We report on the quantum yield (eta) and decay time (tau) measurements at room temperature for the bright red-orange (602 nm) luminescence from new germanium-vacancy (Ge-V) centers in nano- and microcrystalline diamonds synthesized at high pressure and high temperature. The values eta = 3 +/- 1% and tau = 6.2 +/- 0.2 ns were found. The Stokes shift measured as the energy difference between the maxima of the luminescence and luminescence excitation spectra is negligible. The relative intensity of the zero-phonon line constitutes up to 70% from the total intensity of the luminescence. Results of our ab initio DFT calculations for the ground-state electronic and vibrational structure of (Ge-V)(-) in diamond are presented and discussed.
We report an observation of coherent phonons of E-g(1), E-u(1), A(1g)(1), and E-g(2) symmetry generated in a single-crystal film of Bi2Se3 by an intense single-cycle THz pulse. The atomic vibrations reveal themselves through periodic modulation of the refractive index of the film. The largest signal is detected at the frequency of 4.05 THz that corresponds to the E-g(2) mode. The generation of E-g(2) phonons is interpreted as resonant excitation of the Raman mode by the second harmonic of THz-driven nonlinear E-u(1) oscillator, the fundamental frequency of which (2.05 THz) is approximately half that of E-g(2). The origin of nonlinearity in this case is cubic lattice anharmonicity, while generation of E-g(1) (1.1 THz) and A(1g)(1) (2.25 THz) phonons is a manifestation of quartic anharmonicity enhanced by the occasional combination relations between phonon frequencies in Bi2Se3.
The intercalation of H2O, CO2, and other fluid species in expandable clay minerals (smectites) may play a significant role in controlling the behavior of these species in geological C-sequestration and enhanced petroleum production and has been the subject of intensive study in recent years. This paper reports the results of a computational study of the effects of the properties of the charge balancing, exchangeable cations on H2O and CO2 intercalation in the smectite mineral, hectorite, in equilibrium with an H2O-saturated supercritical CO2 fluid under reservoir conditions using Grand Canonical Molecular Dynamics (GCMD) methods. The results show that the intercalation behavior is greatly different with cations with relatively low hydration energies and high affinities for CO2 (here Cs+) than with cations with higher hydration energies (here Ca2+). With Cs+, CO2 intercalation occurs in a 1-layer structure and does not require H2O intercalation, whereas with Ca2+ the presence of a sub-monolayer of H2O is required for CO2 intercalation. The computational results provide detailed structural, dynamical and energetic insight into the differences in intercalation behavior and are in excellent agreement with in situ experimental XRD, IR, quartz crystal microbalance, and NMR results for smectite materials obtained under reservoir conditions.
We construct a distribution function of the strain-tensor components induced by point defects in an elastically anisotropic continuum, which can be used to account quantitatively for many effects observed in different branches of condensed matter physics. Parameters of the derived six-dimensional generalized Lorentz distribution are expressed through the integrals computed over the array of strains. The distribution functions for the cubic diamond and elpasolite crystals and tetragonal crystals with the zircon and scheelite structures are presented. Our theoretical approach is supported by a successful modeling of specific line shapes of singlet-doublet transitions of the Tm3+ ions doped into ABO4 (A=Y, Lu; B=P, V) crystals with zircon structure, observed in high-resolution optical spectra. The values of the defect strengths of impurity Tm3+ ions in the oxygen surroundings, obtained as a result of this modeling, can be used in future studies of random strains in different rare-earth oxides.
A novel triphenylamine derivative-linked ionic liquid unit, 1-(6-((4-(bis(4-(thiophen-2-yl)phenyl)amino)- benzoyl)oxy)hexyl)-3-methyl-imidazolium tetrafluoroborate (TTPAC6IL-BF4), was designed and synthesized successfully, and its corresponding polymer PTTPAC6IL-BF4 was obtained by the electropolymerization method. The highest occupied molecular orbital energy band of TTPAC6IL-BF4 is higher and the onset oxidative potential lower compared with that of 6-bromohexyl 4-(bis(4-(thiophen-2-yl)phenyl)amino) benzoate (TTPAC6Br) without modifying the ionic liquid unit. The results imply that introducing an ionic liquid unit to the side chain is an efficient method to improve the switching time of conjugated polymers and would be inspirational for the design and preparation of novel bifunctional electrochromic polymeric electrolytes.
