Chemical substitution towards a rare-earth borate ultraviolet NLO crystal exhibiting a strong SHG response†

The fabrication of nonlinear optical (NLO) materials that exhibit both a pronounced second harmonic generation (SHG) response and a broad ultraviolet (UV) transmission range remains a big challenge. In this study, we employed a flux method to extract a potential rare-earth borate UV NLO crystal, K_7.5Lu_2.5B₁₅O₃₀, by the chemical cosubstitution strategy. The title compound crystallizes in the trigonal space group R32 (no. 155) with a three-dimensional structural framework consisting of [B₅O₁₀] and [LuO₆] groups. Remarkably, K_7.5Lu_2.5B₁₅O₃₀ possesses a notably short cutoff edge of 198 nm and a wide band gap of 6.3 eV. Moreover, it demonstrates a strong phase-matched SHG efficiency of 1.2 × KDP, which represents an optimal balance between a strong NLO effect and a wide UV transmission range. Besides, theoretical calculations and structural analyses unveil that the NLO properties observed in K_7.5Lu_2.5B₁₅O₃₀ are primarily attributable to the synergistic effect of the [B₅O₁₀] groups and [LuO₆] octahedra. These findings indicate that K_7.5Lu_2.5B₁₅O₃₀ has potential applications as beryllium-free UV NLO materials.