Variable dimensionality of the anion framework in four new borophosphates and fluoroborophosphates with short cutoff edges

Two new borophosphates, Cs₃B₃P₄O₁₆ and Li(NH₄)₂B₃P₄O₁₆, and two new fluoroborophosphates, K₄B₂P₂O₉F₂ and Rb₃B₂PO₅F₄, have been obtained via the high-temperature solution method. Single-crystal X-ray diffraction indicates that all of them exhibit various anion frameworks although they crystallize in the same space group, P2₁/c. Two-dimensional (2D) [B₃P₄O₁₆]_∞ layers and a 3D [B₃P₄O₁₆]_∞ network can be found in Cs₃B₃P₄O₁₆ and Li(NH₄)₂B₃P₄O₁₆, respectively, while isolated [B₂P₂O₉F₂] and [B₂PO₅F₄] exist in K₄B₂P₂O₉F₂ and Rb₃B₂PO₅F₄, respectively. The effect of the alkali metal cation size on the framework structures of Cs₃B₃P₄O₁₆ and Li(NH₄)₂B₃P₄O₁₆ has been discussed in detail. The IR spectra confirm their structural validity. UV-Vis-NIR diffuse reflectance spectroscopy indicates that the new compounds exhibit short cutoff edges. In addition, theoretical calculations were carried out to understand their electronic structures and optical properties.