In CsCdBr3 crystals, the EPR spectra of Fe2+ ion replacing Cd2+ ion in a trigonally distorted octahedron were recorded and studied in the subterahertz frequency range. The zero-field splitting parameter and the value of the g-factor are determined. A comparative analysis of the obtained experimental results with literature data is made.

We present the optical and magneto-optical spectroscopy and electron paramagnetic resonance (EPR) investigations of CaMoO4 single crystals doped with the erbium ions. Telecom-wavelength resonance transition inhomogeneous line width of Er3+ is relatively narrow for oxide crystals which makes this material promising for quantum technologies applications. The hyperfine structure in optical spectra of 167Er3+ isotope is well resolved. Energies and symmetries of wavefunctions of 39 energy levels of Er3+ ions in the crystal-field (CF) of S4 symmetry and g-factors of some CF Kramers doublets were measured and successfully simulated on the basis of CF calculations. The obtained set of CF parameters was used for modeling the hyperfine structure profiles observed in the optical absorption spectra.

EPR spectra of Ho3+ impurity ions were recorded in single crystals of yttrium aluminum garnet (Y3Al5O12, YAG) in the frequency range of 114–410 GHz, at a temperature of 4.2 K. Besides the centers due to unusual substitutions by Y3+ for Al3+ ions (anti-site defects), a trigonal center was found, which indicates the replacement of Al3+ ions by Ho3+ ions in octahedral positions with local symmetry C3i. The magnitude of g-factor, the hyperfine structure constant and the energy interval between the main and the first excited sublevel of the main 5I8 muliplet were determined. A comparative analysis of the formation of satellite centers for crystals grown under different conditions is made.

The EPR spectra of Czochralski grown Y2SiO5:Cr and Y2SiO5:53Cr crystals have been studied. In addition to the previously known spectra of Cr3+, the spectrum of the Cr4+ ion was detected and studied for the first time. The fine structure parameters of tetravalent chromium in Y2SiO5 host were obtained based on the analysis of the angular and frequency-field dependences of the EPR spectra. The conclusion about tetrahedral coordination of Cr4+ ion in this host was made. The concentrations ratio of c(Cr3+)/c(Cr4+) ions in the crystal has been evaluated.

In a single crystal of lutetium aluminum garnet (Lu 3 Al 5 O 12 , LuAG) in the frequency range of 37-210 GHz, at a temperature of 4.2 K, the EPR spectra of non-Kramers Tb 3+ impurity ions were recorded. The measurement results indicate that Tb 3+ ions replace Y 3+ ions in the dodecahedral position with local D 2 symmetry. The value of the g factor, the hyperfine structure constant, and the energy interval between the ground and the first excited nondegenerate sublevels of the ground multiplet are determined. Weak satellite signals were also registered, the origin of which is associated with the formation of antisite defects in the environment of paramagnetic Tb 3+ centers. Keywords: hyperfine interaction, rare earth ions, spin Hamiltonian, non-Kramers ion.

EPR spectra of impurity Ho3+ ions in oriented SrY2O4 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 Ho3+ ions for the Y3+ ions at the structurally nonequivalent sites R1 and R2 with the local Cs point symmetry. The values of g-factors, hyperfine structure constants and the energy gaps between the ground and the first excited non-degenherate crystal-field sublevels of the ground 5I8 multiplet are determined. The observed specific features of the ground states of Ho3+ 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 SrY2O4:Ho and inelastic neutron scattering spectra of SrHo2O4 crystals.

EPR spectra of Ho3+ impurity ions were recorded in single crystals of yttrium aluminum garnet (Y3Al5O12) in the frequency range of 114-170 GHz, at a temperature of 4.2 K. The measurement results indicate the substitution of Ho3+ ions for Y3+ ions in the dodecahedral position with D2 local symmetry. The magnitude of the g-factor, the hyperfine structure constant, and the energy interval between the main and the first excited nondegenerate sublevels of the main multiplet 5I8 are determined. Weak satellite signals have also been registered, the origin of which is associated with the formation of antisite defects in the environment of Ho3+ paramagnetic centers.

EPR spectra of impurity Ho3+ ions in oriented SrY2O4 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 Ho3+ ions for the Y3+ ions at the structurally nonequivalent sites R1 and R2 with the local Cs 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 5I8 multiplet are determined. The observed specific features of the ground states of Но3+ 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 SrY2O4:Ho and inelastic neutron scattering spectra of SrHo2O4 crystals.

EPR spectra of impurity Ho3+ ions in oriented SrY2O4 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 Ho3+ ions for the Y3+ ions at the structurally nonequivalent sites R1 and R2 with the local Cs 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 5I8 multiplet are determined. The observed specific features of the ground states of Но3+ 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 SrY2O4:Ho and inelastic neutron scattering spectra of SrHo2O4 crystals.

