In this work, we study the possibility the use of Nd3+, Yb3+:CeF3/CeO2 nanoparticles in ratiometric luminescence thermometry. In order to explain the mechanism of the luminescence temperature sensitivity, we physically characterized the samples by means of transmission electron microscopy (TEM), X-ray diffraction (XRD), laser spectroscopy, and electron paramagnetic resonance (EPR).In particular, Nd3+, Yb3+:CeF3 nanoparticles were synthesized via co-precipitation method and annealed in air at 600 degrees C for 0, 15, 30, 60, and 120 min to obtain double-phase Nd3+, Yb3+:CeF3/CeO2 nanoparticles as well as single-phase Nd3+, Yb3+:CeO2 ones (at 120 min). The physical diameter of the samples gradually increases from 19 +/- 2 (doped CeF3) to 409 +/- 18 nm (doped CeO2). It was suggested, that the double-phase samples consist of sintered doped CeF3 and CeO2 nanoparticles having average grain diameter around 65 nm. The single-phase CeO2 sample also consists of sintered CeO2 nanoparticles, suggestively. The luminescence intensity ratio (LIR) was analyzed in the 80-320 K range (LIR = INd/IYb, where 848-925 nm ( 4 F 3/2 - 4 I 9/2 ) Nd3+ and 925-1048 nm ( 2 F 5/2 - 2 F 7/2 ) Yb3+). The maximal relative temperature sensitivity was achieved for Nd3+, Yb3+:CeO2 sample (-0.2 %/K), which is very competitive value. The LIR function has a simple linear temperature dependency in the broad 80-320 K which allows uniquely identifying the temperature at least in the studied broad temperature range. The mechanism of temperature sensitivity was suggested.

Here, the facile dry synthesis of composite CeO2/CeF3 nanoparticles doped with Er3+ probe ions using ammonium bifluoride (NH4HF2) is reported. This method allows synthesizing composite nanoparticles with different ratio of CeO2 and CeF3. The structural properties of the composite nanoparticles were studied using XRD, SEM, and TEM techniques, which confirmed the formation of composite CeO2/CeF3 nanoparticles. The luminescence spectra show that the intensity of the Ce3+ peak increases after fluorination procedure, revealing the fact that the amount of Ce3+ is increased. In particular, the total 4f-4f luminescence intensity of CeO2/CeF3:Er3+(0.1 at.%) nanoparticles is 3.5 time higher compared to CeO2:Er3+(0.1 at.%) ones under both UV or resonant pumping. Probably it can be associated with the increased concentration of trivalent rare-earth ions in a cubic-distorted environment. The same tendency is observed for the excitation spectra. EPR measurements showed that fluorination of CeO2 nanoparticles doped with different concentration of Er3+ ions leads to the formation of trigonal sites in ceria nanoparticles.

In this work, the physical characterization of LiGdxY1−xF4 (x = 0.05, 0.3, 0.7, and 1.0) and LiGdF4:Eu3+ microparticles was performed. The distribution coefficient of LiGdxY1−xF4 (x = 0.05) was determined for the first time (0.84). Based on kinetic characterization data, the LiGdF4 sample was chosen for further Eu3+ doping (0.1 and 1.0 at.%). For the LiGdF4:Eu3+ sample, Eu3+ emission was clearly observed under the excitation of Gd3+. This fact indicates an effective energy transfer from Gd3+ to Eu3+. The temperature-dependent spectral characterization of the LiGdF4:Eu3+ (1.0%) sample revealed that in the 30–250 K temperature range, a broad emission peak is evidenced. Its intensity sharply increases with the temperature decrease. We made a suggestion that this phenomenon is related to the irradiation-induced defects. The integrated luminescence intensity ratio of this broad peak and the Eu3+ emission were taken as temperature-dependent parameters. The sensitivity values are very competitive, and the first maximum occurs at 174 K (3.18%/K). The kinetic characteristics of both Gd3+ and Eu3+ did not demonstrate a notable temperature dependence. The LiGdF4:Eu3+ sample showed the possibility of being used as an optical temperature sensor, operating in the cryogenic temperature range.