We report on the first high-resolution optical spectroscopy study of LiYF4:Ho in an external magnetic field. Peculiarities in the hyperfine structure of holmium spectral lines are discussed for the cases H||c and H⊥c (H = 0.53 and 0.87 T). The spectra reveal a strong interaction between crystal-field levels, mediated by Zeeman and hyperfine terms in the Hamiltonian. A study of the magnetic-field-dependent isotope shifts in 7Li0.1 6Li0.9YF4:Ho (0.1 at.%) single crystals delivers an estimate of the difference in magnetic g factors for holmium centers with all 6Li isotopes in the nearest surrounding of Ho3+ (g(0)) and the centers having one 7Li isotope there (g(1)):g(1) − g(0) = 0.01 ± 0.005.
In the paper the content of individual elements (Fe, Co, Zr, Ca and F) contained in nanocomposites FeCoZr ferromagnetic alloy in the CaF2 transparent ceramics dielectric matrix, depending on the content of the metal phase x was determined by the X-ray diffusion microanalysis (EDX) method. The nanocomposites were made by sputtering by argon ions. Investigations of changes in the chemical composition of nanocomposites under the influence of high-temperature treatments were carried out using the thermogravimetry method in the temperature range from 25 °C to 1000 °C with a temperature increase rate of 10 °C/min. On the basis of the research, a model of changes of the structural-phase state of nanogranular layers of ferromagnetic alloy Fe45Co45Zr10 in the transparent ceramics CaF2 matrix occurring under the influence of high-temperature treatments was proposed.
The damage and structural state of the surface layer of Al–Li–Mg samples composed of Al–5% Mg–2% Li (wt %) under pulsed action of power streams of high-temperature deuterium plasma and highenergy deuterium ions in the Plasma Focus (PF) device have been investigated. The radiation power density was q ~ 106 W/cm2; the pulse duration was 50–100 ns. Pulsed thermal heating and rapid cooling is established to lead to the melting and solidification of a thin surface layer of the alloy for several tens of nanoseconds. At the same time, in the superheated surface layer of the alloy, microcavities of a spherical shape are formed which is associated with intense evaporation of lithium into micropores within the heated layer. Thermal stresses caused by abrupt heating, melting, and cooling of a thin surface layer of metal result in formation of microcracks in the near-surface zone of the samples. The evaporation by the power electron beam of the elements of the anode material of the PF device (copper and tungsten) and their subsequent deposition onto the irradiated surface of the investigated samples in the form of droplets of submicron size are noted. It is shown that the thermal and radiationstimulated processes generated in the alloy under the action of pulsed energy fluxes in the implemented irradiation regime lead to the redistribution of elements in the surface layer of the aluminum solution, contributing to an increase in magnesium content and the formation of magnesium oxide on the surface.
A model of the thermo-field electron emission from the metal cathode with a thin insulating surface film at temperatures of 200–400 K is developed. An expression for the film emission efficiency in the gas discharge is obtained. The efficiency is equal to the fraction of electrons emitted into the film from the metal substrate, which enter the discharge volume and increase the effective secondary-electron emission yield of the cathode. It is shown that the thermo-field mechanism of electron emission influences noticeably the ignition voltage of the low-current discharge with such cathode at rather low temperatures exceeding the room temperature by less than 100 K.
Copper borate Cu3(BO3)2 is a complex compound with a layered crystallographic structure in which the Jahn-Teller active and magnetic copper Cu2+ ions occupy 16 nonequivalent positions in the unit cell displaying controversial magnetic behavior. In this paper, we report on the infrared and Raman spectroscopic studies of the lattice dynamics and the electronic structure of 3d9 copper states below the fundamental absorption band. The lattice dynamics is characterized by a large number of phonons due to a low P1 space-group symmetry and a large unit cell with Z = 10. An unusually rich set of phonons was found in the low-energy part of the infrared and Raman spectra below 100 cm−1, which we tentatively assign to interlayer vibrations activated by a crystal superstructure and/or to weak force constants for modes related to some structural groups. Several phonons show anomalous behavior in the vicinity of the magnetic phase transition at TN = 10 K, thus evidencing magnetoelastic interaction. No new phonons were found below TN, which excludes the spin-Peierls type of the magnetic transition. In the region of electronic transitions, a strong broad absorption band centered at ∼1.8 eVis observed, which we assign to overlapping of transitions between the 3d9 states of Cu2+ ions split by the crystal field in nonequivalent positions. The fundamental charge-transfer absorption band edge has a complex structure and is positioned around ∼2.8−3.0 eV.
The work presents a study of manganese-doped copper metaborate (Cu, Mn) B2O4 using optical spectroscopy. The temperature of the antiferromagnetic phase transition T-N = 19 K has been defined according to the absorption spectra. Polarization studies (Cu, Mn) B2O4 in isotropic ab-plane show the presence of linear antiferromagnetic dichroism in the magnetically ordered state previously observed in pure copper metaborate CuB2O4. This measurement allows to find the magnetic phase transition into an elliptical structure at the temperature T* = 7.0 K.
