RbLa(SO ₄ ) ₂ : A KBBF-like Deep-UV Transparent Rare Earth Sulfate with Strong SHG Response and High Thermal Stability Enabled by Multicomponent Structural Design

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.