K6NaCaRe2(B5O10)3 (Re = Y, Lu): Two NLO Compounds Exhibiting a Short Absorption Edge in the A7–xA’xAeRe2(B5O10)3 (x = 0, 1) Family

The discovery and synthesis of new NLO materials in the ultraviolet (UV) region are crucial to developing laser technology. The chemical substitution strategy is an effective pathway to design potential UV or DUV NLO crystals. Herein, two new compounds, K6NaCaY2(B5O10)3 and K6NaCaLu2(B5O10)3, have been synthesized using KB5O8·4H2O as the template. Their crystal structures feature a three-dimensional (3D) [Re2(B5O10)3]∞ framework consisting of [B5O10] groups and [ReO6] octahedra, and the remainder of metal cations fill the void to control charge balance and maintain the stability of the crystal structure. Both K6NaCaY2(B5O10)3 and K6NaCaLu2(B5O10)3 crystallize in the acentric space group R32, so they exhibit an SHG effect, and the value is ∼0.5 × KDP (KH2PO4). Meanwhile, they show a phase-matching ability. Due to the participation of alkali metals, alkaline earth metal, and [B5O10] unit, the title compounds possess short absorption edges of ∼230 nm. In addition, they have high thermal stability and can be stable up to 931 °C. These results confirm the effectiveness of the chemical substitution strategy for designing NLO compounds, and then the two compounds can potentially serve as good UV NLO materials.