Temperature-dependent luminescence and crystal-field levels of Eu ³⁺ in orthorhombic KGdF₄

Luminescent properties of Eu³⁺ ions in orthorhombic KGdF ₄ nanocrystals are investigated with high resolution emission spectra measured under the selective excitation of a tunable laser at the temperature of 20 K. Only the emission from one kind of site is detected, and the crystal-field levels of Eu³⁺ ions in this site, up to ⁵D₂ multiplet, have been identified according to the recorded emission spectra. The symmetry of the site occupied by Eu³⁺ ions is discussed based on the numbers of emission lines of ⁵D ₀-⁷F_J transitions. A temperature-dependent luminescence is observed under the excitation of ⁵D₂ at 465.5 nm. The emission from ⁵D₁ is nearly absent at 20 K, while plays an important role in the emission spectrum at 300 K. This indicates that the direct relaxation from ⁵D₂ to ⁵D ₀ dominates at low temperature. The different processes of the relaxation of ⁵D₂ at 20 and 300 K are discussed with the measured decay curves of the emission from ⁵D₀.