The primary objective of materials science is the study of the properties of the existing materials and the development of new ones. In the present day, the use of solid-state lasers abroad has prompted material scientists globally to develop novel or enhance the properties of the existing active gain media. Materials comprising rare earth elements have historically proven effective in scintillation applications. The synthesis of LaLu1-xYbxO3 monodisperse nanopowders with a particle size of 50-62 nm via the Pechini complexing citrate method has been the subject of investigation using electron paramagnetic resonance (EPR) spectroscopy and photoluminescence (PL) measurements. The EPR data indicate that the Yb3+ ions predominantly create a single type of center, whereby the Yb3+ ion substitutes for the Lu3+ ion. The calculated EPR g-factors are estimated to be g1 = 3.85, g2 = 2.15, g3 = 1.55 and the 171 Yb isotope hyperfine interaction constants are A1 = 600 x 10-4 cm-1 , A2 = 700 x 10-4 cm-1 , A3 approximate to 1800 x 10-4 cm-1 . The principal values of the g factors and the hyperfine tensors indicate that the Yb site in LaLu1-xYbxO3 nanopowders exhibits orthorhombic symmetry. The PL measurements in the wavelength range of 350-1800 nm, with excitation provided by either a xenon lamp or a diode laser, indicate the presence of a broad emission band, centered at 404 nm, in addition to the well-known emission spectrum in the IR region (920-1100 nm) arising from Yb3+ ions.

Strontium titanate (SrTiO3) is the most known material from the family of quantum paraelectrics. Thanks to its extremely "soft" lattice, its functionality can be easily tuned by applying both external stimuli (pressure, strain, electric field) and through doping or isotope exchange. In this paper, we present the results of a detailed study of two Mn2+ centers in Mn-doped SrTiO3 single crystals using both continuous wave and pulsed electron paramagnetic resonance (EPR) spectroscopy at frequencies from 9.5 to 427 GHz and temperatures from 5 to 296 K. The first center is created by a Mn2+ ion at the Sr2+ lattice site in an off-center position. Its spectroscopic characteristics were determined for both fast and slow motion regimes of the impurity ion. In particular, all spin transitions allowed by the Mn2+ spin were well resolved in the slow motion regime. The second center is created by a Mn2+ ion at the Ti4+ position in the center of the oxygen octahedron. It has been established that the surrounding of this ion undergoes strong distortion when cooled below the phase transition temperature Tc = 105 K, stimulated by the rotation of the oxygen octahedron. The present data also perfectly explain the previously obtained EPR data from measurements of SrTiO3:Mn ceramics at low microwave frequencies (9-10 GHz).

The paper presents data on the study of optical absorption, photoluminescence and EPR of lithium-sodium tetragermanate crystals (LiNaGe4O9) doped with Mn ions. Optical spectra were measured in the range 11000-45000 cm(-1) and temperatures 77-300 K. They are attributed to manganese in the +4 charge state. From the analysis of optical spectra, the crystal field strength Dq = 2175.2 cm-1 and Racah parameters A = 4065 cm(-1), B = 813 cm(-1), C = 3239 cm(-1) were determined for the Mn4+ ions. The value of the Huang-Rhys factor SFHR = 17.56 indicates a strong electron-phonon interaction of the Mn4+ ion. The presence of Mn4+ ions was further confirmed by measurement of EPR spectra at microwave frequencies 8.9 and 320 GHz. EPR data show that Mn4+ ions occupy two structurally nonequivalent lattice sites with the formation of two Mn4+ centers: Mn1 and Mn2. From the angular dependences of the EPR spectra, the components of the g factor and the hyperfine interaction tensor A, the parameters of the axial D and rhombic E crystal field were determined for both centers. The axial constant D is large for both centers: 0.644 and 0.560 cm(-1), respectively. Analysis of optical and EPR spectroscopic data testifies that Mn4+ ions replace the Ge4+ ions in oxygen octahedrons within the host. Structurally nonequivalent centers are attributed to manganese ions located in undistorted lattice sites (Mn1) and in sites perturbed by the Li-Na antisite defect (Mn2).

