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

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 phase-matchable in the visible and UV region and feature large SHG responsed that are approximately 5, 5.5, and 5.6 times that of KH₂PO₄ (KDP), respectively. Additionally, these title compounds had wide transparent regions from UV to near IR and larger birefringence, suggesting that these crystals were promising UV NLO materials. And their electronic structures and optical properties were calculated based on DFT methods.