K₁₅La₇(BO₃)₁₂: A KBBF-Like Nonlinear-Optical Material with a Short Cutoff Edge and a Strong Second-Harmonic-Generation Response

Nonlinear-optical (NLO) crystals capable of expanding the spectral regions of solid-state lasers are urgently needed by many high-technological applications. However, in the deep-ultraviolet (DUV) region, there are few crystals available, except KBe₂BO₃F₂ (KBBF), which still suffers from the layered growth habit and high toxicity for raw materials. Herein, through the use of rare-earth (RE) La³⁺ to substitute Be²⁺ cations, a novel KBBF-like borate without beryllium has been successfully synthesized. K₁₅La₇(BO₃)₁₂ (KLBO) crystallizes in the noncentrosymmetric space group <i>Pn</i> (No. 7) and features a three-dimensional framework composed of [La₁₂B₇O₇₈]_∞ layers, which are further linked together through interlayer [BO₃] groups. The property measurements show that KLBO not only possesses an ideal NLO performance including a short UV-cutoff edge (λ_cutoff < 190 nm) and a remarkable second-harmonic-generation response (∼2.5 × KH₂PO₄) with phase-matching ability, but also overcomes the layered habit of KBBF. Besides, theoretical calculations indicate that the good NLO performance is primarily attributed to the BO₃ units and LaO₈ polyhedra. These results demonstrate that KLBO has the potential to be a UV or DUV NLO crystal.