Charge transfer from Eu²⁺ to trivalent lanthanide co-dopants: Systematic behavior across the series

Electron transfer processes between lanthanide activators are crucial for the functional behavior and performance of luminescent materials. Here, a multiconfigurational ab initio study reveals how direct metal-to-metal charge transfer (MMCT) between the Eu²⁺ luminescence activator and a Ln³⁺ co-dopant (Ln³⁺ = Ce³⁺, Pr³⁺, Nd³⁺, Pm³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, and Yb³⁺) systematically dictates the luminescence and optical properties of CaF₂. The combination of the structures and energies of the electronic manifolds, the vibrational force constants, and the structural properties of the donor and acceptor in the host determines the predictions of five different behaviors of CaF₂:Eu²⁺, Ln³⁺ co-doped materials after MMCT absorption: formation of stable traps, MMCT emission, emission quenching, Ln³⁺ emission, and Eu²⁺ emission.