Discovery of a new bimetallic borate with strong optical anisotropy activated by π-conjugated [B2O5] units

Birefringent materials with high optical anisotropy have been identified as a research hotspot owing to their significant scientific and technological significance in modern optoelectronics for manipulating light polarization. Researchers studying borate systems have discovered that adding π-conjugated units placed in parallel can significantly increase the birefringence of crystalline solids; some examples include [BO₃] units, [B₂O₅] units, and [B₃O₆] units. However, there are not many borates with strictly parallel configurations of π-conjugated [B₂O₅] units. In this study, a new bimetallic borate Sr₂Cd₄(B₂O₅)₃ with near-parallel arrangement of π-conjugated [B₂O₅] units was discovered. Sr₂Cd₄(B₂O₅)₃ possesses the maximum number density of [B₂O₅] units, shortest dihedral angle of [B₂O₅] units (between the two [BO₃]), and largest degree of [CdO₆] octahedral distortion among all the currently known Sr-Cd-B-O tetragonal system borates, making it demonstrate a large birefringence of 0.102 at 532 nm. Theoretical analysis proves that π-conjugated [B₂O₅] anions are the primary source of the large birefringence of Sr₂Cd₄(B₂O₅)₃.