The crystal structure of lead tetraborate, PbO.2B2O3, has been refined using single-crystal X-ray diffraction data (Mo Ka radiation, A = 0.71069 A). Crystal data at room temperature: Mr = 362.43, orthorhombic, P21nm (C72v), a = 4.251 (2), b = 4.463(3), c = 10.860(3) A, V = 206.04 A3 with Z = 2, μ = 402.6 cm-1, Dx = 5.88 Mg m-3, F(000) = 316, final R = 0.022, wR = 0.025 over 655 reflections with I > 2.5σ(I). Atomic coordinates are in general agreement with those previously reported for the isostructural compound, SrO.2B2O3, by Perloff & Block [Ada Cryst. (1966), 20, 274-279]. All the borons are tetrahedrally coordinated with a three-dimensional network formed from O atoms that are common to either two or three tetrahedra. The tetrahe-dra show deformation because the B-O bonds involving the two-coordinated O atoms are much shorter than those involved with three-coordinated O atoms. The Pb atoms are situated in empty tunnels running along [010] left by the network of tetrahedra. The Pb atoms display a highly asymmetric distribution of Pb-O bonding, with the five shortest bonds covering the range 2.483 (5)-2.664 (5) A, being all situated to one side of the Pb atom. Preliminary investigations of the non-linear optical behaviour of lead tetraborate are also discussed. The results indicate that doping with barium should lead to a new non-linear optical material that is both phase-matchable and has a high optical non-linearity.
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