The pulse electron paramagnetic resonance (EPR) is widely used in different branches of material and life sciences, including promising applications in quantum information processing and quantum sensing. Here, we study the effect of the high polarizability of KTaO3 and SrTiO3 quantum paraelectrics on local electric and magnetic field components of microwaves (MW) at Fe3+ and Mn2+ paramagnetic ions. The measurements are performed with a commercial EPR spectrometer using dielectric and split-ring resonators. It is found that the power of MW pulses used in coherent spin manipulation at nanoseconds timescale decreases to milliwatts as compared to the tens–hundreds of watts usually used for spins in conventional materials. The amplification of MW fields is related to the very high dielectric permittivity (up to 25 000 in SrTiO3) of quantum paraelectrics at GHz frequencies and temperatures below 20 K. This creates the large induced polarization and, thus, huge displacement current and in turn the secondary MW magnetic field. Numerical simulations support the observation of the enhanced magnetic MW field in the high-permittivity sample. The low MW power for excitation of spin transitions in quantum paraelectrics eliminates the requirement of expensive high-power MW equipment. This approach also allows to globally control spin qubits in tandem with integrated devices based on conventional semiconductor MW circuits working at mW powers. It is suggested that quantum paraelectrics can also be used as substrates for deposition of nanoparticles or films of other materials, which would be manipulated by the low-power MW pulses.

Mn4+, Cr3+, and Fe3+ electron paramagnetic resonance and 47,49Ti and 207Pb nuclear magnetic resonance (NMR) measurements are carried out in PbTiO3 single crystals at temperatures from 300 K down to 20 K. It was found that the tetragonal crystal-field parameter of the Mn4+ and Cr3+ impurity ions undergoes an unexpected change in its temperature dependence at T < 170 K: it increases with decreasing temperature to 150-170 K and then strongly decreases with further temperature lowering. Electric-field gradient at Ti site measured from 47,49Ti NMR shows nonuniform change with decreasing temperature: it sharply increases with decreasing temperature to 150-170 K and is practically temperature independent below these temperatures. Moreover, substantial broadening of NMR linewidth is seen for Ti and Pb nuclei at T < 170 K due to essential increase of lattice parameters fluctuations. All these facts may indicate proximity of a transition to another phase with barely perceptible crystal-structure modification as reported in the paper by Kobayashi et al. [Phys. Rev. B 28, 3866 (1983)] while, actually, it was not detected in NMR. The experimental data are compared with those obtained from ab initio calculations.

This paper reports a detailed study of Li+ incorporation and electron and hole trapping in Li+-codoped Y3Al5O12:Ce garnet scintillation crystals, using the combination of the local probe EPR and NMR methods with optical characterization.

This work reports the results of EPR and NMR study of the Ce3+ incorporation in LaAlO3 single crystals grown by the micro-pulling-down method in the range of the Ce concentrations in the solid solution La1-xCexAlO3 from x = 0.001 up to x = 1.0. From EPR measurements, Ce3+ g tensor parameters were determined as a function of Ce concentration. The g tensor has an orthorhombic symmetry even in the trigonal phase (x < 0.1) suggesting that the incorporation of Ce at La site lowers the lattice symmetry near this ion. The local properties of the La1-xCexAlO3 crystals were further studied by Al-27 and La-139 high-resolution NMR measurements. It was found that La-139 chemical shift has the Fermi contact interaction origin. It linearly increases with the Ce concentration from 0 ppm up to 165 ppm at x = 0.5. Due to this strong Fermi contact interaction, separated peaks corresponding to different Ce-O-La spin transfer passways are resolved in the La-139 NMR spectra. On the other hand, no Fermi contact interaction is visible in Al-27 NMR spectra. However, these spectra contain a satellite peak whose intensity linearly increases with an increase of the Ce concentration leaving position of this peak unchanged. This was interpreted as manifestation of the crystal structure modification in the vicinity of Ce ions in agreement with EPR data. Thus, optical properties of Ce3+ in LaAlO3 will be determined namely by the local crystal structure near this ion.

