Investigations of the weak trigonalGd3+ESR center in alkaline-earth-fluoride crystals

Detailed electron-spin-resonance investigations have been made of the weak trigonal ${\mathrm{Gd}}^{3+}$ center in alkaline-earth fluorides (${\mathrm{CaF}}_2$, ${\mathrm{SrF}}_2$, and ${\mathrm{BaF}}_2$). We have been successful in detecting partially resolved fluorine superhyperfine structures of the weak trigonal ${\mathrm{Gd}}^{3+}$ center in ${\mathrm{SrF}}_2$ and ${\mathrm{BaF}}_2$. Based on the results of analyses of the observed superhyperfine structures, it is concluded that the weak trigonal center is involved with all the eight first-shell (nearest-neighbor) fluorines with the charge compensator, believed to be a ${\mathrm{F}}^{-}$ ion, situated at the interstitial body center of the fluorine cube adjacent to the ${\mathrm{Gd}}^{3+}$ ion along the [111] direction. A complete set of the crystal-field fine-structure splitting constants of the ${\mathrm{Gd}}^{3+}$ center has been determined for the first time in ${\mathrm{SrF}}_2$. Despite extensive searching, the weak trigonal center has not been observed in ${\mathrm{CaF}}_2$.