The structure of fullerite C<Subscript>60</Subscript> intercalated with molecular oxygen

A (O₂) _x C₆₀ sample with a high content of oxygen (x ≥ 0.4) and free of technological solvent impurities was obtained by precipitation from solution. For the first time, the results of the determination of the x coefficients using ¹³C NMR and elemental analysis were compared. It was shown by Raman spectroscopy, mass spectrometry, and NMR that the inclusion of oxygen into fullerite was accompanied by a decrease in the frequency of O=O stretching vibrations by no less than 12 cm⁻¹ compared with gaseous O₂. Nevertheless, oxygen exists in the molecular form in (O₂)_0.4C₆₀ and is released in the form of O₂ as the sample is heated to 373 K. The number of oxygen molecules occupying octahedral pores closets to the fullerene molecule takes on all the possible values, from 0 to 6. At room temperature, the (O₂) _x C₆₀ sample lost oxygen much more slowly than similar products prepared by diffusion saturation of pure fullerite with oxygen.     

Dislocation mobility in C<Subscript>60</Subscript> fullerite crystals

The dependences of the path of leading dislocations in indentation rosette rays on the load, the loading time, and the indentation temperature in the range 260 < T ≤ 373 K were studied for C₆₀ fullerite crystals. The dislocation mobility parameters are estimated: the exponent m characterizing the stress dependence of the dislocation velocity depends on the structural perfection of the crystal and ranges from 2.3 to 24.5, the activation energy for dislocation motion ΔH ₀ ? (0.4–0.5) eV, and the velocity of leading dislocations in indentation rosette rays v _l ? 10^?5?10^?4 cm/s. The data from micro-and macromechanical experiments are shown to agree with each other. The dislocation mobility is assumed to be controlled by the dislocation interaction with local barriers.     

Superparamagnetism of magnetite particles in C<Subscript>60</Subscript> fullerite powder

A new magnetic material, C₆₀ fullerite powder doped by magnetite (Fe₃O₄) nanoparticles, is obtained by heating a mixture of fullerite and iron(III) acetylacetonate. It is shown that the material offers superparamagnetic properties. Surface bonding between the nanoparticles and the fullerite is established.     

Luminescence and thermally stimulated recombination processes in Li<Subscript>6</Subscript>Gd(BO<Subscript>3</Subscript>)<Subscript>3</Subscript>:Ce<Superscript>3+</Superscript> crystals

The luminescence and thermally stimulated recombination processes in lithium borate crystals Li₆Gd(BO₃)₃ and Li₆Gd(BO₃)₃:Ce have been studied. The steady-state luminescence spectra under X-ray excitation (X-ray luminescence), temperature dependences of the intensity of steady-state X-ray luminescence (XL), and thermally stimulated luminescence (TSL) spectra of these compounds have been investigated in the temperature range of 90–500 K. The intrinsic-luminescence 312-nm band, which is due to the ⁶ P _J → ⁸ S _7/2 transitions in Gd³⁺ matrix ions, dominates in the X-ray luminescence spectra of these crystals; in addition, there is a wide complex band at 400–420 nm, which is due to the d → f transitions in Ce³⁺ impurity ions. It is found that the steady-state XL intensity in these bands increases several times upon heating from 100 to 400 K. The possible mechanisms of the observed temperature dependence of the steady-state XL intensity and their correlation with the features of electronic-excitation energy transfer in these crystals are discussed. The main complex TSL peak at 110–160 K and a number of minor peaks, whose composition and structure depend on the crystal type, have been found in all crystals studied. The nature of the shallow traps that are responsible for TSL at temperatures below room temperature and their relation with defects in the lithium cation sublattice are discussed.     

Three types of photomagnetic effects in Sp<Subscript>3</Subscript>Cr(C<Subscript>2</Subscript>O<Subscript>4</Subscript>)<Subscript>3</Subscript> crystals

Three different photomagnetic effects caused by ultraviolet light in paramagnetic crystals based on molecules of spiropyrans (Sp) Sp₃Cr(C₂O₄)₃ and SpI have been revealed and separated: (1) in the high-temperature range (30–300 K), the photomagnetic effect in Sp₃Cr(C₂O₄)₃ is determined by the charge transfer between chromium ions and spiropyran molecules; (2) in the low-temperature range (2 K), the photomagnetic effect in Sp₃Cr(C₂O₄)₃ is due to the photoisomerization of spiropyran molecules, the change in the crystal field, and the splitting of the levels of Cr³⁺ ions in zero field; and (3) in the temperature range 2–20 K, the generation of radiation-induced paramagnetic defects contributes to the magnetic moment of the organic sublattice Sp⁺.     

