A Promising Short-Wave UV NLO Crystal with an Optimized KBBF-like Structure and Well-Balanced Properties Achieved via a Chemical Substitution Strategy

Balancing a short ultraviolet (UV) cutoff edge, a sufficient birefringence, and a strong second-harmonic generation (SHG) response is crucial yet challenging in the quest for beryllium-free deep-UV nonlinear optical (NLO) crystals. Herein, we present the synthesis of an optimized KBe₂BO₃F₂ (KBBF)-like fluorinated borate crystal, Rb₃Sr₃Li₂Al₄B₆O₂₀F (RSLABOF), through a chemical substitution strategy achieved by substituting Be²⁺ cations in the KBBF structure with Al³⁺ and Li⁺ cations. RSALBOF crystallizes in R32 (No. 155), inheriting the structural merits of KBBF and achieving a well-balanced property, including a strong SHG efficiency equivalent to 1.4 times that of KDP, a short UV absorption cutoff edge (<200 nm), and a favorable birefringence value of 0.064 at 1064 nm. Moreover, the layered structure of RSLABOF features a reinforced interlayer bonding facilitated by Sr-O bonds, which is approximately 4.7 times stronger than that in KBBF, alleviating the propensity for layering growth. Theoretical calculations revealed that the significant SHG intensity primarily originates from the in-layer uniformly aligned [BO₃] groups in RSLABOF. These observations highlight the potential of RSLABOF crystals as beryllium-free short-wave UV NLO crystals.