KNa2La2(BO3)3: a shortite-type lanthanide borate exhibiting strong nonlinear optical activity induced by isolated [BO3] triangles and distorted [LaO9] polyhedra

Abstract

The targeted synthesis of ultraviolet (UV) nonlinear optical (NLO) materials with strong second-harmonic generation (SHG) activity and wide transparency window remains a formidable challenge. Herein, a prospective rare-earth borate UV NLO crystal, KNa₂La₂(BO₃)₃, was extracted by spontaneous crystallization using a flux method. The title compound crystallizes into the acentric orthorhombic space group Amm2 (no. 38), analogous to those of shortite, and affords a three-dimensional framework built from planar [BO₃] triangles and distorted [LaO₉] motifs. The optical spectroscopy analyses demonstrate that KNa₂La₂(BO₃)₃ displays a short cutoff edge of 212 nm, accompanied by a wide band gap of 5.3 eV. Notably, it achieves an enhanced phase-matching SHG efficiency of 2.6 × KDP, comparable with those of the other NLO rare earth borate oxide crystals. Besides, theoretical analyses revealed that the optimized optical polarizability of KNa₂La₂(BO₃)₃ is predominantly ascribed to the cooperative effects of the isolated [BO₃] and distortive [LaO₉] structural units. This finding showcases a promising UV NLO crystal and sheds light on further exploration of rare earth metal-based NLO materials with fascinating performances.