Luminescence of isoelectronic lanthanide pairs (Ln2+/Ln3+): A relative compression factor for isoelectronic Ln 4fn levels

Abstract

Luminescent trivalent lanthanides (Ln3+) are utilised extensively in photonic and optoelectronic devices. Luminescent divalent lanthanides (Ln2+) are less frequently applied, despite their desirable 5d electronic properties and similar 4f electronic properties. We report on the luminescence of several isoelectronic lanthanide pairs (Ln3+/Ln2+) that possess the same 4fn configurations but different nuclear charges. The intraconfigurational and interconfigurational transitions of divalent Nd, Sm, Eu, Dy, Ho, Tm and Yb in NaMgF3 are compared to those transitions of trivalent counterparts in NaMgF3 and similar hosts. The 4fn(2S+1LJ) levels of Ln2+ ions are compressed by a factor ξ, relative to the isoelectronic Ln3+. The factor varies within each 4fn configuration with the energy of excited 2S’+1LJ’ states. We define an alternative factor ξ′ that corrects for the intra-ion dependence by introducing a power law dependence on the excited state energies. The inter-ion dependence correlates with the spin-orbit coupling parameters of the isoelectronic ions.