In this work, we used a simple dry synthesis of composite nanostructures of $\mathrm{CeO}_{2} / \mathrm{CeF}_{3}$ doped with $\mathbf{E r}^{3+}$ or $\mathbf{N d}^{3+} / \mathbf{Y b}^{3+}$ using ammonium bifluoride $\left(\mathrm{NH}_{4} \mathrm{HF}_{2}\right)$ and annealing in air, respectively. This synthesis procedure makes it possible to obtain double-phase samples with different ratios of $\mathrm{CeO}_{2}\left(\mathrm{Ce}^{4+}\right)$ and $\mathrm{CeF}_{3}\left(\mathrm{Ce}^{3+}\right)$. As for the structure of $\mathrm{CeO}_{2} / \mathrm{CeF}_{3}: \mathbf{E r}^{3+}(\mathbf{0}.1$ at.%), the luminescence intensity of $\mathbf{C e}^{3+}$ and $\mathbf{E r}^{3+}$ ions increases 3.5 times after the fluorination procedure both with UV and resonance pumping, which indicates an increase in the concentration of $\mathrm{Ce}^{3+}$ and $\mathrm{Er}^{3+}$. In the $\mathrm{CeF}_{3}: \mathrm{Nd}^{3+}$ ($0.1 \mathrm{at}. \%$), $\mathbf{Y b}^{3+}$ (0.5 at. $\%$) nanoparticles, under $\mathrm{Nd}^{3+}$ excitation $\mathbf{Y b}^{3+}$ emission is not observed. However, after formation of double-phase $\mathrm{CeO}_{2} /$ CeF 3 : $\mathbf{N d}^{3+} / \mathbf{Y b}^{3+}$ samples, both $\mathbf{N d}^{3+}$ and $\mathbf{Y b}^{3+}$ emissions are observed.

A CsCaF 3 single crystal doped with Nd 3+ ions is studied using electron paramagnetic resonance spectroscopy. A paramagnetic Nd 3+ center of trigonal symmetry is recorded. The parameters of the corresponding spin Hamiltonians, the ground states and their wave functions are determined. Structural models of the observed complexes are proposed. The experimental results are analyzed in comparison with those for the same paramagnetic ion in other lattices.

The EPR spectroscopy and magnetization measurements were used to study the effect of annealing conditions on the local structure of Era+ ions in CeO2 : 1% Era+ nanoparticles. The nanoparticles were synthesized using the coprecipitation technique from an aqueous solution of cerium nitrate and hexamethylenetetramine. A correlation was found between the EPR spectra of the Era+ ions, magnetization and luminescence, depending on the annealing atmosphere, which proved the crucial role of oxygen vacancies in the origin of magnetism in CeO2 nanoparticles. EPR lines due to trigonal centers were clearly detected in CeO2 : 1% Era+ nanoparticles annealed in a vacuum, while no such lines were found for similar nanoparticles annealed under argon or air atmospheres.

Crystals of the UV lasers active media $\mathrm{Ce}^{3+}: \mathrm{LiCaAlF}_{6}$, which are characterized by the formation of several types of $\mathrm{Ce}^{3+}$ ion centers, have been studied. Presence of nonequivalent impurity centers was confirmed by detection of zero-phonon lines for two centers in low-temperature luminescence spectra, as well as from EPR spectra for various magnetic field orientations relative to the crystallographic axes of the samples. We have shown that for $\mathrm{Ce}^{3+}: \mathrm{LiCaAlF}_{6}$ crystals, with an increase in the concentration of $\mathrm{Ce}^{3+}$ ions in the melt, crystallization occurs in such a way that the concentration of impurity centers of lower symmetry increases to a greater extent than centers of higher symmetry. Moreover, laser oscillation belongs to precisely this type of center.

To measure magnetostriction in LiTmF4 and LiDyF4 single crystals, the acoustic resonance method was used. It is shown that the combination of capacitive dilatometry and the acoustic resonance method makes it possible to measure not only the field dependence of the crystal dimensions but also the field dependence of the sound speed.

