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 CaF$_2$ and BaF$_2$ fluoride crystals doped with 0.1 mol% ErF$_3$ 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 CaF$_2$ compared to BaF$_2$ crystals was observed. Such difference in absorption intensity of 4.7 times of erbium $^4$G11/2 manifold in UV, and 7.5 times in green emission from $^4$S$_3/2$ manifold, could be due to the distinction in the host crystals cationic radius ($Δ_rCa,Ba$) and the dopant-host ionic radius ($Δ_rCa-Er$, $Δ_rBa-Er$). Those $Δ$r differences influence the structure and lead to the following symmetry formation: In CaF$_2$, the C$_4v$ and C$_3v$ isolated centers were identified, with the determined Er$³⁺$- F$^−_i$ bond lengths of 2.734 Å and 4.735 Å respectively; In BaF$_2$, only C$_3v$ isolated centers were identified with the determined Er$³⁺$- 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.