Comprehensive study on different crystal field environments in highly efficient NaLaF₄:Er³⁺ upconversion phosphor

Complex fluorides, especially rare-earth doped NaREF₄ (RE = Y³⁺, La³⁺ or Gd³⁺), are promising materials for the upconversion luminescence mostly due to low phonon energy of their matrices and multisite nature of the crystalline lattice. Although multisite formation in hexagonal NaREF₄ structures has generally been proved, the actual number of the active sites in different structures varies from two (NaGdF₄) to seven (NaYF₄). The aim of this work has been to study multisite formation in NaLaF₄:Er³⁺. For this purpose low-temperature site-selective spectroscopy measurements in hexagonal NaLaF₄:Er³⁺ have been performed. Excitation at different wavelengths corresponding to the excitation of ⁴F_7/2 level of Er³⁺ ions has revealed three distinct luminescence spectra in the green spectral region associated with ⁴S_3/2 → ⁴I_15/2 electronic transition. The number of the spectra has been sufficient to model experimentally measured luminescence spectra at any excitation wavelength as a linear combination of the distinct spectra. The analysis of the structure of the material and the results of site-selective spectroscopy signify the presence of at least three different crystalline field environments where Er³⁺ ions incorporate. Upon site-selective excitation of Er³⁺ located at a specific site energy transfer to erbium ions located at other sites has been observed in both the upconversion and downconversion luminescence processes. The enhanced energy transfer between the different sites in NaLaF₄:Er³⁺ signifies the importance of the multisite nature of the structure, which is a key factor for an efficient upconversion luminescence.