BaFCl single crystals doped with Sm³⁺ ions were studied by using the EPR method. Several types of paramagnetic Sm³⁺ centres were found. The parameters of the corresponding spin Hamiltonians were determined. Structural models and ground states of the observed centres are proposed.

The Yb³⁺ paramagnetic center of the trigonal symmetry (“oxygenâ€� paramagnetic center T₂) in CaF₂ and SrF₂ single crystals is studied by EPR and optical spectroscopy. The Stark level energies of the Yb³⁺ multiplets are established from absorption, luminescence and excitation luminescence spectra and the crystal field parameters are calculated.

Results of EPR and optical spectroscopic investigation of the trigonal paramagnetic Yb³⁺ ion in SrF₂ (‘oxygen’ paramagnetic center — T₂) are presented. The energy level scheme of the center is determined from its optical spectra and the parameters of the crystal field potential are calculated.

Electron paramagnetic resonance, electron-nuclear double resonance, and optical spectroscopy of the tetragonal Yb³⁺ center in KMgF₃ are reported here. The results of these experiments allow us to conclude that a previously given structural model as well as the interpretation of the optical spectrum of this center are incorrect. A model is presented and experimentally and theoretically justified. In particular, the values of the hyperfine and transferred hyperfine interaction parameters were determined as well as an experiment-based energy-level scheme. Its parametrization is performed by including simultaneously the crystal field and the spin-orbit interaction within the ⁷F term. Furthermore, a theoretical analysis of the transferred hyperfine interaction (THFl) parameters is presented. It is further shown from optics and from microscopic calculations of the THFI parameters that g_∥ and g_⊥have opposite signs and that the rule of correspondence between the cubic g factor and g̃=1/3(g_x+g_y+g_z) does not depend on the relative magnitude of the cubic and low-symmetry crystal field acting on the rare-earth ion.

ENDOR measurements on the ¹⁹F^- nuclei in the first four shells of KZnF₃ containing Dy³⁺ ions in the cubic site are reported. The values and signs of the hyperfine and transferred hyperfine interaction parameters are determined. The local deformation of the crystal lattice in the vicinity of the impurity ion is estimated. The theoretical analysis of the THFI parameters for the first coordination shell of the F^- ions has been carried out. For the Dy³⁺ ion the influence of spin polarization of the closed 5s and 5p shells is considered for the first time. Spin polarization is shown to play a significant role in the mechanism of rare-earth ion-ligand coupling. 

The tetragonal Er³⁺ ion associated with the interstitial F^− ion along the [100] axis in CaF₂ is studied using ENDOR. The parameters of the transferred hyperfine interaction and of the nuclear Zeeman interaction of the surrounding fluorine ions are determined. Anomalously large values of the pseudo-nuclear Zeeman effect on the F^− nuclei are found. The theoretical analysis of these parameters is carried out in a frame of operator techniques in the theory of transferred hyperfine interactions. A number of useful relations for practical calculations of the values of the local field at ligand nuclei are reported.