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 KBe 2 BO 3 F 2 (KBBF)-like fluorinated borate crystal, Rb 3 Sr 3 Li 2 Al 4 B 6 O 20 F (RSLABOF), through a chemical substitution strategy achieved by substituting Be 2+ cations in the KBBF structure with Al 3+ and Li + cations. RSALBOF crystallizes in R 32 (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 [BO 3 ] groups in RSLABOF. These observations highlight the potential of RSLABOF crystals as beryllium-free short-wave UV NLO crystals.