Structural Origin of Very Large Second-Harmonic Generation of a Layered Perovskite, Na_0.5Bi_2.5Nb₂O₉

A series of rare-earth metal cation-doped n = 2 Aurivillius-type layered perovskites, Na0.45Bi2.5RE0.05Nb2O9 (RE = Eu, Sm, and Pr), have been synthesized by standard solid-state reactions. Single crystals of the model compound Na0.5Bi2.5Nb2O9 have been grown at 1250 °C to redetermine the structure to explain the origin of the optical properties for the reported compounds. Detailed structural analyses reveal that Na0.5Bi2.5Nb2O9 and Na0.45Bi2.5RE0.05Nb2O9 are isostructural to each other and crystallize in the noncentrosymmetric polar orthorhombic space group Cmc21 (no. 36). Powder second-harmonic generation (SHG) measurements indicate that while Na0.5Bi2.5Nb2O9 reveals a very large SHG efficiency of 9 times that of KH2PO4, that of Na0.45Bi2.5RE0.05Nb2O9 decreases abruptly upon doping. A careful structural analysis suggests that the net moment arising from the alignment of polyhedra of an asymmetric lone pair cation, Bi3+, is crucial for the very large SHG efficiency of Na0.5Bi2.5Nb2O9. Complete characterizations including infrared and ultraviolet–visible spectroscopy analysis, thermogravimetric analysis, energy-dispersive analysis by X-ray, and photoluminescence measurements along with electronic structure calculations for the title materials are presented.