Low-temperature-flux syntheses of ultraviolet-transparent borophosphates Na₄MB₂P₃O₁₃ (M = Rb, Cs) exhibiting a second-harmonic generation response

The first non-centrosymmetric mixed-alkali-metal borophosphates, Na₄MB₂P₃O₁₃ (M = Rb 1, Cs 2), were obtained using a low-temperature flux method. Single-crystal X-ray diffraction studies of 1 and 2 reveal that the two compounds are isostructural, both crystallizing in the orthorhombic space group Pna2₁; their structures consist of novel 1D borophosphate chains constructed from B₂P₃O₁₄ fundamental building units, assembled into a 3D framework by alkali metal cations. Second-harmonic generation (SHG) measurements show that 1 and 2 are type-I phase-matchable, with SHG responses ca. 0.35 and 0.42 times that of KH₂PO₄, respectively. The cut-off edges of 1 and 2 are ca. 276 and 267 nm, respectively, which suggests that they are potential ultraviolet nonlinear optical materials. Density functional theory calculations were employed to shed light on the band structure and density of states as well as the electron density distribution.