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

• Site-selective spectroscopy in NaLaF4:Er3+ has revealed at least three nonequivalent sites occupied by Er3+.
• Site-selective excitation of Er3+ leads to energy transfer between erbium ions located at different sites.
• Broad excitation bands located at the shorter-wavelength side of the electronic transitions of Er3+ are of vibronic nature.

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 4F7/2 level of Er3+ ions has revealed three distinct luminescence spectra in the green spectral region associated with 4S3/2 → 4I15/2 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.

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