The mixture of argon and mercury vapor with temperature-dependent composition is used as the background gas in different types of gas discharge illuminating lamps. The aim of this work was to develop a model of the low-current discharge in an argon-mercury mixture at presence of a thin insulating film on the cathode and to investigate the influence of film on the discharge ignition voltage at low ambient temperatures. When discharge modeling, we used the obtained earlier expression which describes dependence of the mixture ionization coefficient on temperature. When there was a thin insulating film on the cathode the model took into account that positive charges are accumulated on its surface during the discharge. They generate an electric field in the film sufficient for the field emission of electrons from the metal substrate of the electrode into the insulator and some of them can overcome the potential barrier at the film outer boundary and go out in the discharge volume improving emission characteristics of the cathode. Calculations showed that at a temperature decrease the electric field strengthes in the discharge gap and the voltage in it are increased due to reduction of the saturated mercury vapor density in the mixture followed by the decrease of its ionization coefficient. Existence of a thin insulating film on the cathode surface results in an increase of the cathode effective secondary electron emission yield which compensates the reduction of the mixture ionization coefficient value. The results of discharge characteristics modeling demonstrate that in case of the cathode with an insulating film the discharge ignition becomes possible at a lower inter-electrode voltage. This ensures outdoor mercury lamp turning on at a reduced supply voltage and increases its reliability under low ambient temperatures.
In this study the process of a hot laboratory rolling of a round bar on the flat rolls was studied by laboratory
experiments and numerical modelling in order to evaluate the effect of boundary conditions and simulation techniques
on the model predictions. The computer simulations of rolling process were performed by two different techniques
based on Finite Element Method (FEM). The first technique solves three dimensional problems. The second one is
based on the sequential solution of a series of generalized plane problems. Each technique was used for solving of
isothermal forming task and non-isothermal one. The results were compared with laboratory rolling performed with
different reductions and at different temperatures. It was found that the difference of initial temperature is incon-
sequential to the prediction of strain and strain-rate distributions. This observation was confirmed experimentally.
Single crystal of TlCl was doped with NIR photoluminescent univalent bismuth cations by prolonged immersion in liquid bismuth metal. The ion exchange Tl+ + Bi0 ↔ Tl0 + Bi+ at the crystal surface with subsequent Bi+ migration to the bulk are expected to drive the doping process. Contrary with Bi‐doped TlCl crystals, grown by Bridgman method, the ion exchange does not produce the additional nonluminescent bismuth‐containing centers. The investigation of photoluminescence emission and excitation spectra lead to the conclusion, that Bi+ is the main NIR emissive center in Bi‐doped TlCl.
The work investigates some tribological parameters of the surfaces of friction pairs made of steel, with mineral layers. A comparative study of the friction coefficient of 18CrNiMo7-6 steel specimens with and without a mineral coating is performed. The lowest friction coefficient value was achieved for samples with mineral coating, without HFC hard surfacing before creating a mineral layer, and it was approximately 15% lower in comparison with the samples with HFC hard surfacing, but without mineral coating. The friction coefficient in the temperature range of 30…140 0С, with constant displacement speed of samples with mineral coatings is practically unchanged for every sample type, the variation range does not exceed 0.02, as contrasted with the friction coefficient of the samples with hard surfacing, but without mineral coating. The value of the coefficient of volumetric wear of an aluminum oxide toroid during rotation with disks with some types of mineral coatings was at the level of detection limit (less than 1.2x10-9 mm3/Nm), which is significantly less than the coefficient of volume wear of a toroid during rotation with disks without coating (4-7)х10-8 mm3/Nm.
The effect of isotopic modification of diamond lattice on photoluminescence (PL) and optical absorption spectra of ensembles of SiV− centers was studied. Thin epitaxial diamond layers were grown by a microwave plasma CH4/H2 mixtures using methane enriched to 99.96% for either 12C or 13C isotopes, while the Si doping was performed by adding a small percentage of silane SiH4 into the plasma. Temperature dependent SiV−ZPL spectra in absorption were measured at 3–80 K to monitor the evolution of the ZPL fine structure. It is found that the SiV− ZPL at 736.9 nm observed in PL for 12C diamond at T = 5 K, exhibits a blue shift of 1.78 meV, to 736.1 nm in 13C diamond matrix. Narrow ZPL with the width (FWHM) of 0.09 meV (21 GHz) was measured in absorption spectra at T = 3–30 K in the Si‐doped 13C diamond. Besides the charged SiV− center, the absorption of the neutral SiV0 defect at 946 nm wavelength has also been detected. From changes observed in SiV− phonon band structure in PL with isotopic modification, the band at 64 meV was confirmed to be a local vibration mode (LVM) involving a Si atom.
