Phonon-induced hypersensitive transitions in lanthanide double perovskites

Journal article, 2026

Lanthanide ions exhibit characteristic emission originating from intra-4 f transitions. Among them, hypersensitive transitions (HSTs) are distinguished by their pronounced spectral tunability. Conventional HST emitters typically rely on dilute doping and host-to-4 f charge transfer, which are often limited by defect-induced quenching and transfer losses. Here, we observe a highly efficient HST channel independent of a host-to-4 f transfer in undoped, intrinsically luminescent europium-based double perovskites, and we identify the mechanisms that govern both the enhancement and loss of HST efficiency. In this system, a thermally activated doubly degenerate phonon mode of the Eu-Cl6 octahedra relaxes the parity selection rule, thereby strengthening the HST emission. The HST oscillator strength is enhanced by a factor of 2–3 relative to that of the magnetic-dipole transition. Direct excitation into the ⁵LJ level bypasses reverse energy transfer and yields a maximum quantum efficiency of nearly 70%. These findings provide insights into the design of efficient and spectrally tunable emission in lanthanide systems.