Lone electron-pair enhancement of SHG responses in eulytite-type compounds: AII3MIII(PO4)3 (A = Pb, M = Bi; A = Ba, M = Bi, La)

Abstract

A new eulytite-like orthophosphate Pb₃Bi(PO₄)₃, was synthesized by the high temperature solution growth (HTSG) method and characterized by X-ray single crystal diffraction, powder diffraction and electron microscopy. It crystallizes in the acentric cubic space group (I3d). The pure-phase powders of Pb₃Bi(PO₄)₃, Ba₃La(PO₄)₃ and Ba₃Bi(PO₄)₃ were also prepared by the high temperature solid reaction method. Second harmonic generation (SHG) measurements revealed that the optical response signals of Pb₃Bi(PO₄)₃ and Ba₃Bi(PO₄)₃ are about 3.0 and 0.5 times that of KH₂PO₄ (KDP), respectively, and both are phase-matchable. The SHG signal of Ba₃La(PO₄)₃ is very weak. Our studies indicate that the enhancement of the nonlinear optical response is mainly due to the contribution from the irregular PbO₆ and BiO₆ octahedra due to the 6s² lone electron-pair stereochemical activity in eulytite-like orthophosphates. Additionally, the calculated band structure and density of states also provide evidence that the charge transfers within irregular octahedral of Pb/BiO₆ make a significant contribution to the nonlinear optical response.