A new nonlinear optical borate, Na0.5Pb2[B5O9](OH)1.5 · 0.5H2O, from the orthorhombic hilgardite family

Abstract

A new polar representative of the hilgardite structural family, Na0.5Pb2[B5O9](OH)1.5 · 0.5H2O (space group Pnn2), is synthesized under hydrothermal conditions. The crystal structure of the compound synthesized is similar to the structure of the previously studied polar compound Na0.5Pb2[B5O9]Cl(OH)0.5 and is intermediate between the latter compound and the centrosymmetric hydrate Pb2[B5O9](OH) · 0.5H2O. The additional peak revealed in the electron density in the vicinity of the Pb(1) atom is attributed to the stereochemically active lone electron pair of this atom. The lone electron pair is oriented toward the two most distant oxygen atoms involved in the Pb(1) coordination environment, namely, O(7) and O(2), which are located in a boron-oxygen framework layer in the ac plane, not toward the (00z) channel occupied by water molecules. The nonequivalence in the stereochemical activity of two lead atoms [Pb(1) > Pb(2)] is similar to that observed in the nonlinear optical borate Pb2[B4O5(OH)4](OH)2 · H2O related to BiB3O6.