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 Pn (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 (λcutoff < 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.