Low-temperature helium defectoscopy and interaction of helium with ions of cerium gadolinium ceramics Ce<Subscript>0.8</Subscript>Gd<Subscript>0.2</Subscript>O<Subscript>1.9</Subscript> with a submicrocrystalline structure

The concentration of unbound neutral anion vacancies in cerium gadolinium ceramics Ce_0.8Gd_0.2O_1.9 with a submicrocrystalline structure has been determined using low-temperature helium defectoscopy at temperatures ranging from 613 to 773 K and at saturation pressures from 0.05 to 12 MPa. It has been found that the energy of dissociation of impurity-vacancy complexes is 1.1 ± 0.2 eV, and the energy of dissolution of helium in defects is −0.8 ± 0.2 eV. The obtained results have been compared both with the experimental data on the energy of interaction between helium and ions of the cerium gadolinium ceramics and with the results of the quantum-chemical calculations. It has been demonstrated that the anomalously low value of the energy of helium dissolution in the studied ceramic samples is determined by the chemical interaction of helium with the nearest environment of the cerium cations.     

Mass-spectrometric analysis of diffusion and solubility of helium in cerium-gadolinium ceramics with a submicrocrystalline structure

Diffusion and solubility of helium in Ce_0.8Gd_0.2O_{1.9 − δ} ceramics (δ = 0, 0.015) with a submicrocrystal structure are studied by thermodesorption of helium from preliminarily saturated (in the gas phase) crystals at temperatures of 613 and 673 K in the saturated pressure range 0–21 MPa. It is shown that, in this ceramics (δ = 0), the defect-trap diffusion mechanism operates. The main positions for dissolution are neutral anion vacancies formed as a result of thermal dissociation of impurity-vacancy complexes and saturated up to ∼1 × 10¹⁹ cm⁻³ at P = 6 MPa and T = 673 K. The dissociation energy of the complex and the energy of helium dissolution in the neutral anion vacancy are estimated at ∼2 eV and below −0.3 eV, respectively.     

The interaction of nitrogen with vacancies in molybdenum

The recovery of a nitrogen bombarded molybdenum single crystal is studied with thermal helium desorption spectrometry (THDS). A new recovery stage at T = 900 K is found, which is ascribed to the dissociation of nitrogen occupied monovacancies (NV). The dissociation is a first order process with activation energy E^D = 2.55 ± 0.05 eV; the binding energy of a nitrogen atom to a vacancy is estimated to be 1.35 ± 0.05 eV.     

The activation energy of single vacancy formation in platinum

The formation energy of single vacancies in platinum (purity 5 N) has been measured resistometrically in thin wires, quenched exponentially from temperatures 650 ÷ 1150 °C. Attention has been paid to the establishment of the effective temperature, corresponding to the total concentration of vacancies. In pure samples, the influence of this correction on E^F is rather small, whereas the uneven distribution of temperature along the specimen length is of greater importance. The size-effect correction partially improves the curvature of the Arrhenius plot at low temperatures. The quenching rate in helium gas was high enough to retain almost all vacancies present in the solid solution. For evalution of experimental data the method of least squares was used, taking into account the changing scale of the Arrhenius plot. The most probable value of formation energyE ^F is 1·31±0·05 eV; it is compared with results of other authors and the reason of large spread of experimentally found formation energies is discussed.     

Detection of surface defects under low-energy scattering

Computer simulation of Ga⁺ ion (E ₀ = 40–100 eV) scattering on a GaAs film with defects in the form of vacancies and K⁺ ions (E ₀ = 40–300 eV) on a V-target containing cerium has been carried out in the framework of the multiparticle interaction model. The simulation results show that low-energy scattering can be used as a tool for detection of surface defects.     

CeAlO<Subscript>3</Subscript> crystals: Preparation and study of their electrical and optical characteristics

Crystals of cerium aluminate with perovskite structure were obtained using the cold-crucible technique. The electrical and optical properties of cerium aluminate were studied in air in the range 300–1300 K. The main characteristics of CeAlO₃ at T=300 K are a follows: electrical conductivity σ=10^?7 S/cm, dielectric permittivity ?=3000–10000 (both measured at a frequency of 1000 Hz), thermal band-gap width ΔE=2.3±0.5 eV, and optical width δE=2.65±0.25 eV, which decreases at a rate of ?0.62×10^?3 eV/K with increasing temperature in the 300-to 1500-K interval.     