Defect Structure and Up-conversion Luminescence Properties of ZrO<Subscript>2</Subscript>:Yb<Superscript>3+</Superscript>,Er<Superscript>3+</Superscript> Nanomaterials

The up-converting ZrO₂:Yb³⁺,Er³⁺ nanomaterials were prepared with the combustion and sol–gel methods. FT-IR spectroscopy was used for analyzing the impurities. The crystal structures were characterized with X-ray powder diffraction and the mean crystallite sizes were estimated with the Scherrer formula. Up-conversion luminescence measurements were made at room temperature with IR-laser excitation at 977 nm. The IR spectra revealed the conventional and OH⁻ impurities for the combustion synthesis products. The structure of the ZrO₂:Yb³⁺, Er³⁺ nanomaterials was cubic except for the minor monoclinic and tetragonal impurities obtained with the sol–gel method. The materials showed red (650–700 nm) and green (520–560 nm) up-conversion luminescence due to the ⁴F_9/2→⁴I_15/2 and (²H_11/2, ⁴S_3/2)→⁴I_15/2 transitions of Er³⁺, respectively. The products obtained with the combustion synthesis exhibited the most intense luminescence intensity and showed considerable afterglow.     

Magnetic and thermal properties of single-crystal NdFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript>

The neodymium ferroborate NdFe₃(BO₃)₄ undergoes an antiferromagnetic transition at T _N = 30 K, which manifests itself as a λ-type anomaly in the temperature dependence of the specific heat C and as inflection points in the temperature dependences of the magnetic susceptibility χ measured at various directions of an applied magnetic field with respect to the crystallographic axes of the sample. Magnetic ordering occurs only in the subsystem of Fe³⁺ ions, whereas the subsystem of Nd³⁺ ions remains polarized by the magnetic field of the iron subsystem. A change in the population of the levels of the ground Kramers doublet of neodymium ions manifests itself as Schottky-type anomalies in the C(T) and χ(T) dependences at low temperatures. At low temperatures, the magnetic properties of single-crystal NdFe₃(BO₃)₄ are substantially anisotropic, which is determined by the anisotropic contribution of the rare-earth subsystem to the magnetization. The experimental data obtained are used to propose a model for the magnetic structure of NdFe₃(BO₃)₄.     

Recent results on hyperpolarized<Superscript>3</Superscript>He−<Superscript>4</Superscript>He liquid mixtures

Laser optical pumping in low magnetic field provides very high nuclear polarizations in gaseous helium mixtures, and is used to prepare polarized liquid. Wall relaxation in glass cells is effiently reduced using cesium coatings, and bulk longitudinal relaxation times are measured. In highly magnetized samples, dipolar fields control the spin dynamics in anisotropic volumes and weak external magnetic field inhomogeneities. Long lived magnetostatic modes are observed by pulsed NMR. Detailed analysis of their frequency and damping gives information on magnetization density and spin diffusion coefficient in polarized mixtures. Experiments are performed above 0.2 K on mixtures with³He concentrations of order a few percents or larger. When phase separation occurs, the³He-rich phase retains a high polarization.     

Spin Kinetics of Liquid <Superscript>3</Superscript>He in Contact with a DyF<Subscript>3</Subscript> Micropowder at Ferromagnetic Ordering of Dy<Superscript>3+</Superscript> Ions

The spin kinetics of liquid ³He in contact with a mixture of LaF₃ (99.67%) and DyF₃ (0.33%) micropowders at temperatures of 1.5–3 K has been studied by pulsed nuclear magnetic resonance (NMR). The DyF3 is a dipolar dielectric ferromagnet with the phase transition temperature T_c= 2.55 K, whereas the diamagnetic fluoride LaF₃ is a diluting substance for the optimal observation conditions of ³Не NMR in powder pores. The magnetic phase transition in DyF₃ is accompanied by a considerable change in the character of fluctuations of the magnetic moments of dysprosium ions, which affect the spin kinetics of ³Не in contact with the substrate. Significant changes in the relaxations rates of the longitudinal and transverse magnetizations of 3Не have been discovered in the region of magnetic ordering of the solid matrix. The technique of studying the static and fluctuating magnetic fields of a solid matrix at low temperatures using liquid ³He as a probe has been proposed.     

