Structural Origin of Very Large Second-Harmonic Generation of a Layered Perovskite, Na0.5Bi2.5Nb₂O₉

A series of rare-earth metal cation-doped n = 2 Aurivillius-type layered perovskites, Na 0.45 Bi 2.5 RE 0.05 Nb 2 O 9 (RE = Eu, Sm, and Pr), have been synthesized by standard solid-state reactions. Single crystals of the model compound Na 0.5 Bi 2.5 Nb 2 O 9 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 Na 0.5 Bi 2.5 Nb 2 O 9 and Na 0.45 Bi 2.5 RE 0.05 Nb 2 O 9 are isostructural to each other and crystallize in the noncentrosymmetric polar orthorhombic space group Cmc 2 1 (no. 36). Powder second-harmonic generation (SHG) measurements indicate that while Na 0.5 Bi 2.5 Nb 2 O 9 reveals a very large SHG efficiency of 9 times that of KH 2 PO 4, that of Na 0.45 Bi 2.5 RE 0.05 Nb 2 O 9 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, Bi 3+, is crucial for the very large SHG efficiency of Na 0.5 Bi 2.5 Nb 2 O 9 . 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.