[B5O9]Br (space group Pnn2) are analyzed with the use of data obtained from two (precision and conventional) single-crystal X-ray diffraction experiments. The electron-density peaks associated with the stereochemically active lone electron pair are located in free space at two equally probable positions in the vicinity of the Pb(2) atom on the side of the two most distant atoms, Br(1) and Br(2). The stereochemical activity of two nonequivalent lead atoms in the crystal structure increases upon changing over from the Pb(1) atom to the Pb(2) atom. This is in agreement with the behavior of the lone electron pairs in the hilgardite-like hydrated borate Na0.5Pb2[B5O9](OH)1.5 · 0.5H2O and another nonlinear optical borate, namely, Pb2[B4O5(OH)4](OH)2 · H2O, which is related to the compound BiB3O6. The most pronounced nonlinear optical properties of the lead bromo-borate Pb2[B5O9]Br as compared to other orthorhombic hilgardites and lead borates are associated with the presence of highly polarized Pb–Br bonds. In this case, the electron density of the lone electron pair of the Pb(2) atom enhances the polarization effect. PACS numbers: 61.10.Nz, 61.66.Fn DOI: 10.1134/S1063774507050070
INTRODUCTION At present, borate systems have been considered very promising for use in the design of new nonlinear optical materials in the form of single crystals, glasses, and glass composites [1]. The scientific principles underlying the design of these materials have been provided by recent crystal chemical investigations, which have revealed new crystals with a high optical nonlinearity in borate and mixed haloid borate systems. It has been established that the record-high optical nonlinearity is exhibited by a number of lead-containing and bismuth-containing borates, such as the bismuth triborate BiB3O9 [2] and the structurally related hydrated lead diborate Pb2[B4O5(éç)4](éç)2 · ç2é [3], as well as the lead haloid borates belonging to the hilgardite structure type, including anhydrous bromo-pentoborate and chloro-pentaborate of the general formula Pb2[B5O9]Hal (where Hal = Cl or Br) [4, 5]. All these compounds are characterized by a substantially higher optical nonlinearity as compared to that exhibited by well-known nonlinear optical crystals, such as barium diborate, lithium triborate, lithium tetraborate, and lead tetraborate. The unusually high optical nonlinearity of lead-containing and bismuth-containing borates can be
associated with bo
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