μ+SR study of vacancies in thermal equilibrium in ferromagnets

Muon spin precession frequencies and transverse relaxation rates have been measured on demagnetized iron, cobalt, and FeCo alloys (3 at%–50 at% Co) between room temperature and the Curie temperatureT _c. The increase of the relaxation rate in iron between 930 K and 1010 K could be quantitatively attributed to the trapping of positive muons by vacancies in thermal equilibrium, resulting in an enthalpy of monovacancy formation ofH _1V ^F =(1.7±0.1) eV. the smallest vacancy concentrations detected are = 10⁻⁸.     

Monovacancy formation energy in copper,silver, and gold by positron annihilation

Monovacancy formation energies in copper, silver, and gold have been deduced from the temperature variation of the peak counting rate in the angular correlation curve of positron annihilation radiation from these metals. The counting rate was temperature dependent over the entire temperature range, including temperatures so low that no trapping of positrons at vacancies is effective. At these temperatures the increase in counting rate results from thermal expansion of the lattice. By separating this thermal expansion effect from the vacancy trapping effect at higher temperatures, we obtained values for the monovacancy formation energyE _1v for copper, silver, and gold to 1.29±0.02 eV, 1.16±0.02 eV, and 0.97±0.01 eV, respectively.     

Electrical resistivity and length variations of iron and nickel during low temperature electron irradiations

Abstract

Low concentrations of Frenkel pairs are produced in polycrystalline iron and nickel targets during irradiations with 1.9 to 2.5 MeV electrons at 5 or 20 K. During irradiation (and subsequent thermal annealing for the nickel target) we simultaneously measure on the targets the changes of electrical resistivity, and of length with two different linear systems: a capacitive and an inductive device.

The measured ratios of relative length to electrical resistivity increases are [(Δl/l)/Δρ]_Fc = and [(Δl/l)/Δρ]_Nj = 1020 ±120(Ωcm)^?1. Values of Frenkel pair formation volumes of ν _Fc = 1.35 ± 0.25 and ν _Ni = 2.06 ± 0.30 atomic volumes are proposed.     

Kinetics of copper,nickel, cobalt and iron sulfidation by a modified “pellet” method. The effect of reaction at the metal/scale interface

This paper presents the results of studies on sulfidation of copper at temperature 600–700 K, nickel at 870–890 K, cobalt at 1060–1140 K and iron at 970–1170 K in sulfur vapor at 101 kPa with the use of a modified Wagner's pellet method. It has been stated that in the case of copper and nickel the initial heating of metal samples with a corresponding sulfide greatly influences the formation of hexagonal Cu₂S or Ni_{3 ± y}S₂ which are the thermodynamically stable phases under the reaction conditions. If these sulfides are lacking the scale morphology, the sulfidation rates are different. In the case of cobalt and iron the process of sulfure dissolution does not affect the sulfidation rate. The modified pellet method permits the measurement of the sulfidation kinetics of these metals which has not been possible with the use of the classical one.     

Self-diffusion of oxygen in superstoichiomertric uranium dioxide in the range of the superion phase transition

The self-diffusion of oxygen in the superion transition range (1300–3000 K) of superstoichiometric uranium dioxide UO_{2 + x} is studied by the method of molecular dynamics using the pair interaction potential recovered from data for the thermal expansion of the UO₂ lattice. It is shown that three portions can be distinguished in the temperature dependence of the coefficient of oxygen self-diffusion in UO_{2 + x} , lnD = f(1/T), for all the compositions studied (x = 0, 0.008, and 0.030). These portions, each being described by the Arrhenius relationship, correspond to the crystalline, transition, and superion states of UO_{2 + x} . At low temperatures (1300–1820 K), the activation energies of oxygen diffusion for the above compositions are, respectively, 2.66 ± 0.44, 1.33 ± 0.10, and 1.00 ± 0.09 eV. At the beginning of the transition region, these activation energies rise to 3.40 ± 0.11, 2.24 ± 0.10, and 1.66 ± 0.60 eV. In the superion state, the activation energy of oxygen diffusion for all the compositions is the same, 1.25 ± 0.15 eV, within the error limit. As the oxygen content in UO_{2 + x} grows, the phase transition temperature decreases considerably and may reach 1600 K at x = 0.2. Comparison with experimental data for the low-temperature oxygen diffusion coefficient and with the data of UO₂ simulation using graphic processors shows good agreement of the results. By comparing the concentration dependences of the oxygen diffusion coefficient that are obtained by magnetic dynamics simulation with experimental data, it is shown that quantitative calculation of these dependences in the case of UO_{2 + x} can be carried out only for compositions with x < 0.03 if the given type of potential is used.     

