Deep-Ultraviolet Nonlinear Optical Crystal Cs Al (B O ) O: A Benign Member of the Sr Be (BO ) O Family with [Al (B O ) O] Double Layers.

For the explorations of deep ultraviolet (DUV) nonlinear optical (NLO) borates, a type of important optoelectronic material, the (BO ) 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 Al (B O ) O (CABO), was designed and synthesized by simultaneously replacing the (BO ) groups and Be cations for (B O ) units and Al cations in Sr Be (BO ) 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 (B O ) O] double layers. Remarkably, CABO melts congruently and does not contain the toxic beryllium, which is favorable for bulk-size crystal growth and practical applications.