We present the results of temperature- and polarization-dependent high-resolution optical spectroscopy studies of DyFe3(BO3)4 performed in spectral ranges 40-300cm-1 and 3000-23000cm-1. The crystal-field (CF) parameters for the Dy3+ ions in the P3121 (P3221) phase of DyFe3(BO3)4 are obtained from calculations based on the analysis of the measured f-f transitions. Recently, quadrupole helix chirality and its domain structure was observed in resonant x-ray diffraction experiments on DyFe3(BO3)4 using circularly polarized x rays [T. Usui, Y. Tanaka, H. Nakajima, M. Taguchi, A. Chainani, M. Oura, S. Shin, N. Katayama, H. Sawa, Y. Wakabayashi, and T. Kimura, Nat. Mater. 13, 611 (2014)10.1038/nmat3942]. Using the obtained set of the CF parameters, we calculate temperature dependencies of the electronic quadrupole moments of the Dy3+ ions induced by the low-symmetry (C2) CF component and show that the quadrupole helix chirality can be explained quantitatively. We also consider the temperature dependencies of the bulk magnetic dc-susceptibility and the helix chirality of the single-site magnetic susceptibility tensors of the Dy3+ ions in the paramagnetic P3121 (P3221) phase and suggest the neutron and resonant x-ray diffraction experiments in a magnetic field to reveal the helix chirality of field-induced magnetic moments.
Professor Yuri E. Gorbatywas born 30 July 1932 in the city Grozny, in the Soviet Union. He has graduated from the Mendeleev Institute of Chemical Technology,Moscow, in 1955. He has got his Candidate of Sciences (Ph.D.) degree in 1963 for his work on “Non-equilibrium crystallization of the three-componentmelts”, and later in 1988 he was awarded a Doctor of Sciences degree for the work “The effect of temperature and pressure on the nearest ordering in liquid and supercritical water”. Between these two dates and then later in his scientific career Yuri E. Gorbaty has become one of the leading experts in the field of experimental studies of the structure and properties of fluids, especially aqueous fluids at high temperatures and pressures, by methods of IR and Raman spectroscopy and by X-ray diffraction.
We study the effect of periodic, spatially uniform temperature variation on mechanical properties and structural relaxation of amorphous alloys using molecular dynamics simulations. The disordered material is modeled via a non-additive binary mixture, which is annealed from the liquid to the glassy state with various cooling rates and then either aged at constant temperature or subjected to thermal treatment. We found that in comparison to aged samples, thermal cycling with respect to a reference temperature of approximately half the glass transition temperature leads to more relaxed states with lower levels of potential energy. The largest energy decrease was observed for rapidly quenched glasses cycled with the thermal amplitude slightly smaller than the reference temperature. Following the thermal treatment, the mechanical properties were probed via uniaxial tensile strain at the reference temperature and constant pressure. The numerical results indicate an inverse correlation between the levels of potential energy and values of the elastic modulus and yield stress as a function of the thermal amplitude.
The change in the structure of solid hydrogen upon compression along the isotherm of 100 K near the transition to the conducting state has been investigated within the density-functional theory. The dependences of pressure and electrical conductivity on the hydrogen density have been calculated. The pressure range from 602 to 836 GPa has been found where the first peak of the pair correlation function arises at a distance of 0.92 Å, which corresponds to the interatomic distance in the molecular H3+ ion. Notably, this distance does not change with an increase in density. A sharp increase in the electrical conductivity is also observed.
We propose and develop a classical density functional theory for the description of a minor amount of water dissolved in ionic liquid in the vicinity of an electrode. In addition to the electrostatic energy and lattice-gas mixing entropy terms, the utilised grand canonical potential contains several phenomenological terms/parameters that describe short-range interactions between ions of ionic liquid, water molecules and the electrode. Some of these have been earlier introduced in the theory of electrical double layer in pure ionic liquids. Based on this, we investigate the role of the remaining ’specific interaction’ parameters e those that characterize possible (i) specific interaction of ions and molecules with the electrode, which are responsible for their specific adsorption; and (ii) hydrophilicity/hydrophobicity of ions. As a result we obtain water electrosorption isotherms as a function of the potential drop across the electrical double layer, investigate its asymmetry with respect to the sign of electrode potential, and establish the relationship between the sign of this asymmetry and hydrophobicity/hydrophilicity of cations and anions. We also calculate the effect of water electrosorption on the double layer differential capacitance which brings clear new features to its voltage dependence, some of which have been already experimentally observed.