EPR spectra of Ho 3+ impurity ions were recorded in single crystals of yttrium aluminum garnet (Y 3 Al 5 O 12 ) in the frequency range of 114-170 GHz, at a temperature of 4.2 K. The measurement results indicate the substitution of Ho 3+ ions for Y 3+ ions in the dodecahedral position with D 2 local symmetry. The magnitude of the g-factor, the hyperfine structure constant, and the energy interval between the main and the first excited nondegenerate sublevels of the main multiplet 5 I 8 are determined. Weak satellite signals have also been registered, the origin of which is associated with the formation of antisite defects in the environment of Ho 3+ paramagnetic centers. Keywords: hyperfine interaction, rare earth ions, spin Hamiltonian, non-Kramers ion.

A YAlO3 crystal with an admixture of iron was studied by broadband EPR spectroscopy. Two types of paramagnetic centers were found. The energy level scheme of one of them is a singlet and a doublet with a splitting of 215.2 GHz. This center is presumably associated with the Fe2+ ion. The second center was identified as the Tb3+ ion, which entered the crystal as an uncontrolled impurity. The localization of the impurity ion in the YAlO3 crystal lattice is determined for each case. Within the framework of the existing theory, calculations are carried out and spectral parameters are determined.

EPR spectra of impurity Ho3+ ions in oriented SrY2O4 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 Ho3+ ions for the Y3+ ions at the structurally nonequivalent sites R1 and R2 with the local Cs 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 5I8 multiplet are determined. The observed specific features of the ground states of Но3+ 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 SrY2O4:Ho and inelastic neutron scattering spectra of SrHo2O4 crystals.

The results of the study of the temperature dependence of attenuation and velocity of ultrasonic waves are presented for CaF2:Cr and CaF2:Ni crystals in which Cr2+ and Ni2+ ions substitute calcium ions, forming Jahn–Teller (JT) complexes [CrF8]6– and [NiF8]6–. The Cr2+ and Ni2+ ions in the fluorite structure have threefold orbital degeneracy in the ground state, are described by the T $$ \otimes $$ (e + t2) Jahn–Teller effect (JTE) problem, and have an adiabatic potential energy surface (APES) defined in the five-dimensional space of trigonal and tetragonal symmetrized coordinates whose lower sheet is a simply connected surface with orthorhombic minima separated by potential energy barriers. The temperature dependence of the ultrasonic attenuation and velocity exhibits anomalies typical of the relaxation contributions of systems of JT complexes in the low-temperature region. The effect of tunneling relaxation mechanisms (direct and two-phonon transitions) on the JT contribution to the complex elastic moduli and to the error in determining the relaxation time is analyzed. Based on the approach that takes into account both tunneling and activation mechanisms of relaxation, the activation energies and the constants characterizing these relaxation mechanisms are determined in CaF2:Cr2+ and CaF2:Ni2+ crystals.

The temperature dependences of attenuation and the velocity of ultrasonic waves at frequencies of 26–158 MHz in a CaF2 fluorite crystal at the substitution of Jahn–Teller Cr2+ centers for calcium ions have been studied. An abnormally high relaxation rate, which is two orders of magnitude higher than the relaxation rate in other previously studied CaF2:Ni2+ and SrF2:Cr2+ fluorites, has been found in the system of Jahn–Teller complexes in the low-temperature region. It has been shown that the global minima of the adiabatic potential energy surface of the Cr2+ $${\text{F}}_{8}^{ - }$$ complexes in the CaF2:Cr2+ crystal also have orthorhombic symmetry but are separated by significantly lower potential energy barriers than in CaF2:Ni2+ and SrF2:Cr2+ crystals. It has been found that tunneling relaxation mechanisms (direct and two-phonon transitions) are dominant, rather than thermal activation, in CaF2:Cr2+ in the temperature range where the Jahn–Teller effect is manifested in an ultrasonic experiment. The parameters characterizing these relaxation mechanisms have been determined.

The PbGa2S4: Tm3+ crystal was investigated by tunable high-frequency EPR spectroscopy. Two thulium centers located in positions with different local symmetry were observed in the frequency range of 9.8-170 GHz. Both spectra with the resolved hyperfine structure correspond to the transition between sublevels of the main quasi-doublet Tm3+ with crystal field splitting of 6.93 GHz and 1.42 GHz. The times of phase memory and spin-lattice relaxation of thulium ions were measured by the spin-echo method for various values of the magnetic field.

Методом широкополосной ЭПР-спектроскопии изучен кристалл YAlO3 с примесью железа. Обнаружено два типа парамагнитных центров. Энергетическая схема уровней одного из них представляет собой синглет и дублет с расщеплением между ними 215.2 GHz. Этот центр предположительно связан с ионом Fe2+. Второй центр идентифицирован как ион Tb3+, вошедший в кристалл как неконтролируемая примесь. Для каждого случая определена локализация примесного иона в кристаллической решетке YAlO3. В рамках существующей теории сделаны расчеты и определены спектральные параметры. Ключевые слова: широкополосный электронный парамагнитный резонанс, ортоалюминат иттрия, примесные центры, некрамерсовые ионы.

Tetragonal Cr4+ centers (3d2 electron configuration, spin S=1) in the Li2CaSiO4 crystal at different frequencies were detected and studied. The value of splitting at a zero magnetic field was determined. In the area where the positions of the two Cr4+ transitions coincide, the additional signal with inverse phase was observed. For uncontrolled impurity Mn2+ ions (S=I=5/2) the spin Hamiltonian parameters and localization are determined