Deep‐Ultraviolet Nonlinear Optical Crystal Cs2Al2(B3O6)2O: A Benign Member of the Sr2Be2(BO3)2O Family with [Al2(B3O6)2O]2− Double Layers

Abstract For the explorations of deep ultraviolet (DUV) nonlinear optical (NLO) borates, a type of important optoelectronic material, the (BO 3 ) 3− group has been long regarded as the sole microscopic optically‐active unit, and toxic Be‐containing raw materials are frequently‐adopted. Herein, a new DUV NLO crystal, Cs 2 Al 2 (B 3 O 6 ) 2 O (CABO), was designed and synthesized by simultaneously replacing the (BO 3 ) 3− groups and Be 2+ cations for (B 3 O 6 ) 3− units and Al 3+ cations in Sr 2 Be 2 (BO 3 ) 2 O, which possesses a favorable structure, through a chemical co‐substitution approach. CABO exhibits a considerable DUV NLO capability because of the wide band gap and large birefringence originating from the [Al 2 (B 3 O 6 ) 2 O] 2− double layers. Remarkably, CABO melts congruently and does not contain the toxic beryllium, which is favorable for bulk‐size crystal growth and practical applications.