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, Cs₂ Al₂ (B₃ O₆ )₂ O (CABO), was designed and synthesized by simultaneously replacing the (BO₃ )^3- groups and Be²⁺ cations for (B₃ O₆ )^3- units and Al³⁺ cations in Sr₂ Be₂ (BO₃ )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 [Al₂ (B₃ O₆ )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.