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

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 (I (4) over bar 3d). 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 6s(2) 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.