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

Abstract

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.