The B-site-doping method of barium titanate (BaTiO3) is one of the promising route to prepare lead-free materials with enhanced dielectric and piezoelectric properties. Lead-free (Ba0.85Ca0.15)(Zr0.1-xSnxTi0.9)O-3 [BCZT:Sn] (x = 0, 0.02, 0.04 and 0.06) ceramics were synthesized using the sol-gel method. The effects of Sn content on the energy-storage performance and electric conduction mechanisms of BCZT ceramic were systematically investigated. The energy storage performance investigation showed that the recoverable energy storage has been enhanced with Sn doping rate, the composition doped x = 0.02 (BCZT: 2Sn) depicted the highest recoverable energy density and efficiency (W-rec = 19 mJ/cm(3), eta = 81.65%). The electrical properties of the BCZT:Sn ceramics were investigated using the impedance spectroscopy technique at temperature range of 25-450 degrees C. The net impedance of the samples showed a significant enhancement as the Sn content increases, owing to the lattice distortion created by the relative difference in the radius of Sn4+ and Zr4+ and different outer electronic shells. The AC conductivity was measured and analyzed as a function of frequency and temperature. Obtained activation energy values were associated with possible conduction mechanisms. (C) 2021 Elsevier Ltd. All rights reserved.

We present a study of KTaO3:Er (approximate to 0.05%) single crystals based on a multifaceted approach including the use of optical absorption, far-infrared reflectivity, electron paramagnetic resonance, photoluminescence spectra, and ab initio simulations. We describe briefly the fundamental consequences of Er doping, in particular, stiffening of TO1 soft mode of KTaO3. We provide information about energy level structures controlling f - f optical transitions in Er3+, on formation of minor cubic octahedral and major orthorhombic Er3+ centers. It is revealed that the temperature shift of narrow zero-phonon emission lines is strikingly unusual being much larger than that typical for trivalent rare-earth impurities.

Single crystals of LiCaAlF6 undoped and Eu, Na co-doped were studied by electron paramagnetic resonance, radioluminescence and thermally stimulated luminescence techniques applied in a correlated manner. The undoped samples exposed to X-ray irradiation exhibited two hole-like charge trapping centers creation, the molecular ions of the form: ClF- and F2^- - F2^- dimer. Their trap depths and frequency factors were determined as follows: Et1=1.7 eV and Et2=1.1. eV for trap depths and f ~ 10^13 s-1 for frequency factor, respectively. It was found that the europium preferable charge state is 2+ in the LiCaAlF6:Eu,Na samples, however, some amount of the Eu3+ is also present. Moreover, there were two Eu2+ centers: the dominating Eu2+(Ca) and the low-content Eu2+(Li). The amount of the latter is easily governed by the sodium admixture while the former is insensitive to the Na co-doping. Eu and Na co-doping affected the defects distribution and incorporation in the LiCaAlF6 host.

The origin and thermal stability of charge trapping centers were studied in Li2MoO4 cryogenic scintillators by correlated electron paramagnetic resonance (EPR) and thermally stimulated luminescence (TSL) measurements. Up to five electron and three hole trapping centers were detected in the crystals X-ray irradiated at 77 K. The electron traps were ascribed to MoO43- molecular complexes, unperturbed and perturbed by neighboring defects, while the hole traps - to O- lattice ions perturbed by lithium and molybdenum vacancies. The thermal stability of the trapping centers was studied and the depths of electron and hole traps were determined. The TSL peaks observed in the 77-300 K temperature range were related to the depletion of the detected paramagnetic centers. Influence of the revealed centers on the scintillation light yield is discussed.

A PbMoO4 single crystal as a perspective detector for neutrinoless double beta decay experiment was grown by the Czochralski technique from high purity raw materials including archaeological lead. Charge trapping and energy transfer phenomena in this crystal have been studied by electron paramagnetic resonance (EPR) and wavelength-resolved thermally stimulated luminescence (TSL). EPR revealed several impurities, which however do not participate in the charge trapping processes: Gd3+, Cr3+, Mn2+. Self-trapped electron and (MoO4)3--VPb centers creation under laser light was confirmed. At least six glow peaks related to the thermal release of the charge carriers were observed having maxima at 41 K, 53 K, 83 K, 90 K, 105 K and 118 K. The peak at 41 K was ascribed to the de-trapping of self-trapped electrons. The 53 K one, in particular, is created by re-trapping processes. Partial cleaning procedure followed by the initial rise method allow to determine the trap depths and frequency factors for the 90 K and 105 K glow peaks. They have been referred to the (MoO4)3--VPb decay. A three-component analysis of the spectra could explain the observed 100 nm red shift of the thermo-luminescence emission maximum.

