Study of Incorporating Cesium into Copper Hexacyanoferrate by Density Functional Theory Calculations

Copper hexacyanoferrate (Cu-HCF), a Prussian blue analogue, has the ability to incorporate Cs+ ions into its structure; it is thus a candidate to selectively extract radioactive Cs+ from water and to sequester it. We use density functional theory calculations to probe the incorporation of Cs+, K+, and Na+ ions into Cu-HCF. Specifically, we determine their incorporation energies, energies for the exchange of alkali ions, lattice stabilities, and the strengths of the bonds between the transition metal and CN ligand. We find that all of these are more favorable for Cs+ than for either Na+ or K+. In addition, we analyze the electronic structure to explain the favorability of Cs+ incorporation. Cu-HCF is found to switch from a ferromagnetic structure to an antiferromagnetic structure as the number of incorporated alkali ions increases.