Concentration dependence of EPR spectra in mixed (BaF$_2$)$_{1-x}$(CeF$_3$)$_x$ crystals

In the EPR spectra of the (BaF$_2$)$_{1-x}$(CeF$_3$)$_x$ system, additional lines appear as the concentration $x$ in the mixture increases from 10$^{-3}$ to 10$^{-2}$, some of which correspond to tetragonal centers with $g_{\parallel}$ = 0.725 and $g_\perp$ = 2.52. These lines are similar to those detected previously in mixed (BaF$_2$)$_{1-x}$(CeF$_3$)$_x$(LaF$_3$)$_y$ systems with a low cerium concentration ($x\approx$ 10$^{-3}$) and in the CaF$_2$ : Ce$^{3+}$ crystal grown with added Na$^{+}$ ions.