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.