Water Molecule-Driven Structural Evolution from Na2ZnCl4 to Na2ZnCl4·3H2O: A Route to Promising Deep-UV Nonlinear Optical Crystals

Exploring structure and property modulation strategies is crucial for developing functional crystal materials with superior properties. In this study, Na2ZnCl4·3H2O and Na2ZnCl4·2H2O crystals were synthesized using an aqueous solution evaporation method, and Na2ZnCl4 was synthesized using a high-temperature solution method. By varying the temperature, these compounds exhibit intriguing structural transformations and diverse structures following the removal of the water molecules. All these compounds possess a wide band gap, making them suitable for deep-ultraviolet (DUV) applications. Notably, the theoretical calculation results show that the noncentrosymmetric (NCS) compound Na2ZnCl4·3H2O exhibits a significant nonlinear optical (NLO) coefficient d33 = 1.73 pm/V in its largest direction, as determined by theoretical calculations, which is expected to be a candidate for additional periodic phase (APP) materials. This research provides significant implications for designing short-wavelength optical crystals and suggests that the incorporation of water molecules can significantly alter and enrich the crystal structures.