Superhyperfine structure of the ESR spectra of Gd$^{3+}$ impurity ions in LiYF$_{4}$ double fluoride

Electron spin resonance spectra of Gd$^{3+}$ ions forming a small ($\sim$0.0001 at.%) impurity in a LiYF$_{4}$ single crystal have been investigated in a wide temperature range from liquid helium to room temperature. A number of the fine-structure components of the spectrum exhibit a pronounced superhyperfine structure depending on the orientation of the external magnetic field with respect to the crystallographic axes. The superhyperfine structure was not observed in earlier ESR studies of double fluorides with a Gd$^{3+}$ impurity.