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.