K ₃ Yb ₂ (BO ₃ ) ₃ and Rb ₃ Yb ₂ (BO ₃ ) ₃ : Two Rare-Earth Borate Ultraviolet Nonlinear Optical Crystals

By incorporating rare-earth and alkali metals into borates, two noncentrosymmetric borate crystals, K 3 Yb 2 (BO 3 ) 3 and Rb 3 Yb 2 (BO 3 ) 3, were successfully obtained via a high-temperature solution method. Both K 3 Yb 2 (BO 3 ) 3 and Rb 3 Yb 2 (BO 3 ) 3 deliver second harmonic generation (SHG) responses of 0.39 and 0.22 × KH 2 PO 4 (KDP) and moderate birefringence of 0.029 and 0.035 at 546 nm. The UV cutoff edges of the two title crystals are 237 and 240 nm, which correspond to band gaps of 4.68 and 4.61 eV. Both K 3 Yb 2 (BO 3 ) 3 and Rb 3 Yb 2 (BO 3 ) 3 demonstrate high thermal stability, with decomposition temperatures of 942 and 885 °C, respectively. In addition, to better demonstrate the relationship between microscopic groups and macroscopic optical performance, the first-principles calculations were carried out, which revealed that the optical polarizability of K 3 Yb 2 (BO 3 ) 3 and Rb 3 Yb 2 (BO 3 ) 3 originates from the synergy between the BO 3 units and the rare-earth polyhedra.