K3Yb2(BO3)3 and Rb3Yb2(BO3)3: Two Rare-Earth Borate Ultraviolet Nonlinear Optical Crystals

By incorporating rare-earth and alkali metals into borates, two noncentrosymmetric borate crystals, K3Yb2(BO3)3 and Rb3Yb2(BO3)3, were successfully obtained via a high-temperature solution method. Both K3Yb2(BO3)3 and Rb3Yb2(BO3)3 deliver second harmonic generation (SHG) responses of 0.39 and 0.22 × KH2PO4 (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 K3Yb2(BO3)3 and Rb3Yb2(BO3)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 K3Yb2(BO3)3 and Rb3Yb2(BO3)3 originates from the synergy between the BO3 units and the rare-earth polyhedra.