The development of high-performance deep-ultraviolet (deep-UV) second-order nonlinear optical (NLO) crystals that simultaneously combine noncentrosymmetric crystal structure, broad transparency, strong second-harmonic generation (SHG) activity, and high thermal and chemical stability remains a formidable challenge due to intrinsic trade-offs among these properties. Herein, we present a multicomponent structural design strategy that enables the rational synthesis of a new KBe2BO3F2 (KBBF)-like rubidium lanthanum sulfate, RbLa(SO4)2. In this crystal, the highly polarizable LaO10 unit substitutes for BeO3F to enhance polarization, the non-π-conjugated SO4 group replaces BO3 to maximize the SHG response, and the alkali metal Rb+ cation improves optical transparency and structural robustness. Consequently, RbLa(SO4)2 exhibits excellent deep-UV transparency, featuring an absorption cutoff below 190 nm, along with pronounced SHG responses about 120 times that of Y-cut quartz at 880 nm and 1.5 times stronger than that of KH2PO4 at 1064 nm. Moreover, the crystal shows exceptional thermal stability, maintaining structural integrity up to 1000 °C, making it among the most thermally robust NLO sulfates reported to date. Experimental and theoretical analyses reveal that the synergistic alignment of polarizable LaO10 polyhedra and SO4 tetrahedra underpins their high NLO performance. This study provides a promising design paradigm for rare earth sulfate-based deep-UV NLO crystals.
Tao et al. (Mon,) studied this question.