The maximum of the color-conferring charge-transfer (CT) band in Prussian Blue (PB) varies with the electrochemically introduced cation M(z+) incorporated (as "supernumerary") for charge neutrality, and the dependence on particular properties of the M(z+) has been sought. With alkali-metal ions, the CT-maximum shifts are in the same sequence as the PB mass changes on M+ insertion; the effect on the CT ground state of the intra-lattice interaction of an M+ with the ferrocyanide CN- moiety (competing with cation hydration), is then implicated in shifts of the maxima, as the ferrocyanide is the donor center in the optical CT. More definitely, for M2+ and Ag+, solubility-products of the insoluble M(z+) ferrocyanides (that provide direct indicators of the intra-lattice M(z+)-[Fe(II)(CN)(6)](4- interactions) show a strong correlation with the spectral shifts. The determining interaction of M(z+) with ferrocyanide within PB is enhanced in some cases by the accessibility of M(z+) oxidation states +/- 1 different from the common values. PB lattice energies and the ground states of the optical CTs thus appear closely interlinked. The electrochemical uptake of appreciable amounts of the M(z+) within the lattices was confirmed by XPS.

It is currently important to achieve and understand adjustments of optical properties: "guest cation" induced CT spectral shifts in Prussian Blue are shown to be driven (via its specific effect on the Fe(CN)6 CT-donor entity) by the cation lattice-energy interaction, as inferred from microgravimetry of introduced alkali-metal ions, and from independent solubility correlations for other intercalated cations.