K3B4PO10 and K2MB4PO10 (M = Rb/Cs): rare mixed-coordinated borophosphates with large birefringence

Borophosphates are of current interest and great importance as ultraviolet (UV) and deep-UV optical functional materials. However, most of the borophosphates exclusively contain tetrahedral coordination of B and P atoms, i.e. BO4 and PO4. Few compounds contain mixed-coordinated groups, i.e. BO4, BO3 and PO4. A new strategy of large B/P ratios has been suggested to effectively synthesize three mixed-coordinated borophosphates, K3B4PO10 and K2MB4PO10 (M = Rb/Cs). Their structures all show 2D 2∞[B4PO10]3− sheets, which are separated by the alkali metal cations. The thermal behavior measurements indicate that K3B4PO10 and K2MB4PO10 (M = Rb/Cs) melt congruently and a crystal of K3B4PO10 with a size up to 4 × 2 × 1 mm3 has been grown. Property measurements show that they all possess deep-ultraviolet (UV) cut-off edges and a relatively large birefringence and are potential deep-UV birefringent crystals. In particular, the measured birefringence of K3B4PO10 is about 0.0445@532 nm, which is the largest in borophosphates with reported birefringence. This study indicates that B-rich borophosphates are promising systems to synthesize materials with large birefringence.