Lead–strontium borate halides with hilgardite-type structure and their SHG properties

Lead-containing members of the hilgardite family of borate halides, M 2B 5O 9 X ( M=Ca, Sr, Ba, Pb; X=Cl, Br) exhibit an abnormally large nonlinear optical response. In order to establish any underlying crystal-chemical rationale for this we have carried out detailed crystallographic characterisations of the representative members of this series, Sr 2B 5O 9Cl and Pb 2B 5O 9Cl, using powder neutron diffraction. Both adopt the orthorhombic hilgardite structure type, in space group Pnn2. The full solid solution range Pb 2− x Sr x B 5O 9Cl has also been prepared, and studied by X-ray Rietveld and second harmonic generation (SHG) measurements. The SHG response shows a linear decrease with increasing x, whereas the unit cell shows an increasing orthorhombic distortion. However, the crystallographic results suggest no significant or systematic changes in the nature of the borate sublattice between Sr 2B 5O 9Cl and Pb 2B 5O 9Cl. We conclude that the enhanced SHG response in Pb-containing hilgardites is due predominantly to the presence of the polarizable nature of Pb 2+, in particular the presence of a stereochemically active lone pair. Sr 2B 5O 9Cl, Pb 2B 5O 9Cl and the solid solution range Pb 2− x Sr x B 5O 9Cl has been prepared, and studied by powder neutron and X-ray diffraction and SHG measurements.