From ZnF₂ to ZnF₂(H₂O)₄: Partial Substitution Achieves Structural Transformation and Nonlinear Optical Activity while Keeping Short Ultraviolet Cutoff Edge

Exploring nonlinear optical (NLO) materials with short ultraviolet cutoff edges are significant for developing an all-solid-state laser. Here, a noncentrosymmetric zinc fluoride hydrate, ZnF2(H2O)(4), was synthesized by a hydrothermal method. It crystallizes in the polar space group of Pca2(1). The compound consists of the central Zn2+ combined with F- and coordination water to form the [ZnF2(H2O)(4)] octahedra, and each octahedron is isolated from each other to form a 0-dimensional structure. As an acentric compound, ZnF2(H2O)(4) shows a phase-matchable second-harmonic-generation (SHG) activity with an intensity about 0.5 times that of KH2PO4. More attractively, it also shows a short ultraviolet cutoff edge below 200 nm, which is rare in reported halide hydrate systems. Interestingly, from ZnF2 to ZnF2(H2O)(4), the partial substitution of the coordinated F atoms by H2O molecules leads to the structural transformation from centric to acentric with SHG activity off to on. Structural analyses, NLO activity, and theoretical calculations are presented in this work.