F+ center, an electron trapped at oxygen vacancy (VO), was investigated in the oxygen deficient Y3Al5O12 (YAG) crystals by EPR. The measurements were performed at temperatures 5-450 K and frequencies 9.4-350 GHz with using both the continue wave and pulse EPR technique. The pulse electron-nuclear double resonance was applied to resolve the hyperfine interaction of the trapped electron with surrounding nuclei. The measurements show that at low temperatures, T 200 K, we observed delocalization of the electron into the conduction band with the activation energy about 0.4-0.5 eV that resulted in substantial narrowing of the EPR spectral line with simultaneous change of its shape from the Gaussian to Lorentzian due to diminish up to zero of the Fermi contact hyperfine field at 27Al and 89Y nuclei. Such temperature behavior of the F+-center electron in YAG is completely similar to behavior of a donor electron in a semiconductor. Our findings is further supported by measurements of the conductivity and dielectric properties. In particular, these data show that the conduction electrons are not homogeneously distributed in the crystal: there are high-conductive regions separated by poorly-conductive dielectric layers. This leads to the so-called Maxwell-Wagner dielectric relaxation with huge apparent dielectric constant at low frequencies.

The present work reports results of the electron paramagnetic resonance (EPR), optical absorption, radio- and photoluminescence (RL and PL) complex study of the Lu2Si2O7:Pr and Lu2Si2O7:Ce pyrosilicate crystals. In both crystals, the EPR spectra demonstrate the presence of characteristic signals originating from the Yb3+, Er3+, Nd3+, Dy3+, Gd3+ and V3+ ions existing in the material as uncontrolled impurities. The corresponding spectra (except for Gd3+) have been analysed in detail and g- and hyperfine tensors are determined for all these ions for the first time in the lutetium pyrosilicate lattice. Optical absorption, RL and PL measurements in the Lu2Si2O7:Pr crystal have shown only the characteristic Pr3+ transitions. In addition, the Pr3+- Pr3+ energy transfer was observed and confirmed experimentally.

Electron and hole trapping is studied in the Ce3+- and Pr3+-doped Lu2Si2O7 scintillation single crystals (LPS:Ce and LPS:Pr) by electron paramagnetic resonance (EPR). Detailed EPR measurements of the x-ray irradiated LPS crystals reveal that holes generated by irradiation are predominantly trapped at oxygen lattice ions creating O- centers. The same x-ray irradiation creates also electron-type centers, which are attributed to Lu2+ ions, where the trapped electron at the Lu lattice ion is stabilized by a nearby defect, such as the oxygen vacancy and Ir3+ impurity ion. Both the hole and electron centers can be thus considered as a bound small polaron, which makes the charge trapping in a scintillation mechanism quite competitive. The hole O- and electron Lu2+ centers show thermal stability well above room temperature. Thermal decays of their concentrations correlate well with the appearance of the thermally stimulated luminescence glow peaks at 470-550 K. The presence of the same intrinsic traps in the Ce- and Pr-doped LPS crystals suggests that the difference in the light yield of these crystals is an intrinsic property of the Ce3+ and Pr3+ activator centers in the LPS lattice. An origin of charge traps in this pyrosilicate structure and their role in the scintillation mechanism is compared with the results previously described in the literature on orthosilicates.

Charge trapping and energy transfer processes are investigated in PbMoO4 single crystals by electron paramagnetic resonance (EPR) and wavelength-resolved thermally stimulated luminescence (TSL) in a correlated manner. New signals produced by two differently perturbed Mo5+ centers (Mo2 and Mo3) were observed in EPR spectra measured in the crystals after 420 nm light irradiation. Two sets of spin-Hamiltonian parameters, g tensor and the Mo-95,Mo-97, Pb-207 (super) hyperfine tensors, have been determined and analyzed in terms of crystal field and LCAO-MO theories. A significant overlap of the Mo 5d(1) and 6s6p ligand Pb orbitals was deduced for the Mo3 whereas the Mo2 center seemed to be not or very slightly affected by the lead orbitals. The obtained TSL results allowed to suppose the existence of at least six glow peaks produced by the de-trapping of charge carrier traps either of intrinsic nature or somehow stabilized by nearby accidental impurities. The peaks having maxima at 51 K, 79 K, and 89 K, in particular, were attributed to the Mo2 and Mo3 centers thermal destruction due to the observed correspondence between the kinetic parameters (trap depths and frequency factors) determined separately for the glow peaks and the EPR intensity thermal decay curves of these centers. The Mo2 EPR decay curve is rather complex experiencing two-step trend probably due to trapped electron recombination with some holes released below 60 K. It was further confirmed by the TSL emission maximum relatively large red shift (similar to 100 nm) compared to the much smaller offset measured in radioluminescence. This phenomenon, observed also in samples obtained from extra-pure starting materials, was explained by three-component origin of the spectra, each having its own thermal fading rate. One of them ceased to exist above 60 K. The discussion of the obtained results is provided in comparison with other representatives of the scheelite tungstate and molybdate-based single crystals.

