Structure and optical non-linearity of PbO.2B₂O₃

The crystal structure of lead tetraborate, PbO.2B₂O₃, has been refined using single-crystal X-ray diffraction data (Mo Kα radiation, λ = 0.71069 Å). Crystal data at room temperature: M_r = 362.43, orthorhombic, P2₁nm (C⁷_2v), a = 4.251 (2), b = 4.463 (3), c = 10.860 (3) Å, V = 206.04 ų with Z = 2, μ = 402.6 cm⁻¹, D_x = 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 [Acta 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 tetrahedra 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) Å, 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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