Lattice defects of V3Si single crystals studied by positron annihilation

The sensitivity of positrons to point defects created by the irradiation of V₃Si with neutrons is demonstrated. We found no indication of thermal vacancies by thermal equilibrium measurement up to 1273 K which indicates that the monovacancy formation enthalpy for V₃Si isH _1V ^F ≧(1.84±0.14) eV. Investigations within the range of homogeneity for excess vanadium suppot the idea that substitutional defects are the dominating defect type, whereas for excess silicon a direct confirmation of existing structural vacancies as the dominating defect type is given.     

Determination of the cation site-occupation parameter in a cobalt ferrite from Synchrotron-Radiation diffraction data

Energy-tuned synchrotron radiation (SR) was used to collect Bragg diffraction intensity data from a single crystal of a cobalt ferrite (nominally CoFe₂O₄). X-Ray energies were chosen just below the Fe and Co K absorption edges where large changes in anomalous dispersion enhance differences in scattering between the two metals. Observations were analyzed by least-squares methods to determine the value of the parameter, X, that describes the distribution of cobalt and iron ions on the octahedral and tetrahedral sites of the cubic spinel structure. Refinements based on over 290 SR data gave X = 0.83 ± 0.01 (83% of the cobalt ions located on octahedral sites); with only 30 specially selected SR data, this value could be fixed at 0.80 ± 0.07. Estimates of dispersion corrections to the X-ray scattering factors for iron near the Fe K edge were obtained through observations of SR diffraction from a magnetite crystal. Results do not differ appreciably from predicted values in the range 6358–7105 eV, through the precision in estimates of the imaginary term is poor at the lower energies.     

Kinetics of silicon interaction with textured tantalum ribbons

The kinetics of silicon interaction with textured tantalum ribbons having a predominantly (100)-oriented surface has been studied by Auger electron spectroscopy. For T<700 K, silicon builds up on the surface. Within the 900<T<1000-K interval, after the formation of a monolayer coating, the excess silicon enters into layer-by-layer growth of the silicide TaSi₂. Within 1150<T<1320 K, this excess silicon reacts with tantalum to produce in a layer-by-layer manner the Ta₅Si₃ silicide; this compound is unstable and decomposes for T>1320 K to form Ta ₄Si, with only one third of the original silicon monolayer being left on the surface. The activation energy of Ta₅Si₃ decomposition is 4.3±0.2 eV. The activation energy of Si thermal desorption at low coverages is 5.4±0.2 eV. Fiz. Tverd. Tela (St. Petersburg) 39, 1484–1490 (August 1997)     

Diffusion and solubility of electrically active iron atoms in gallium arsenide

Iron diffusion in GaAs at arsenic pressure 1 atm is studied. The temperature dependences of the diffusion coefficient and solubility of electrically active iron atoms in GaAs are determined. The dependences can be described by the Arrhenius equations with the following parameters: D ₀ = 1.61 cm²/s and E = (2.16 ± 0.47) eV (for diffusion) and N _S ⁰ = 4.62 ⋅ 10²³ cm⁻³ and E _S = (1.61 ± 0.16) eV (for solubility). The results obtained are compared with the earlier published data. The concentration of electrically active iron atoms is shown to be about 2 times lower than the total iron concentration in GaAs. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 39–41, November, 2008.     

Low thermal expansion behavior and transport properties of Ni and Ge co-doped manganese nitride materials at cryogenic temperatures

A series of Ni and Ge co-doped manganese nitride materials were fabricated by mechanical ball milling followed by solid-state sintering. Their thermal expansion properties and electrical and thermal conductivities were investigated in the temperature range of 77–300 K. The results show that Ni and Ge co-doped manganese nitride materials have negative thermal expansion (NTE), and the operation-temperature window of NTE shifts toward the lower temperature region and the variation of linear thermal expansion (ΔL/L _(300K)) in the operation-temperature window of NTE decreases with increasing Ni content. The combination of these two factors results in a low coefficient of thermal expansion (CTE) at cryogenic temperatures. The average CTE of Mn₃(Cu_0.2Ni_0.4Ge_0.4)N drops to ‘zero’ in the temperature range of 190–77 K. The values of electrical and thermal conductivities of the Ni and Ge co-doped manganese nitride materials are in the ranges of 2–3×10³ (ohm cm)⁻¹ and 1.6–3.4 W (m K)⁻¹, respectively.     

