Inorganic Chemistry, Vol.57, No.8, 4726-4738, 2018
A Family of Layered Phosphates Crystallizing in a Rare Geometrical Isomer of the Phosphuranylite Topology: Synthesis, Characterization, and Computational Modeling of A(4)[(UO2)(3)O-2(PO4)(2)] (A = Alkali Metal) Exhibiting Intralayer Ion Exchange
Single crystals of eight new layered uranyl phosphates were grown from alkali chloride fluxes: Cs1.4K2.6[(UO2)(3)O-2(PO4)(2)], Cs0.7K3.3[(UO2)(3)O-2(PO4)(2)], Rb1.4K2.6[(UO2)(3)O-2(PO4)(2)], K-4[(UO2)(3)O-2(PO4)(2)], K2.9Na0.9Rb0.2[(UO2)(3)O-2(PO4)(2)], K2.1Na0.7Rb1.2[(UO2)(3)O-2(PO4)(2)], Cs1.7K4.3[(UO2)(5)O-5(PO4)(2)], and Rb1.6K4.(4)[(UO2)(5)O-5(PO4)(2)]. All structures crystallize in the monoclinic space group, P2(1)/c and contain uranyl phosphate layers with alkali metals located between the layers for charge balance. Ion exchange experiments on Cs0.7K3.3[(UO2)(3)O-2(PO4)(2)], Rb1.4K2.6[(UO2)(3)O-2(PO4)(2)], and K-4[(UO2)(3)O-2(PO4)(2)] demonstrated that Cs and Rb cations cannot be exchanged for K cations; however, K cations can be readily exchanged for Na, Rb, and Cs. Enthalpies of formation were calculated from density functional theory (DFT) and volume-based thermodynamics (VBT) for all six structures. A value for the enthalpy of formation of the phosphuranylite sheet, [(UO2)(3)O-2(PO4)(2)](4-), was derived using single-ion additive methods coupled with VBT. DFT and VBT calculations were used to justify results of the ion exchange experiments. Cs0.7K3.3[(UO2)(3)O-2(PO4)(2)], Rb1.4K2.6[(UO2)(3)O-2(PO4)(2)], and K-4[(UO2)(3)O-2(PO4)(2)] exhibit typical luminescence of the uranyl group.