Instability of the local environment of Mn2+ in BaF2

Excitation and luminescence spectra and luminescence lifetime of Mn2+ ion in BaF2 crystals at 77 K have been investigated for the first time. Mn2+ ions in the crystal are coordinated by six and eight fluorine ions, have a trigonal environment, and form exchange-coupled pairs. Several types of centers of the Mn2+ ion are formed mainly because of the large difference in the Mn2+ and Ba2+ ionic radii, which causes instability of the local structure around the activator and its strain, and exchange striction. An increase in the impurity concentration enhances these factors, thus changing the relative concentration of various centers.