Emission and excitation spectra of ZnO:Ga and ZnO:Ga,N ceramics

The spectral characteristics of ZnO:Ga and ZnO:Ga,N ceramics prepared by uniaxial hot pressing have been investigated. At room temperature, the edge (exciton) band at 3.12 eV dominates in the luminescence spectra of ZnO:Ga, while a wide luminescence band at 2.37 eV, which is likely to be due to zinc vacancies, is observed in the spectra of ZnO:Ga,N. Upon heating, the edge band maximum shifts to lower energies and the bandwidth increases. The extrapolated position of the edge-band maximum at zero temperature, E _m (0) = 3.367 ± 0.005 eV, is in agreement with the data for thin zinc oxide films. The luminescence excitation spectra in the range from 3 to 6.5 eV are reported and the mechanism of energy transfer to excitons and luminescence centers is considered.     

O−−-Lücken-Dipole in Rubidiumhalogenidkristallen

Dielectric loss measurements are reported for RbCl, RbBr, and RbJ crystals containing O^??ions. The frequency and temperature dependence can be approximated by a Debye curve. The dipole moment is interpreted as being due to O^?? ions associated with anion vacancies. Activation energies for reorientation of the dipole are 0.60 eV for RbCl, 0.56 eV for RbBr, and 0.50 eV for RbJ. The deviations of the experimental dielectric loss curves from a Debye curve are interpreted. The dielectric losses decrease and the center uv absorption changes during annealing due to association of the centers into aggregates. A detailed analysis of the kinetics for RbBr indicates the formation of O^??-pair centers as the first step. The activation energy for association isE ₂=0.79 eV.     

Luminescence of LaBr3: Ce,Hf crystals under photon excitation in the ultraviolet,vacuum ultraviolet,and X-ray ranges

This study has been carried out using synchrotron radiation, time-resolved luminescence ultraviolet and vacuum ultraviolet spectroscopy, optical absorption spectroscopy, and thermal activation spectroscopy. It has been found that, in scintillation spectrometric crystals LaBr₃: Ce,Hf characterized by a low hygroscopicity, along with Ce³⁺ centers in regular lattice sites, there are Ce³⁺ centers located in the vicinity of the defects of the crystal structure. It has also been found that the studied crystals exhibit photoluminescence (PL) of new point defects responsible for a broad band at wavelengths of 500–600 nm in the PL spectra. The minimum energy of interband transitions in LaBr₃ is estimated as E _g ～ 6.2 eV. The effect of multiplication of electronic excitations has been observed in the range of PL excitation energies higher than 13 eV (more than 2E _g ). Thermal activation studies have revealed channels of electronic excitation energy transfer to Ce³⁺ impurity centers.     

Direct experimental study of the equilibrium diffusion of carbon atoms between the (100)Mo surface and the bulk

A direct experimental study of the diffusion of carbon atoms between the (100)Mo surface and the bulk has been carried out at process temperatures in the range 1400–2000 K, and the total balance of carbon atoms in the system has been determined. The difference in the activation energies of carbon dissolution and precipitation ΔE=E _S 1?E _{1 S} has been found under conditions of a dynamic equilibrium between both processes. This difference determines the temperature dependence of the degree of surface enrichment with carbon in reference to the bulk. The activation energy of the dissolution of carbon atoms has been determined in special experiments (E _S 1=3.9 eV), and the activation energy of the precipitation of carbon atoms E _{1 S} has been calculated (E _{1 S} =1.9 eV), which turns out to be close to the energy of carbon bulk diffusion in molybdenum.     

