Breakdown of Crystallographic Site Symmetry in Lanthanide‐Doped NaYF 4 Crystals

High‐resolution photoluminescence spectroscopy employing Eu 3+ as the structural probe unambiguously revealed a universal breakdown of crystallographic site symmetry in lanthanide‐doped disordered crystals. The spectroscopic site symmetries of Eu 3+ descend from crystallographic O h to C s (or C 2 ) in α‐NaYF 4 , and from crystallographic C 3 h to C s in β‐NaYF 4 , which were further verified by crystal‐field level fitting.