Design and Synthesis of a Rare Earth Borate Ultraviolet Nonlinear Optical Crystal K ₃ Lu ₂ (BO ₃ ) ₃

Borate is considered to be a promising nonlinear optical (NLO) material due to its structural diversity, good ultraviolet transmittance, and excellent NLO properties. In this paper, a novel noncentrosymmetric rare earth borate crystal K 3 Lu 2 (BO 3 ) 3 (KLBO) was successfully synthesized by the spontaneous crystallization method in a K 2 O–Lu 2 O 3 –B 2 O 3 ternary system. Its crystal structure belongs to the orthorhombic Pna2 1 space group, which is composed of a three-dimensional skeleton composed of [BO 3 ] triangular groups and [LuO 6 ]/[LuO 7 ] polyhedra. The second harmonic generation (SHG) effect of KLBO under 1064 nm laser excitation is about 0.7 times that of the standard compound KDP. Notably, KLBO exhibits a short UV cutoff of 215 nm, a large bandgap of 5.64 eV, and a moderate birefringence of 0.037 at 1064 nm. Additionally, first-principles calculations reveal that the NLO characteristics are predominantly determined by the fundamental building blocks [BO 3 ] plane triangles and distorted [LuO 6 ] octahedron. These findings make KLBO show good application prospects in the research field of ultraviolet (UV) NLO crystal materials.