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

For the explorations of deep ultraviolet (DUV) nonlinear optical (NLO) borates, a type of important optoelectronic material, the (BO3)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, Cs2Al2(B3O6)2O (CABO), was designed and synthesized by simultaneously replacing the (BO3)3− groups and Be2+ cations for (B3O6)3− units and Al3+ cations in Sr2Be2(BO3)2O, 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 [Al2(B3O6)2O]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.