We report the detailed study of the magnetocaloric effect (MCE) in a dipolar-Heisenberg magnet LiGdF4 using magnetization measurements performed on a single crystal sample. Entropy variation on isothermal demagnetization from the magnetic field up to 3 T is determined in the temperature range 2–10 K for two principal directions of the applied field (parallel and perpendicular to the tetragonal c-axis of the crystal). The MCE is found to be highly anisotropic, with the cooling efficiency being up to twice higher at H∥c. The results are nicely interpreted in the frame of a conventional molecular field approach taking into account considerable anisotropy of the paramagnetic Curie–Weiss temperature. These results are compared to earlier studies of MCE in powder samples of LiGdF4 (Numazawa et al., 2006) as well as with analogous data for other well known magnetocaloric materials. Our findings may open new possibilities to enhance the efficiency of magnetic refrigeration in the liquid helium-4 temperature range.

This work presents the results of experimental and theoretical studies of the magnetic properties of the LiErF4 single crystal and powder samples at low temperatures and applied field range of 0-9 T. The magnetization was calculated in the framework of the exchange-charge model taking into account dipole-dipole and electron-deformation interactions, with the calculation of the electron-deformation parameters. The theoretical analysis presents quantitative agreement in the temperature range of 2-300 K with the magnetization measurements of the LiErF4 samples.

The studied $\mathrm{YF}_{3}: \mathrm{Eu}^{3+}, \mathrm{Nd}^{3+}$ and $\mathrm{YF}_{3}: \mathrm{Eu}^{3+}$ nanoparticles demonstrated, that annealing in air at $400{ }^{\circ} \mathrm{C}$ for 4 hours increases the rise times by about 2 times and the luminescence decay times by about 1.2 times. Stronger temperature dependence of decay time was observed for samples without annealing. The addition of $\mathrm{Nd}^{3+}$ leads to the increase of temperature sensitivity of spectral characteristics of $\mathrm{YF}_{3}: \mathrm{Eu}^{3+}$ nanoparticles

The local environment of Yb3+ ions doped into ceria (CeO2) nanoparticles, synthesized by two different methods, was determined using EPR spectroscopy. It was found that the charge compensator for the Yb3+ ion located in the nearest environment can be a vacancy, a hydroxyl group, or a fluorine ion. The EPR spectra of Yb3+ ions at concentrations of 0.1 mol% and 0.5 mol% in ceria nanoparticles, prepared using the coprecipitation technique from an aqueous solution, are mainly due to cubic sites, which indicate the remote location of the charge compensators. The EPR lines resulting from trigonal sites in these samples suggested that the charge compensation is associated with a hydroxyl group. The FTIR measurements correlate with this supposition. After annealing the samples in a vacuum furnace, EPR lines from trigonal sites related to oxygen vacancies were observed. The second type of ceria nanoparticle was prepared by annealing CeF3 nanoparticles in air. EPR spectra of Yb³⁺ ions at concentrations of 0.1 mol% and 0.5 mol% in these samples revealed that the lines from trigonal sites were mainly associated with fluorine ions in the nearest environment.

YF3: (Eu3+, Nd3+) nanoparticles (orthorhombic phase, D~130 nm) were synthesized via the co-precipitation method, with subsequent hydrothermal treatment and annealing. The Eu3+ τdecay linearly descends with the increase of temperature in the 80–320 K range. The τdecay (T) slope values of the annealed YF3: Eu3+ (2.5 and 5.0 mol.%) nanoparticles were the highest (110·10−4 and 67·10−4, μs/K) in the whole 80–320 K range, respectively. Thus, these samples were chosen for further doping with Nd3+. The maximum Sa and Sr values based on the LIR (IEu/INd) function were 0.067 K−1 (at 80 K) and 0.86%·K−1 (at 154 K), respectively. As mentioned above, the single-doped YF3: Eu3+ (2.5%) nanoparticles showed the linearly decreasing τdecay (T) function (5D0–7F1 emission). The main idea of Nd3+ co-doping was to increase this slope value (as well as the sensitivity) by increasing the rate of τdecay (T) descent via the addition of one more temperature-dependent channel of 5D0 excited state depopulation. Indeed, we managed to increase the slope (Sa) to 180·10−4 K−1 at 80 K. This result is one of the highest compared to the world analogs.

