Wide-band EPR spectroscopy of SrY$_2$O$_4$:Ho$^{3+}$ crystal

EPR spectra of impurity Но$^{3+}$ ions in oriented SrY$_2$O$_4$ single-crystals are registered at the temperature 4.2 K in the frequency range from 70 to 180 GHz. The results of measurements evidence for the substitution of Но$^{3+}$ ions for the Y$^{3+}$ ions at the structurally nonequivalent sites $R1$ and $R2$ with the local $C_s$ point symmetry. The values of $g$-factors, hyperfine structure constants and the energy gaps berween the ground and the first excited non-degenherate crystal-field sublevels of the ground $^5I_8$ multiplet are determined. The observed specific features of the ground states of Но$^{3+}$ ions (non-Kramers doublets with the zero-field splittings of 4.30 and 1.67 cm$^{-1}$) open a possibility to identify transitions in optical spectra of SrY$_2$O$_4$ : Ho and inelastic neutron scattering spectra of SrHo$_2$O$_4$ crystals.