Deep-Ultraviolet Transparent Zr/Hf Fluorides with Second-Harmonic Generation Effects Induced by Alkali-Metal Modulation

Zirconium/hafnium fluorides have recently garnered considerable interest as potential optical materials, owing to their short ultraviolet (UV) cutoff edges. Transforming centrosymmetric (CS) structures into noncentrosymmetric (NCS) analogues through cationic regulation has proven to be an effective approach for the development of nonlinear-optical (NLO) materials. Herein, various Zr/Hf-based fluorides with different alkali-metal cations, including K8M5F28(H2O) (M = Zr(1), Hf(2)), CsNaHfF6 (3), and LiK10M6F35(H2O)2 (M = Zr(4), Hf(5)), were synthesized. 1–3 belong to centrosymmetric space groups, whereas 4 and 5, incorporating the Li+ cation with a small radius, exhibit NCS crystal structures. 1 and 2 feature chain-like anionic structures. 3 presents a zero-dimensional (0D) anionic structure constructed from isolated [HfF6]2– octahedra. Compounds 4 and 5 adopt three-dimensional (3D) anionic networks. The introduction of Li+ enhances structural distortion and dipole alignment, resulting in phase-matchable (PM) second-harmonic-generation (SHG) responses. Notably, all compounds exhibit UV absorption edges below 200 nm, confirming superior UV transparency. Overall, these results indicate that alkali-metal regulation is an efficient approach to trigger symmetry breaking while concurrently enabling SHG activity and maintaining short UV absorption edges.