The memory effect in the form of hysteresis has been detected in the measured dependence of the intensity of the photon echo in YLiF 4 and LuLiF 4 samples with Er 3+ impurity ions on the orientation, strength, and variation direction of the magnetic field. The prehistory of the location of a sample in the magnetic field with a certain direction and strength is written and stored for no less than 6 h at a temperature of 2 K. The effect crucially depends on the orientation of the optical axis of the sample with respect to the external magnetic field.

Microsized and nanosized powders of terbium tetrafluoride were synthesized. The decrease of Curie temperature of the nanosized sample was observed: TC = 2.88(4) K for the microsized sample, TC = 2.78(4) K for the nanosized sample. The field dependences of the magnetization of the microsized sample in magnetic fields up to 9 T at temperatures from 2 to 300 K were measured and calculated.

A CsCaF3 single crystal doped with Nd3+ ions is studied using electron paramagnetic resonance spectroscopy. A paramagnetic Nd3+ center of trigonal symmetry is recorded. The parameters of the corresponding spin Hamiltonians, the ground states and their wave functions are determined. Structural models of the observed complexes are proposed. The experimental results are analyzed in comparison with those for the same paramagnetic ion in other lattices.

Nd3+ (0.3 mol.%), Yb3+ (0, 1, 2, 3 and 5 mol.%): LiYF4 phosphors were grown by the Bridgman–Stockbarger technique. The luminescence intensity ratio (LIR) of Nd3+ (4F3/2–4I9/2, ~866 nm) and Yb3+ emission (2F5/2–2F7/2, ~980 nm) was taken as a parameter. The energy exchange between 4F3/2 (Nd3+) and 2F5/2 (Yb3+) occurs via phonons, which elucidates the LIR temperature dependence. The influence of the cross-relaxation process on the temperature sensitivity was estimated as negligible. The LIR function depends on the Yb3+ concentration at a fixed 0.3 mol.% Nd3+. The maximum Sa and Sr value were reached for Nd3+ (0.3%), Yb3+ (1.0%): LiYF4 (Sa = 0.007 K−1 at 320 K) and Nd3+ (0.3%), Yb3+ (5.0%): LiYF4 (Sr = 1, 1.03%*K−1 at 260 K), respectively.

This work presents the results of experimental and theoretical studies of the magnetic properties of the LiYbF4 single crystal and powder samples, and magnetoelastic properties of the LiYbF4 single crystal samples, at low temperature and applied field range of 0-9 T. The magnetostriction experimental data for the single crystal of LiYbF4 is published for the first time. The magnetization and magnetostriction were calculated in the framework of the exchange-charge model taking into account dipole–dipole and electron-deformation interactions, with the calculation of the electron-deformation parameters. The theoretical analysis presents quantitative agreement in the temperature range of 2–10 K with the magnetization and magnetostriction measurements of the LiYbF4 samples.

The opportunity of growing of several centimeters in length LiGdF4 single crystals by Bridgman-Stockbarger technique was demonstrated and the optically perfect LiGdF4 single crystals with 8 mm and 55 mm in length were grown. The phase composition, thermal conductivity, basic optical and spectral-kinetic properties of LiGdF4 single crystals were studied for the first time. For LiGdF4 crystal, the resulting linear thermal expansion co-efficients alpha is 11% larger, than for both LiYF4 and LiYbF4 crystals. The obtained thermal conductivity dependence k(T) is more pronounced compared to LiLuF4 and LiYF4. The absorption spectrum of the LiGdF4 crystal shows absorption bands corresponding to the 8S7/2 - (6Dj, 6Ij, and 6Pj) transitions of Gd3+ ions. The measured room temperature lifetime of 6Pj multiplet of Gd3+ ions was equal to 4.25 +/- 0.05 ms. The values of the temperature gradient of the LiGdF4 refractive index do not strongly depend on the wavelength.