From orange to red: understanding the abnormal intensity of the 5D0→7F4 emission of GdF3:Eu3+ microcrystals via crystal field tuning

This study investigates the anomalous photoluminescence of Eu³⁺-doped GdF₃ microcrystals, with particular emphasis on the unexpectedly intense ⁵D₀→⁷F₄ emission band. By correlating structural refinement with spectroscopic analysis, we demonstrate that Eu³⁺ ions occupy a nine-coordinated distorted tricapped trigonal prism site of C_s symmetry. This specific crystal-field environment, coupled with the intrinsically low polarizability of fluoride ligands, suppresses dynamic coupling effects and accentuates forced electric dipole contributions. Consequently, higher-rank crystal-field components dominates, favoring the ⁵D₀→⁷F₄ emission over the conventionally hypersensitive ⁵D₀→⁷F₂ band. These findings establish a direct structure–property relationship, positioning this atypical emission profile as a sensitive fingerprint of local symmetry in low-polarizability hosts.