A3Sr2P7O21 (A = Rb, Cs): Two Polyphosphates Based on Different Types of P–O Chains and Ring Structures

Two polyphosphates containing two types of polymerization of the [PO4] groups, Rb3Sr2P7O21 and Cs3Sr2P7O21, were grown through a spontaneous nucleation method. Single-crystal X-ray diffraction data were collected in order to determine their structures. Interestingly, Rb3Sr2P7O21 is the first example of two kinds of [PO3]∞ linear chains coexisting in one phosphate structure. However, in the structure of Cs3Sr2P7O21, the isolated [P4O12] ring and the 1D [PO3]∞ chain can be observed, which is also rare in phosphates. After careful structural analysis, the alkali-metal cations have an effect on the polymerization of the [PO4] groups and make Rb3Sr2P7O21 and Cs3Sr2P7O21 crystallize in different space groups. What is more, IR spectra, UV–vis–NIR diffuse reflectance spectroscopy data, and first-principles theoretical calculations were adopted to determine the optical properties and the structure–properties relationship of the compounds.