From Centrosymmetry to Noncentrosymmetry: Precise Structural Regulation and Characterization on ZnHPO₃?2H₂O Polymorphs

Polymorphs of ZnHPO3 & BULL;2H(2)Owith centrosymmetry(Cmcm) and noncentrosymmetry (C2)structures were prepared by modified solution evaporation and seed-crystal-inducedsecondary nucleation methods. In Cmcm-ZnHPO3 & BULL;2H(2)O, the zinc atoms are only octahedrally coordinated,while in C2-ZnHPO3 & BULL;2H(2)O, they feature both tetrahedral and octahedral coordination. Asa result, Cmcm-ZnHPO3 & BULL;2H(2)O features a 2D layered structure with lattice water molecules locatedin the interlayer space, while C2-ZnHPO3 & BULL;2H(2)O features a 3D electroneutral framework of tfa topology connected by Zn(1)O-4, Zn(2)O-6, and HPO3 units. The UV-visible diffusereflectance spectra associated with Tauc's analyses give adirect bandgap of 4.24 and 4.33 eV for Cmcm-ZnHPO3 & BULL;2H(2)O and C2-ZnHPO3 & BULL;2H(2)O, respectively. Moreover, C2-ZnHPO3 & BULL;2H(2)O exhibits a weak second harmonicgeneration (SHG) response and a moderate birefringence for phase matching,indicating its potential as a nonlinear optical material. Detaileddipole moment calculation and analysis confirmed that the SHG responsemainly derived from the HPO3 pseudo-tetrahedra. Polymorphs of ZnHPO3 & BULL;2H(2)O withcentrosymmetry (Cmcm) and noncentrosymmetry (C2) structures were prepared by modified solution evaporationand seed-crystal-induced secondary nucleation methods.