Lanthanoid Ion Color

Synonyms

Lanthanide ions; Lanthanides; Rare earths; Rare-earth elements; Rare-earth ions

Definition

The lanthanoids (often designated Ln) are the 15 elements with atomic numbers 57 (lanthanum) to 71 (lutetium).

Colors, Electron Configurations, and Energy Levels

Color

Most of the lanthanoid ions exhibit rather pale characteristic colors when introduced into transparent solids or in water solutions, the most important being the Ln³⁺ state (Table 1). These colors arise from electronic transitions between the ionic ground state and energy levels derived from 4f electron configurations lying between 1.77 and 3.10 eV above it, giving absorption maxima in the visible wavelength range (700–400 nm). Of more practical importance is color produced when ions excited to higher energy levels fall back to these 4f-derived levels and thence to the ground state, giving rise to characteristic visible emission spectra, which are used in many applications including fluorescent printing inks used as...