Understanding the Polymorphism of A₄[(UO₂)₃(PO₄)₂O₂] (A = Alkali Metals) Uranyl Phosphate Framework Structures

In this study we combine experimental synthesis and density functional theory (DFT) calculations to gain insight into the polymorphism of A₄[(UO₂)₃(PO₄)₂O₂] (A = Na, K, Rb, Cs) uranyl phosphate structures. Single crystals of a new 3D uranyl phosphate, Cs₄[(UO₂)₃(PO₄)₂O₂], were grown by molten flux methods using a CsCl flux. DFT calculations, using the DFT+U method, were carried out to study the difference between this new 3D uranyl phosphate and a family of recently described layered uranyl phosphates. Variation of the computed properties with changes in U_eff values are also studied. The DFT results agree with the experimental observations, showing that the Cs-containing 3D polymorph and the K-containing layered polymorphs are more stable than their respective layered and 3D polymorph. We show an increase in the difference between the total energies of the layered and 3D polymorphs and an increase in the band gaps with increasing U_eff value. Volume-based thermodynamics was also applied to calculate the total energies of the different polymorphs, showing consistently higher stability of the layered polymorphs compared to the 3D polymorphs. For each of the studied polymorphs, we calculated the electronic, optical, and bonding properties. We also show an anisotropy in the absorption indexes along the three crystallographic directions of the polymorphs, which is especially noticeable in the layered polymorphs. We attribute the difference in the density of states to the different coordination of the U atoms in the layered and 3D polymorphs. We attribute the preferred formation of the 3D Cs polymorph to the substantial increase in the U–A bond strength, which is more pronounced than the differences in the bond strength between structures for other atomic pairs.