Breakdown of Crystallographic Site Symmetry in Lanthanide-Doped NaYF4 Crystals

摘要

Trivalent lanthanide ions (Ln~(3+)) are well-known for their luminescent properties and have been utilized for decades in television sets and fluorescent lights. More recently, Ln~(3+)-doped inorganic nanoparticles, emerging as a new class of bioprobes, have attracted revived interest for their promising applications in bioimaging and biosensing owing to their superior features such as intense, long-lived, and multicolor emissions. The optical transitions of Ln~(3+) are sensitive to their local coordination, and the emission intensity of Ln~(3+)-based compounds strongly depends on the crystal structure and crystal-field (CF) surroundings around Ln~(3+). Therefore, Ln~(3+) ions are often used as probe to analyze the local structure of cations in luminescent materials. However, for one family of inorganic crystals with disordered structures, such as molybdates and tungstates in which two or more cations statistically occupy the same lattice site, the symmetry of spectroscopic sites for the dopant Ln~(3+) ions was observed to deviate from that of crystallographic sites. Because the microscopic model of such structural disorder is established from a least-squares fitting of single-crystal X-ray diffraction (XRD) data by standard crystallography analysis, which takes into account those cations randomly occupying the same lattice site as a virtual "average" ion with their respective probabilities, the actual local symmetry of the dopant in the disorder site can not be revealed from the crystallographic data. To date, only one report has attempted to resolve the apparent symmetry distortion between the crystallographic sites and spectroscopic sites by diffuse X-ray scattering. Unfortunately, the mechanism behind this breakdown of crystallographic site symmetry in an average structure remains essentially untouched/Nowadays, disordered crystals with distinct optical properties are widely used as host materials for lighting and displays, lasers, or bioassays. An unambiguous spectroscopic revelation of local site symmetry breakdown in this huge family of crystals is crucial to optimizing their optical performance for further applications.