The activation energy for γ-Fe precipitates inCuFe

The precipitation of the fcc γ-phase of iron has been studied by a systematic series of isothermal and isochronal (0–48 h) heat treatments (300–800°C) on a supersaturated solution ofCuFe containing 3 at% Fe. The optimal conditions (450–600°C) for precipitation of the maximum fraction (90%) of iron attainable within 48 h in the form of γ-Fe have been delineated from analysis of the room temperature spectra. The time dependence for formation of γ-Fe precipitates is well described by the equation for long term annealing. An activation energyE _a ≈0.6 eV for the formation of γ-Fe in Cu is obtained. It indicates short range rather than long range diffusion in theCuFe sample studied.     

Thermal conductivity and expansion of PbF2 single crystals

In order to check a phenomenon of the negative correlation between ionic and thermal conductivities of solid substances, we studied the thermal conductivity and expansion of cubic PbF₂ single crystals at 50–300 and 5.6–317 K, respectively. We found that lead difluoride had a thermal expansion coefficient α that was equal to (28.5 ± 0.3)10⁻⁶ K⁻¹ at 300 K, and a thermal conductivity coefficient k(T) was equal to 1.40 ± 0.07 W/(m K) at the same temperature. Thus, the thermal conductivity for PbF₂ is the lowest among fluorite-type MF₂ (M = Ca, Sr, Ba, Cd, Pb) thermal conductivities, whereas its fluoride-ion conductivity is the highest one among MF₂ (M = Ca, Sr, Ba, Cd, Pb) ionic conductors.

     

Mass-spectroscopic study of the diffusion and solubility of helium in submicrocrystalline palladium

By the method of helium thermal desorption from submicrocrystalline palladium presaturated in the gaseous phase, the diffusion coefficient D _eff and solubility coefficient C _eff of helium are measured in the range P=0–3 MPa and T=293–508 K. The pressure dependence of C _eff flattens at high pressures. At low saturation pressures, the temperature dependences of the diffusion and solubility coefficients may be divided into (1) high-temperature (400–508 K) and (2) low-temperature (293–400 K) ranges described by the exponentials D _{1, 2}=D ₀exp (−E _{1, 2} ^D /kT) and C _{1, 2}=C ₀exp (−E _{1, 2} ^S /kT). The energies of diffusion activation are E ₂ ^D =0.0036±0.0015 eV and E ₁ ^D =0.33±0.03 eV, and the solution energies are E ₂ ^S =−0.025±0.008 eV and E ₁ ^S =0.086±0.008 eV in the low-and high-temperature ranges, respectively. Mechanisms behind the diffusion and solution of helium are discussed.     

Elastic leed intensity-energy studies of clean (001), (110) and (111) nickel surfaces

Elastic low energy electron diffraction (LEED) intensity-energy (I-E) measurements for clean (001), (110), and (111) nickel surfaces were obtained at room temperature. Surface composition was monitored by Auger spectroscopy. I-E data from 15 to 220 eV were obtained at normal incidence for the non specular beams and for the specular beams at incidence angles from 4° to 20° on the 0° and 45° azimuths of (001), on the 0° and 90° azimuths of (110), and on the 0° azimuth of (111) nickel. Normalization of the data was performed electronically during data acquisition. Intensities were calibrated with the use of a shielded, biased Faraday collector. The effects of instrumental and experimental uncertainties were examined and minimized to obtain intensities accurate to ± 15 %, energy scales accurate to ± 0.35 eV, and incident and azimuthal angles accurate to ± 0.25° and ± 1.0° respectively.All nickel surfaces have I-E spectra which are characteristic of strong multiple scattering. Angular evolution features for (001) and (110) spectra may be correlated with intraplanar resonances associated with the onset of propagating beams. Only the (001) surfaces were found to have pronounced, sharp resonance features associated with surface barrier resonances and inelastic loss processes. Kinematic analysis of the Lorenzian-shaped I-E peaks on all surfaces in consistent with surface expansion using either an energy-dependent or a constant inner potential of 10.75 ± 0.5 eV. The widths of these same peaks on all surfaces were found to vary as $\text{E}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ above 40 eV and $\text{E}{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}$ below.     

