Luminescent spectroscopy of $\mathrm{Mn^{2+}}$ ions in $\mathrm{CaF_2}$

Excitation and emission spectra as well as the lifetimes of a concentration series of $\mathrm{CaF_2:Mn^{2+}}$ crystals have been studied. The multiple centers of $\mathrm{Mn^{2+}}$ ions in $\mathrm{CaF_2}$ are discovered. The manganese ions are eightfold coordinated and form centres of trigonal symmetry and exchange-coupled pairs consisting of two ions coupled by $C_2$ axis. The trigonal center is a vacancy of the fluorine at the immediate surrounding of $\mathrm{CaF_2:Mn^{2+}}$. The main causes of the existence of multiple centers at the homovalent substitution $\mathrm{Mn^{2+}\to Ca^{2+}}$ are the deformation of local structure around the activator ion due to the large difference in ionic radius between $\mathrm{Mn^{2+}}$ (0.80 Å) and $\mathrm{Ca^{2+}}$ (1.10 Å) and the influence of the striction of exchange-coupled pairs. The importance of using the Trees–Racah and Seniority corrections in the scheme of crystal field theory calculations is demonstrated.