A miniature tunable TEA CO<Subscript>2</Subscript> laser using isotope <Superscript>13</Superscript>C<Superscript>16</Superscript>O<Subscript>2</Subscript> and <Superscript>12</Superscript>C<Superscript>16</Superscript>O<Subscript>2</Subscript>

A miniature tunable TEA CO₂ laser using isotope ¹³C¹⁶O₂ as the active medium is developed to extend the spectral range of CO₂ lasers for further application. The optimization of the energy parameters of the tunable TEA ¹³C¹⁶O₂ laser and the same laser using ¹²C¹⁶O₂ are studied. When a gas mixture (¹³C¹⁶O₂: N₂: He = 1: 1: 3) at a total pressure of 6.4 × 10⁴ Pa is used, the TEA ¹³C¹⁶O₂ laser of a 45-cm³ active volume obtains 51 emission lines in the [00⁰1–10⁰0] and [00⁰1–02⁰0] bands. The maximum pulse energy of the TEA ¹³C¹⁶O₂ laser is about 357 mJ. The same laser using the conventional gas mixture (¹²C¹⁶O₂: N₂: He = 1: 1: 3) at a pressure of 6.66 × 10⁴ Pa is measured to obtain 69 laser emission lines and the maximum pulse energy of laser radiation is about 409 mJ.     

Cleavage of C<Subscript>60</Subscript> fullerite crystals

It is found that, under certain conditions, C₆₀ fullerite crystals can be cleaved along cleavage planes that are close-packed planes of the {111} type. Rigid gas-phase grown crystals exhibit good cleavage properties. In experiments with active compressive deformation, these crystals showed a high yield point τ_y = 2.65 MPa, a “parabolic” stress-strain curve, and brittle fracture after attaining a shear strain of about 8%. The fracture surface was clearly seen to have fragments parallel to the (111) plane. Typical microstructures observed in the cleavage plane are discussed: crystallographic cleavage steps, an indentation pattern, and a dislocation prick rosette. The fact that the activation volume V ? 60b³ is small (b is the Burgers vector of a dislocation) and strain-independent indicates the Peierls character of fullerite deformation or dislocation drag in a dense network of local defects.     

Observation of magnetic coupling between the nuclei of liquid <Superscript>3</Superscript>He and the <Superscript>141</Superscript>Pr nuclei of PrF<Subscript>3</Subscript> crystalline powder

The resonance magnetic coupling between the nuclei of liquid ³He and the ¹⁴¹Pr nuclei of a fine-dispersed powder of PrF₃ Van Vleck paramagnet with the grain size below 45 μm has been discovered at a temperature of 1.5 K with the use of a pulsed NMR technique. The magnetic specific heat of the corresponding spin systems is estimated theoretically.     

Structure of C<Subscript>60</Subscript> polymer coatings deposited via electron-beam dispersion of fullerite

The structure and surface of thin coatings deposited via electron-beam dispersion of the C₆₀ fullerite have been investigated using IR and Raman spectroscopy, mass spectroscopy, and atomic-force microscopy. It has been demonstrated that layers with different contents of the polymerized phase, crystals of the tetragonal polymer phase, and three-dimensional polymeric forms of the C₆₀ fullerene are formed under the conditions providing for irradiation by secondary electrons in vacuum at a substrate temperature of 300 K.     

LiNbO<Subscript>3</Subscript>:Yb<Superscript>3+</Superscript> crystals for optical temperature sensors

Possibilities of using lithium niobate crystals, doped with ytterbium (LN:Yb³⁺) as materials for optical temperature sensor (OTS), are discussed. We consider both the radiative and absorptive characteristics of the crystals, assuming that their temperature dependences are caused by the Boltzmann factor of the initial population of Stark sublevels of the ion. It is shown that the crystals can be used as materials for OTS in the temperature range 150–400 K.     

