Ba3Al2B12O24: a beryllium-free member of the Sr2Be2B2O7 family with a 2∞[B12Al2O28] double-layered structure

Nonlinear optical (NLO) crystals capable of expanding the spectral region of solid-state are of great importance for many high-tech applications, yet their rational structure design remains a great challenge because of the conflicting property requirements among second harmonic generation (SHG) response, ultraviolet (UV) cut-off edge, and birefringence. Herein, based on the chemical disubstitution of the classic NLO crystal Sr₂Be₂B₂O₇ (SBBO), i.e., substituting [BO₃] triangles with larger π-conjugated [B₃O₇] groups and substituting high-toxic [BeO₄] tetrahedra with environment-friendly [AlO₄] tetrahedra, a new high-performance aluminoborate NLO crystal, Ba₃Al₂B₁₂O₂₄, has been successfully designed and synthesized. The theoretical calculations and optical property measurements indicate that Ba₃Al₂B₁₂O₂₄ exhibits not only the largest SHG response among the reported aluminoborates (2.7 × KH₂PO₄), but also a short UV absorption edge (<190 nm) and moderate birefringence. More importantly, Ba₃Al₂B₁₂O₂₄ melts congruently, and a single crystal with a size of 9 × 5 × 2 mm³ has been grown by the top-seeded-solution-growth method. These demonstrate that Ba₃Al₂B₁₂O₂₄ is a promising NLO crystal and the chemical disubstitution is a feasible strategy for the exploration of functional materials.