O−−-Lücken-Dipole in Alkalihalogenidkristallen

Dielectric loss measurements are reported for KBr, KCl, and NaCl crystals containing O^?? ions. The frequency and temperature dependences can be well represented by a Debye curve. The dipole moment found immediately after quenching is interpreted as being due to O^?? ions associated with anion vacancies. Activation energies for reorientation of the ion are E₀=0.59 eV for KCl, 0.54 eV for KBr and 0.61 eV for NaCl. Fluorescence bands were found in freshly quenched KCl crystals at 470 nm and 1.05 μm and are attributed to isolated dipoles. Dielectric losses decreased and the optical absorption changed during annealing due to the collection of centers into aggregates. A detailed analysis of the kinetics for KCl indicates the formation of double dipoles as the first step. The energy of association is E_A=0.85 eV. Earlier measurements of absorption, fluorescence and photochemical coloring are reinterpreted on the basis of the proposed model in which an O^?? center is thought of as a O^?? ion associated with an anion vacancy.     

Ab initio simulation of dissolution energy and carbon activity in fcc Fe

The equilibrium structure and properties of fcc iron with a carbon impurity were simulated ab initio using the WIEN2k software package. A procedure is proposed that enables the simulation of the magnetically disordered state of a system within the density functional theory. In the framework of this procedure, the value of the dissolution energy of carbon was calculated, which was 0.25 eV. Interaction energies between carbon atoms in the first, second, and third coordination spheres of each other were also determined, which were E ₁ = 0.06 eV, E ₂ = 0.1 eV, and E ₃ = 0.005 eV. To verify the reliability of the obtained energy values, the activity of carbon was calculated by the Monte Carlo method. A good qualitative agreement of the calculated activity with the experimental data indicates the reliability of the obtained energy parameters.     

A shock tube determination of the CN ground state dissociation energy and the CN violet electronic transition moment

The CN ground state (X²∑⁺) dissociation energy and the electronic transition moment of the CN violet B²∑⁺ ? X²∑⁺ bands have been simultaneously determined from spectral emission measurements behind incident shock waves. The unshocked test gases were composed of various CO₂-CO-N₂-Ar mixtures, and the temperatures behind the incident shocks ranged from 3500 to 8000°K. The dissociation energy was determined to be 7.89 eV with a statistical precision of ±0.02 eV; a conservative estimate of the absolute error was ±0.13 eV. The value obtained for the Δυ = 0 sequence electronic transition moment was 0.90 ± 0.14, corresponding to an electronic absorption f-number of 0.035 ± 0.005 at a wavelength of 3860 Å. The electronic transition moment variation with internuclear separation was also measured.     

Measurements of double differential cross sections in electron impact ionization of helium and argon

Double differential cross sections (angular distributions and energy loss spectra) have been measured of electrons after ionizing electron collisions with helium at primary energiesE ₀ between 25 eV and about 260 eV and with argon atE ₀=75, 150 and 200 eV. The spectra have been measured with an energy analyzing collector system of constant transmission. It was found that for high collision energies (E ₀≧ 80 eV) the outgoing electrons belong to one of the two energetically well separated groups, either thefast electrons, which are scattered mainly in forward direction or theslow electrons which are distributed isotropically into all angles. At low primary energiesE ₀ no separation into groups is possible. Several findings indicate the qualitative applicability of the binary collision model.     

Ionic conductivity and NMR relaxation study on Li3N single crystal

Both ionic conductivity and ⁷Li NMR relaxation time T₂ were studied on Li₃N single crystal samples containing an oxygen impurity. Owing to the O impurity, the samples showed a high ionic conductivity (σ⊥ = 0.83 S m^?1 and σ_∥= 0.013 S m^?1 at 300 K) and a low activation energy (E_a,⊥ = 0.19 eV and E_a,∥ = 0.24 eV). The intrinsic conduction accompanying vacancy generation within the Li₂N layer was observed in the high temperature region. In the extrinsic region only a motion of the Li ion located in the Li₂N layer is responsible for both σ_⊥ and σ_∥. The site exchange motion of two kinds of Li ion has little importance to the ionic conduction.     

Investigation of impurity levels in thin polycrystalline SmS films

Data obtained in the study of the behavior with temperature of the electrical resistance of thin polycrystalline SmS films (thickness ～0.5–0.8 μm) performed in the temperature region 4.2–440 K have been used to correct the band structure model of this material. It has been shown that the main impurity levels in thin polycrystalline SmS films are levels corresponding to localized states close to the conduction band bottom, as well as the impurity donor levels E _i which belong to Sm ions filling vacancies in the S sublattice. The tail of localized states has been found to extend up to the energy of impurity donor levels.     

