From Centrosymmetry to Noncentrosymmetry: Precise Structural Regulation and Characterization on ZnHPO 3 ·2H 2 O Polymorphs.

Polymorphs of ZnHPO 3 ·2H 2 O with centrosymmetry ( Cmcm ) and noncentrosymmetry ( C 2) structures were prepared by modified solution evaporation and seed-crystal-induced secondary nucleation methods. In Cmcm -ZnHPO 3 ·2H 2 O, the zinc atoms are only octahedrally coordinated, while in C 2-ZnHPO 3 ·2H 2 O, they feature both tetrahedral and octahedral coordination. As a result, Cmcm -ZnHPO 3 ·2H 2 O features a 2D layered structure with lattice water molecules located in the interlayer space, while C 2-ZnHPO 3 ·2H 2 O features a 3D electroneutral framework of tfa topology connected by Zn(1)O 4 , Zn(2)O 6 , and HPO 3 units. The UV-visible diffuse reflectance spectra associated with Tauc's analyses give a direct bandgap of 4.24 and 4.33 eV for Cmcm -ZnHPO 3 ·2H 2 O and C 2-ZnHPO 3 ·2H 2 O, respectively. Moreover, C 2-ZnHPO 3 ·2H 2 O exhibits a weak second harmonic generation (SHG) response and a moderate birefringence for phase matching, indicating its potential as a nonlinear optical material. Detailed dipole moment calculation and analysis confirmed that the SHG response mainly derived from the HPO 3 pseudo-tetrahedra.