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 (4)G(11/2) manifold in UV, and 7.5 times in green emission from S-4(3/2) manifold, could be due to the distinction in the host crystals cationic radius (Delta r(Ca,Ba)) and the dopant-host ionic radius (Delta r(Ca-Er), Delta r(Ba-Er)). Those Delta r 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 angstrom and 4.735 angstrom respectively; In BaF2, only C-3v isolated centers were identified with the determined Er3+- F-i(-) bond lengths of 5.380 angstrom. 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.

Automated summary

This study investigates the correlation between optical properties, local symmetry, and crystal structure in CaF2 and BaF2 fluoride crystals doped with ErF3. The differences in optical properties between CaF2 and BaF2 crystals at room temperature are observed and attributed to the differences in cationic radius and dopant-host ionic radius. The determined symmetries and bond lengths of Er3+- F- ions in both crystals are reported. This study provides valuable insight into the development of lanthanide-doped optical materials.