Observation of kaonic hydrogen atom X-ray

We successfully observed clear K-series kaonic hydrogen X-rays for the first time taking advantage of two charged pions tagging technique and a gaseous hydrogen target. The strong-interaction energy shift and width of 1s state of K ⁻p atom were determined to be ΔE _1s = -323 ± 63 (statistical) ± 11 (systematic) eV and Г_1s = 407 ± 208 ± 100 eV. Our result means that the K ⁻p strong interaction near threshold is repulsive and the long-standing kaonic hydrogen puzzle is solved. This revised version was published online in August 2006 with corrections to the Cover Date.     

121Sb and 123Sb NQR and the heterophase structure in the SbSI ferroelectric

The temperature dependences of NQR line frequencies and widths of ¹²¹Sb (for the ±1/2→±3/2 transition) and of ¹²³Sb (for the ±1/2→±3/2 and ±3/2→±5/2 transitions), as well as of the principal components and the asymmetry parameter of the electric-field-gradient tensor at the ¹²³Sb nucleus have been studied in a SbSI crystal in the 115–325 K range. The dynamic and static factors governing the character of these relations are discussed. The ±1/2→±3/2 line in the ¹²¹Sb NQR spectrum splits into a doublet within a narrow (0.5 K) temperature interval near the ferroelectric phase transition (T _c=293 K), which is associated with the formation of a macroscopic heterophase structure in the crystal. Fiz. Tverd. Tela (St. Petersburg) 41, 1286–1292 (July 1999)     

Melting and thermal expansion of iron in uniformly laser-heated diamond anvil cells

Abstract

A technique is described to laser heat samples uniformly under hydrostatic pressure conditions to over 2500 K and 400 kbar with very high accuracy in P and T. I re-measured the melting curve of iron by this technique and obtained excellent agreement with my earlier work using resistive wire heating (Boehler 1986). P-V-T measurements on γ-iron to 200 kbar and 2000 K using synchrotron radiation leads to a strong decrease of the thermal expansion coefficient with pressure, (?lnα/?lnV)_T = 6.5. The zero pressure bulk modulus K₀ decreases with temperature by 0.33 kbar deg^?1. This Yields densities of iron at conditions in the Earth's core which are consistent with shock compression measurements. The potiential of studying mineralogical phase transitions by this method is described.     

Electron stimulated desorption of carbon monoxide from a (111) niobium surface

Electron stimulated desorption of CO from the (111) face of a Nb single crystal produced both CO⁺ and O⁺ ions after adsorption at 150°K on a clean surface. When the surface was heated to above 250 °K only O⁺ ions were observed, and this current disappeared as the temperature was increased to 700 °K. Readsorption (at 150 °K) was inhibited following the 700 °K heating. These data indicate the formation on heating of a tightly bound surface phase with very low ionic desorption cross section. Threshold energies for CO⁺ and O⁺ ion production were 10.0 ± 0.5 eV and 19.0 ± 0.5 eV, respectively. The cross section for electron stimulated depopulation of the O⁺ producing phase was (4 ± 1) × 10^?18 cm² for 100 eV electrons.     

Temperature dependence of positron annihilation in iron and vanadium

The Doppler broadened lineshape is measured as a function of temperature for iron and vanadium; vacancies produce a strong trapping effect in γ-Fe, but a weak effect in V. Threshold temperaturesT _t for trapping are 1280±25 and 1370±30 K. Empirical linear relationships betweenT _t , self-diffusion energyQ ^SD and monovacancy formation energyH _1v ^F are discussed and used to determineH _1v ^F values for γ-Fe and V.     

Electronic structure of ZnS:Co and ZnSe:Co crystals

The x-ray photoelectron spectra of the valence bands and inner Co 2p levels of solid solutions of the wide-gap semiconductors ZnS, ZnS:Co, ZnSe, and ZnSe:Co are investigated. The x-ray photoelectron Co 2p spectrum of the ZnS:Co crystal indicates a 3d configuration of the ground state of cobalt and strong covalence of the cobalt-sulfur bond, but somewhat weaker than for CoS. It is established that the edge of the valence band recedes by 0.7±0.1 eV in transition from ZnSe to ZnS. Fiz. Tverd. Tela (St. Petersburg) 39, 1971–1974 (November 1997)