Superhyperfine structure of the EPR spectra of Nd$^{3+}$ impurity ions in fluorite CaF$_{2}$

EPR spectra of a CaF$_{2}$ single crystal that was grown from melt containing a small addition of NdF$_3$ were studied. Signals corresponding to tetragonal centers of Nd$^{3+}$ ions and cubic centers of Er$^{3+}$ and Yb$^{3+}$ ions were found. Superhyperfine structure (SHFS) in the spectra of the Nd$^{3+}$ ions was observed for the first time in this crystal; parameters of the superhyperfine interaction of the Nd$^{3+}$ ions with the nearest nine fluorine ions were determined. The dependence of the resolution of the Nd$^{3+}$ EPR spectrum SHFS on the incident microwave power at the temperature of $T\approx$ 6 K was studied. Obtained results are discussed and compared with the literature data.