Microscopic understanding of lithium defect on lanthanide luminescence in NaYF4 lattices from first principles

Lithium doping has been widely employed to modulate the photoluminescence of lanthanide-doped crystals. Despite its effectiveness, the underlying mechanism remains debatable. Using cubic sodium yttrium fluoride as a model crystal, first-principles calculations indicate that neutral substitution of host sodium with lithium ions can occur, irrespective of synthesis conditions. Unlike lithium substitution, the formation of lithium interstitials is likely accompanied by the substitution of F- with O2- ions. The lithium substitution shows negligible influence on the electronic structures of the host crystal and the 4f orbital energies of lanthanide ions. By comparison, the lithium-oxygen defect pairs could induce self-absorption losses in lanthanide luminescence or produce ligand-to-metal charge transfer states that could populate lanthanide 4f states.