Analysis of site symmetries of Er³⁺ doped CaF₂ and BaF₂ crystals by high resolution photoluminescence spectroscopy

The understanding of complex relationships between luminescent properties, local symmetry of an emitting center, and the host crystal structure provides a better insight into optical properties of materials. In this work, the alkaline earths CaF₂ and BaF₂ fluoride crystals doped with 0.1 mol% ErF₃ were investigated. The crystals structure has been studied using a synchrotron and laboratory X-ray diffraction. The C_3v and C_4v sites symmetry were determined using erbium probed high resolution emission spectroscopy (HRPL) at low temperature (LT) of 10 K. The considerable difference in room temperature (RT) optical properties for CaF₂ compared to BaF₂ crystals was observed. Such difference in absorption intensity of 4.7 times of erbium ⁴G_11/2 manifold in UV, and 7.5 times in green emission from ⁴S_3/2 manifold, could be due to the distinction in the host crystals cationic radius (Δr_Ca,Ba) and the dopant-host ionic radius (Δr_Ca-Er, Δr_Ba-Er). Those Δr differences influence the structure and lead to the following symmetry formation: In CaF₂, the C_4v and C_3v isolated centers were identified, with the determined Er³⁺- F⁻_i bond lengths of 2.734 Å and 4.735 Å respectively; In BaF₂, only C_3v isolated centers were identified with the determined Er³⁺- F⁻_i bond lengths of 5.380 Å. The present work is the first study that takes into account correlations of optical properties, the local symmetry and the structure in mentioned fluorides crystals, and it could be a step forward in the lanthanide doped optical materials systematics.