From CaBaMF to KBaMF (M = Zr, Hf): Heterovalent Cation-Substitution-Induced Symmetry Break and Nonlinear-Optical Activity.

Designing short-wavelength nonlinear-optical (NLO) crystals is of vital importance for laser applications. Here, the combination of alkaline-earth metals, d transition metals, and F atom has generated two new and isostructural fluorides, CaBaZrF (CBZF) and CaBaHfF (CBHF), which adopt centrosymmetric space group 4/. Taking CBZF and CBHF as the parents, two new fluorides, KBaZrF (KBZF) and KBaHfF (KBHF), with an 2 polar structure were obtained via a heterovalent cation substitution strategy. All four compounds feature ZrF-dodecahedra-built {[ZrF]} chains and show short ultraviolet cutoff edges (<200 nm). KBZF and KBHF show phase-matchable behavior with moderate second-harmonic-generation responses [0.6 and 0.35 × KHPO (KDP)] under 1064 nm laser radiation. This work enriches fluorides as promising short-wavelength NLO materials.