Investigations of two trigonal ($T_1$ and $T_2$) ${\mathrm{Gd}}^{3+}$ ESR centers in treated alkaline-earth-fluoride crystals

A detailed investigation has been made of the two trigonal ${\mathrm{Gd}}^{3+}$ ESR centers ($T_1$ and $T_2$) in alkaline-earth-fluoride (Ca${\mathrm{F}}_2$, Sr${\mathrm{F}}_2$, and Ba${\mathrm{F}}_2$) crystals treated with water vapor at high temperatures. We have observed both the $T_1$ and $T_2$ centers in all three fluoride crystals. Partially resolved superhyperfine structures have been detected for the first time for the $T_1$ and $T_2$ centers. It is established that the charge-compensation structures of the $T_1$ and $T_2$ trigonal ${\mathrm{Gd}}^{3+}$ centers are associated with the ${\mathrm{Gd}}^{3+}$ -${\mathrm{F}}^{-}$ ${\mathrm{O}}_4^{2-}$ and ${\mathrm{Gd}}^{3+}$ -${\mathrm{F}}_7^{-}$ ${\mathrm{O}}^{2-}$ configurations, respectively.