Superhyperfine structure of the EPR spectra of Ce$^{3+}$ ions in LiRF$_4$ (R = Y, Lu, Tm) double fluorides

The EPR spectra of Ce$^{3+}$ impurity ions in LiYF$_4$, LiLuF$_4$, and LiTmF$_4$ double-fluoride single crystals have been investigated at a frequency of $\sim$9.3 GHz in the temperature range 5–25 K. The effective $g$ factors of the ground Kramers doublet of the cerium ions in three crystals are close to each other ($g_\parallel$ = 2.737, $g_\perp$ = 1.475 for LiYF$_4$:Ce$^{3+}$). A superhyperfine structure of the EPR spectrum of Ce$^{3+}$ ions in the LiTmF$_4$ Van Vleck paramagnet has been observed in the external magnetic field $\mathbf{B}$ oriented along the crystallographic axis $c$ ($\mathbf{B}\parallel c$). The superhyperfine structure of the EPR soectra of the Ce$^{3+}$ ions in the LiYF$_4$ and LiLuF$_4$ diamagnetic matrices is resolved for $\mathbf{B}\perp c$. Possible factors responsible for this pronounced difference in the properties of the systems studied have been discussed.