Phosphites: A Nonlinear Optical Materials System with a Wide Band Gap and Moderate Second-Harmonic Generation Effect

Centric Sc(HPO3)(H2PO3)(H2O) and Acentric Sc(H2PO3)3: Two Ultraviolet Scandium Phosphite Optical Crystals

Exploring ultraviolet (UV) nonlinear-optical (NLO) materials is significant for the conversion of a high-frequency laser. Two scandium phosphites, Sc(HPO3)(H2PO3)(H2O) and Sc(H2PO3)3, were successfully synthesized. Centric Sc(HPO3)(H2PO3)(H2O) exhibits a short UV cutoff edge (<200 nm) and a unique double-layer structure of [Sc2(HPO3)2(H2PO3)2(H2O)2]∞. The acentric Sc(H2PO3)3 exhibits a three-dimensional [Sc(H2PO3)3]∞ structure with a large band gap of 4.05 eV, and it demonstrates a moderately phase-matchable second-harmonic-generation response [0.60 × KDP (KH2PO4)] at 1064 nm. The crystal structures, optical properties, and theoretical calculations of the two compounds are discussed. This work will promote the exploration of new NLO phosphite materials.

C(NH2)3Cd(C2O4)Cl(H2O)·H2O and BaCd(C2O4)1.5Cl(H2O)2: Two Oxalate Chlorides Obtained by Chemical Scissors Strategy Exhibiting Low-Dimensional Structural Networks and Balanced Overall Optical Properties

Low-dimensional crystalline materials have attracted much attention due to their special physical and chemical properties. Herein, two new oxalate chlorides, C(NH2)3Cd(C2O4)Cl(H2O)·H2O and BaCd(C2O4)1.5Cl(H2O)2, were synthesized. C(NH2)3Cd(C2O4)Cl(H2O)·H2O presents the unique {[Cd(C2O4)Cl(H2O)]-}∞ zigzag chain, while BaCd(C2O4)1.5Cl(H2O)2 shows a novel {[Cd(C2O4)1.5Cl]2-}∞ layer. They showed large measured band gaps, which were 3.76 and 4.53 eV, respectively, and the latter was the largest band gap in the A-M-C2O4-X (A = Monovalent cationic or alkaline earth metals, X = F, Cl, Br, I) family. They exhibit a large calculated birefringence of 0.075 and 0.096 at 1064 nm, respectively. This study promotes the exploration of synthesizing low-dimensional crystalline materials with balanced overall optical performances by a chemical scissors strategy.

Pb2Cl2(HPO3)(H2O) and Pb3Br2(HPO3)2: Two Phosphite Halides with 3D Structural Networks and Enlarged Birefringence

Compounds constructed by mixed anions are widely favorable for their diverse crystal structures and physical performances. Here, two lead phosphite halides, Pb₂Cl₂(HPO₃)(H₂O) (1) and Pb₃Br₂(HPO₃)₂ (2), were obtained by facile hydrothermal reactions. Both compounds crystallize in the space group Pnma. 1 exhibits a 3D structure composed of HPO₃ units, PbCl₄O₃ pentagonal bipyramids, and PbO₃ pyramids. 2 also shows a 3D framework built by HPO₃ units, PbO₂Br₄ octahedron, PbO₅ and PbO₃Br₂ square pyramids. By introducing halogen anions to the lead phosphite system, 1 and 2 show enhanced birefringences of 0.083 and 0.072 at 1064 nm, respectively, compared with 0.023@1064 nm for Pb₂(HPO₃)₂. In this work, syntheses, crystal structures, optical properties, and theoretical calculation results have been studied in detail.