Structure and optical non-linearity of PbO.2B(2)O(3)

The crystal structure of lead tetraborate, PbO.2B(2)O(3), has been refined using single-crystal X-ray diffraction data (Mo K alpha radiation, lambda = 0.71069 Angstrom). Crystal data at room temperature: M(r) = 362.43, orthorhombic, P2(1)nm (C-2 nu(7)), a = 4.251(2), b = 4.463(3), c = 10.860(3) Angstrom, V = 206.04 Angstrom(3) with Z = 2, mu = 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 > 2.5 sigma(I). Atomic coordinates are in general agreement with those previously reported for the isostructural compound, SrO.2B(2)O(3), 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) Angstrom, 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.