KBa3M2F14Cl (M = Zr, Hf): novel short-wavelength mixed metal halides with the largest second-harmonic generation responses contributed by mixed functional moieties

The development of short-wavelength nonlinear optical (NLO) materials is indispensable and urgently required for further applications. Halides have been disregarded as potential NLO materials with deep-ultraviolet (DUV) cutoff edges due to their weak second-harmonic generation (SHG) response and poor birefringence. Here, two novel and isostructural halides, KBa₃M₂F₁₄Cl (M = Zr (KBZFC), Hf (KBHFC)), possess structures that are formed by isolated MF₇ monocapped triangular prisms and dissociative K⁺, Ba²⁺, and Cl⁻ ions. Compared with reported metal halides that are transparent to the DUV region, KBZFC and KBHFC possess the strongest SHG responses (approximately 1, 0.9 × KH₂PO₄), which are contributed by the synergistic effect of MF₇ (M = Zr, Hf) groups, Ba²⁺ cations, and Cl⁻ ions. The zero-dimensional structures favour sufficient birefringences (0.12, 0.10 @ 1064 nm) for phase-matchable (PM) behaviours. The discovery of KBZFC and KBHFC showcases the potential of NLO mixed metal halides transparent to the DUV region.