Electrical charging of non-conductors in a photoelectron spectrometer; effect of X-ray source voltage

The electrical charging on samples of gadolinium trifluoride, sodium fluoride and fused silica, has been investigated as a function of the voltage in the X-ray source of a photoelectron spectrometer. The results can be expressed in the form,E_c = d + c(V – V₀) + a(1 - e^{?b(V – V₀)}) an equation which expresses the linear dependence of the intensity of the photoelectrons on X-ray intensity and the dependence of the low energy electrons as that corresponding to the equivalent electrical circuit consisting of an emf, a capacitor, and a resistor. With gadolinium trifluoride and sodium fluoride, both of which have a relatively high photoconversion, the electrical potential is positive for all voltages. With fused silica, which has a relatively low photoemission, the electrical potential is negative for the lower voltages. Relative to an assumed value of 83.8 eV for the binding energy for Au(4f_{$\text{7}\text{2}$}) of gold deposited on NaF and GdF₃, the charge corrected values for F(1s) and the Gd(4f_{$\text{sol7}\text{2}$}) orbital binding energies are determined to be 684.57 ± .07 and 10.1 eV, respectively.     

Properties of single vacancies and of divacancies in copper

The electrical resistivity change at high temperatures due to vacancies was determined as a function of temperature. From the data E^F_1V=1.00 eV, E^B_2V=0.45 eV and about C^tot_V=4 × 10^-4 at the melting point results.     

Optical study of Ce ion in gamma-irradiated binary barium-borate glasses

Absorption spectra measurements of cerium-doped binary system from barium-borate glasses have been measured. The effects of dopant concentration of CeO₂ and Al₂O₃ in the concentration range 0.54-2.9 and 4.8-9.2 mol%, respectively, and exposed to different irradiation doses have been measured in the range 1-7 eV and the result have been interpreted in terms of structural concepts. The radiation-induced broad band at 2.25-1.88 eV in the base glass is observed to be suppressed by the presence of cerium due to the transformation of Ce⁴⁺ to Ce³⁺. The released electrons are then used to annihilate positive holes responsible for this band. The resolution of the observed absorption spectra show two to seven induced bands depending on the glass composition. Absorption spectra of the irradiated binary glass system are found to be controlled by the cerium concentration. From the absorption edge studies, the values of optical band gap E_opt and Urbach energy ΔE have been evaluated. The oxidoreduction (redox) reaction Ce³⁺/Ce⁴⁺ is assumed to be related to the glass basicity and the possible complex-ion formation. The oxygen ion activity (O²⁻) is believed to be related to the basicity and to the possible oxygen ion formation in the glass melt, and the redox equilibrium is shifted toward the reduced state.     

Double hysteresis loops induced by Mn doping in Pb0.99Nb0.02(Zr0.95Ti0.05)0.98O3 ferroelectric ceramics

Pb_0.99Nb_0.02(Zr_0.95Ti_0.05)_0.98O₃ (PNZT95/5) ceramics with 1 mol% and without Mn doping were prepared via conventional solid state reaction process. X-ray diffraction patterns show that the PNZT95/5 and Mn-doped PNZT95/5 (PNZTM95/5) ceramics, with composition near the boundary of the ferroelectric phase (FE)/antiferroelectric phase (AFE), have a rhombohedral perovskite structure. The ferroelectric behavior of PNZT95/5 ceramics is strongly affected by Mn doping. Without any aging process the PNZTM95/5 ceramics possess double hysteresis loops (P–E loops), whereas the PNZT95/5 ceramics possess normal single hysteresis loops. Due to the defect dipoles formed by effectively negatively charged Mn³⁺ dopants and positively charged O²⁻ vacancies, the PNZTM95/5 ceramics exhibit the double P–E loops. The defect dipole effect has been proved by investigating the P–E loops under different external fields. As a result, the PNZTM95/5 ceramics become “hardened”, exhibiting a high mechanical quality factor (1300).