Breakdown of Crystallographic Site Symmetry in Lanthanide‐Doped NaYF4 Crystals

High-resolution photoluminescence spectroscopy employing Eu3+ as the structural probe unambiguously revealed a universal breakdown of crystallographic site symmetry in lanthanide-doped disordered crystals. The spectroscopic site symmetries of Eu3+ descend from crystallographic Oh to Cs (or C2) in α-NaYF4, and from crystallographic C3h to Cs in β-NaYF4, which were further verified by crystal-field level fitting. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.