Distinct ionic adsorption sites in defective Prussian blue: a 3D-RISM study

Ferric hexacyanoferrate (FeHCF) or Prussian blue (PB) exhibits selective alkali ion adsorption and has great potential for use in various applications. In the present work, alkali ion (Li⁺, Na⁺, K⁺, and Cs⁺) and water configurations in defective PB (d-PB) were studied by using the statistical mechanics of molecular liquids. The three-dimensional (3D) distribution functions of the ions and water were determined by solving the 3D-reference interaction site model (RISM) equation of systems of a unit lattice of d-PB in electrolyte solutions, i.e., LiCl, NaCl, KCl, and CsCl. The results show the difference in the ion-water configurations and distributions between small (Li⁺ and Na⁺) and large ions (K⁺ and Cs⁺). The adsorption sites of Li⁺ and Na⁺ are located off-center and lie on the diagonal axis. By contrast, the larger ions, K⁺ and Cs⁺, are adsorbed at the center of the unit cell. The degree of dehydration due to the adsorption of alkali ions indicates that there was no water exchange during Li⁺ and Na⁺ adsorption, whereas two and three water molecules were removed after adsorption of K⁺ or Cs⁺ in the unit cell.