Scheme of the absorption and emission levels of Ce3+ and Pb2+ ions in the energy band scheme of the LuAP host (a) and fragment of the LuAP structure with CeLu, CeLu–CeLu, Pb2+ and Pt3+ centers (b).

Incorporation of the V3+ and V4+ ions into the yttrium aluminum garnet Y3Al5O12 (YAG) lattice has been studied by the correlated optical absorption and electron paramagnetic resonance (EPR) measurements. Only the V3+ ions at both octahedral and tetrahedral aluminum sites are found in as-grown crystals. However, annealing in air leads to the transformation of the V3+ to V4+, whereas annealing in the hydrogen atmosphere remains the V3+ concentration almost unchanged. Spin Hamiltonian parameters of the V3+ and V4+ ions at the tetrahedral sites including the zero field splitting and 51V hyperfine constants are determined using high-frequency, up to 300 GHz, EPR measurements. By using the spin Hamiltonian parameters, the ground state energy levels splitting of the V3+ and V4+ ions were calculated in the framework of the crystal field theory. The g and hyperfine tensors suggest that electron density around the tetrahedral V3+ ion is distributed strongly inhomogeneously resulting also in large (40%) reduction of the spin-orbit coupling constant whereas the tetrahedral V4+ ion exhibits weaker covalent bounding with surrounding ligands. Furthermore, the obtained EPR data on vanadium ions allow to improve identification of the optical absorption peaks in YAG:V crystals.

Charge trapping phenomena and recombination centers were studied in three Cs2HfCl6 single crystals of slightly different stoichiometry grown by the vertical Bridgeman method. Electron paramagnetic resonance (EPR) spectra measured both before and after X-ray irradiation show creation of two distinct V-k centers. One of them was Vk(a) already known from a recent work by "R Kral, V. Babin, E. Mihokov, M. Buryi, V. V. Laguta, K. Nitsch, and M. Nikl, Luminescence and Charge Trapping in Cs2HfCl6 Single Crystals: Optical and Magnetic Resonance Spectroscopy Study, J. Phys. Chem. C 121, 12375-12382 (2017)". Its quantity was different in each of the samples studied; the smallest, however, was in the one with the best stoichiometry. The second Vk center presently observed has never been described before. Its existence at an almost undetectable level was observed only in two of the three crystals. Thermally stimulated luminescence (TSL) spectra measured in the three samples were evidently composed of at least 2-3 strongly overlapped components within the 10-500 K temperature range. This suggested the existence recombination centers activated by the depletion of specific charge carrier traps. The corresponding TSL glow curves composed of seven complex peaks demonstrated significant decrease of the peaks amplitude in the sample with the best stoichiometry. Along with decreased radioluminescence amplitude, the combined EPR and TSL study allowed us to assume the reduction of both the recombination and trap center concentration with the increased crystal quality.

The processes of hole localization in the Y3Al5O12 and Lu3Al5O12 single crystals were investigated by electron paramagnetic resonance (EPR) and thermally stimulated luminescence (TSL). It was found that holes created by x-ray irradiation at 77 K are predominantly self-trapped at regular oxygen ions forming O- hole center. This self-trapped hole (STH) center is thermally stable to about 100 K in both YAG and LuAG crystals. At higher temperatures, thermally liberated holes are retrapped at oxygen ions in the vicinity of an acceptor ion such as Mg2+ and Al_{Y} or Al_{Lu} antisite ion that leads to increase of the thermal stability of the trapped hole to app. 150 K. TSL measurements show two composite glow peaks in the temperature range of 77 - 280 K, the temperature positions of which well correlate with the thermal stability of the O- centers. The hole thermal ionization energy was determined from a numerical fit of the TSL peaks within the model of second order kinetics. It is in the range of 0.25 - 0.26 eV for the O- STH center, and increases to 0.41 - 0.45 eV for O- center stabilized by the acceptor. Revealed O- centers can be attributed to O- small polarons formed mainly due to the hole stabilization by short-range interaction with the surrounding lattice.