On the color of the trivalent lanthanide ions

The energy levels of the trivalent lanthanide ions are rather insensitive to the ligand environment of the central metal ion. Since some of these ions have absorption bands in the visible part of the electromagnetic spectrum, they will show a characteristic color. It is therefore strange that the colors of the trivalent lanthanide ions reported in literature are not always consistent. This Letter reconsiders the problem of the color of the lanthanide ions. Based on the positions of the strongest bands in the absorption spectrum of a trivalent lanthanide ion, the color can be predicted.