Structural Modulation of Nitrate Group with Cations to Affect SHG Responses in RE(OH)2NO3 (RE = La, Y, and Gd): New Polar Materials with Large NLO Effect after Adjusting pH Values of Reaction Systems

Abstract

A series of rare-earth hydroxide nitrate crystals (La(OH)2NO3, Y(OH)2NO3, and Gd(OH)2NO3) have been synthesized through adjusting pH values of reaction systems under the subcritical hydrothermal condition. All the titled compounds were isostructural with the noncentrosymmetric space group P21 (No. 4) with layer structure, containing [REO9] (RE = La,Y, and Gd) polyhedra in each layer. The polyhedra were stacked on top of each other and further connected with zigzag strings of edge sharing to form infinite corrugated sheets that parallel to the a–c plane. The [NO3] groups that presented two different orientation (A and B) project into the space between the layers. In this study, the angle θ between two different orientation [NO3] groups was defined. With the decrease of ionic radii from La3+, Gd3+ to Y3+, the θ was increased, which led to different second harmonic generation (SHG) effects on lanthanide hydroxide nitrates. The powder SHG measurements revealed that La(OH)2NO3, Gd(OH)2NO3, and Y(OH)2NO3 were p...