Influence of magnetic ordering on electronic structure of Tb<Superscript>3+</Superscript> ion in TbFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> crystal

Optical absorption spectra of trigonal crystal TbFe₃(BO₃)₄ have been studied in the region of ⁷F₆ → ⁵D₄ transition in Tb³⁺ ion depending on temperature (2–220 K) and on magnetic field (0–60 kOe). Splitting of the Tb³⁺ excited states, both under the influence of the external magnetic field and effective exchange field of the Fe-sublattice, have been determined. Landé factors of the excited states have been found. Stepwise splitting of one of the absorption lines has been discovered in the region of the Fe-sublattice magnetic ordering temperature. This is shown to be due to the abrupt change of equilibrium geometry of the local Tb³⁺ ion environment only in the excited state of the Tb³⁺ ion. In general, the magnetic ordering is accompanied by temperature variations of the Tb³⁺ local environment in the excited states. The crystal field splitting components have been identified. In particular, it has been shown that the ground state (in D ₃ symmetry approximation) consists of two close singlet states of A ₁ and A ₂ type, which are split and magnetized by effective exchange field of the Fe-sublattice. Orientations of magnetic moments of the excited electronic states relative to that of the ground state have been experimentally determined in the magnetically ordered state of the crystal. A pronounced shift of one of absorption lines has been observed in the vicinity of the TbFe₃(BO₃)₄ structural phase transition. The temperature interval of coexistence of the phases is about 3 K.     

Low-temperature instability of uniform spin precession in the <Emphasis Type="Italic">B</Emphasis> phase of pure <Superscript>3</Superscript>He and <Superscript>3</Superscript>He in an aerogel

The magnetic-field dependences of the threshold temperature of the low-temperature instability of uniform spin precession in pure ³He-B and ³He-B in an aerogel have been determined for the bulk mechanism. These dependences appear to be different. The theoretical dependence of the threshold temperature for the pure case has been compared with the experimental dependence. The threshold temperature of the instability for ³He in the aerogel has been estimated for typical experimental conditions.     

Infrared absorption spectra of pure and doped YAl<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> single crystals

Several weak absorption bands have been observed in the optical absorption spectra of pure and rare-earth-doped YAl₃(BO₃)₄ single crystals in the 3350– 3650 cm⁻¹ wave number region. Two of them, peaking at about 3377 cm⁻¹ and 3580 cm⁻¹ in the 8 K spectra, appear in most of the samples. They are tentatively attributed to the stretching mode of OH⁻ ions incorporated in the crystal during the growth. An additional absorption band at about 5250 cm⁻¹ at 8 K has also been detected in almost all samples. The temperature and polarization dependences of these bands, and their possible origin, are discussed.     

Thermal conductivity,heat capacity,and thermodynamic functions of the Bi<Subscript>4</Subscript>Ge<Subscript>3</Subscript>O<Subscript>12</Subscript> single crystal

The thermal conductivity and the heat capacity of a single crystal of bismuth orthogermanate Bi₄Ge₃O₁₂ have been experimentally investigated in the temperature ranges 50–300 and 56–300 K, respectively. The temperature dependences of the phonon mean free path, the characteristic Debye temperature, and the changes in the entropy and enthalpy have been calculated.     

Magnetic state of the structural separated anion-deficient La<Subscript>0.70</Subscript>Sr<Subscript>0.30</Subscript>MnO<Subscript>2.85</Subscript> manganite

The results of neutron diffraction studies of the La_0.70Sr_0.30MnO_2.85 compound and its behavior in an external magnetic field are stated. It is established that in the 4–300 K temperature range, two structural perovskite phases coexist in the sample, which differ in symmetry (groups R[`3]cR\bar 3c and I4/mcm). The reason for the phase separation is the clustering of oxygen vacancies. The temperature (4–300 K) and field (0–140 kOe) dependences of the specific magnetic moment are measured. It is found that in zero external field, the magnetic state of La_0.70Sr_0.30MnO_2.85 is a cluster spin glass, which is the result of frustration of Mn³⁺-O-Mn³⁺ exchange interactions. An increase in external magnetic field up to 10 kOe leads to fragmentation of ferromagnetic clusters and then to an increase in the degree of polarization of local spins of manganese and the emergence of long-range ferromagnetic order. With increasing magnetic field up to 140 kOe, the magnetic ordering temperature reaches 160 K. The causes of the structural and magnetic phase separation of this composition and formation mechanism of its spin-glass magnetic state are analyzed.     

Appearance of anomalous phase in <Superscript>4</Superscript>He crystal in the presence of small <Superscript>3</Superscript>He impurity

The kinetics of formation of an anomalous phase from a solution with a small ³He impurity was studied in the temperature range 0.2–0.7 K up to a supersaturation of ～30 mbar. The phase diagram was determined for normal and anomalous growths. It is found that, in the presence of impurities, the formation of a fast-growing state is retarded. This experimental fact indicates that the dissipative processes in fluids have an appreciable effect on the phase formation kinetics. The retardation is, possibly, caused by the direct interaction of an impurity with the crystal surface.