Effective Debye Temperature Research Articles

Structure and oxidation of W(100) carbides: A helium diffraction study

Abstract Helium atom diffraction from c(2 × 2) and 5 × 1 carbon overlayers has been observed and analysed to determine the surface structures. The high temperature oxidation of the W(100)-(5 × 1)C surface is characterized using elastic specular helium scattering and Auger electron spectroscopy. The oxidation reaction at 1400 K is observed to produce the lower coverage c(2 × 2) and c(3√2 × √2)R45° ordered carbon overlayers, before the formation of a surface oxide layer. Annealing of partially ordered c(2 × 2) structures with and without hydrogen exposure is examined. The helium scattering from the controversial 5 × 1 overlayer is analysed in light of several proposed structural models, and is found to be consistent only with a recently proposed missing row model. Helium diffraction intensity calculations together with temperature dependent intensity measurements determine a helium-surface potential well depth of 8 meV and an effective surface Debye temperature of 1350 K. Finally, helium atom diffraction peak splittings are reported here for the first time for scattering from a stepped c(2 × 2) carbide structure.

Specific heat of the cubic high-T c superconductor Ba 0·6K 0·4BiO 3

We report magnetic susceptibility and specific heat measurements on polycrystalline samples of the 30 K superconductor Ba 0·6K 0·4BiO 3. Normal-state magnetization measurements indicate a Pauli-paramagnetic susceptibility of χ pauli = 2.3 × 10 −5 emu/mole, from which we infer a value for the density of states at the Fermi level of N(0) = 8.6 × 10 21ev −1cm. −3 Specific heat measurements performed between 1.6 K and 40 K indicate that considerable lattice softening occurs at low temperatures; the effective Debye temperature drops from ∼280 K at 35 K to ∼210 K at 4 K, implying that soft phonon modes are present in this compound. This result indicates that conventional phonon-mediated interactions may be responsible for the high transition temperature exhibited by Ba 0·6K 0·4BiO 3.

Effect of a deviation from stoichiometry on the mössbauer parameters of Sn0.8Ge0.2Te during the phase transition

The system of solid solutions (Sn0.8Ge0.2)1−yTey with the values of y = 0.499; 0.505; 0.510 is investigated by means of the Mossbauer effect on the 119Sn nucleus within the temperature range 160 to 300 K. The effective Debye temperature and mean-square displacements of tin atoms from equilibrium are calculated. In the region of phase transition (about 200 K) fractures on the temperature dependencies of the isomer shift and Debye temperature are observed, while no anomalies of the Mossbauer effect probability are noticed. No quadrupole interaction is observed with the symmetry decrease in the lattice. A correlation between the isomer shift and interatomic distance is revealed. The ionicity of the chemical bond in the system SnxGe1−xTe is ≈ 54%, when x = 0.9, and about 40%, when x = 0.1. [Russian Text Ignored].

Specific-heat measurements of HoxY

We report on specific-heat measurements of (Ho/sub x/Y/sub 1/..sqrt../sub x/)Co/sub 2/ in the temperature range from 1.5 to 60 K. Sharp peaks at the Curie temperature T/sub C/ and the spin-reorientation temperature T/sub r/ characterize the temperature dependence for 0.5less than or equal toxless than or equal to1. For 0.2less than or equal toxless than or equal to0.4 the magnetic specific heat C/sub m/ indicates spin-glass-like behavior, which appears to be in agreement with freezing phenomena observed in magnetic measurements. From the analysis of the magnetic entropy S/sub m/, we deduce that the induced Co moment can be referred to as itinerant in the sense of the Stoner theory. In the dilute Ho-concentration range, various anomalies are observed as, e.g., an enhancement of the effective electronic specific heat ..gamma.. and the effective Debye temperature FTHETA/sub D/ (of about 400% and 100%, respectively) and significant upturns of the C/sub p/T versus T/sup 2/ graphs together with a pronounced reduction of the magnetic entropy. These anomalies are discussed in terms of spin fluctuations;the loss of the magnetic entropy for x<0.2 may point to an instability of the Ho moment

Carbon‐, alumina‐, and silica‐supported iron sulfide catalysts

AbstractIron sulfide catalysts supported on various carbon materials, on alumina and on silica were compared for their thiophene hydrodesulfurization (HDS) activity measured in a flow micro reactor at atmospheric pressure. The iron content was varied from 0.2–5.0 iron atoms per nm2 support surface area. Mössbauer spectroscopy and X‐ray photoelectron spectroscopy (XPS) were used to characterize the nature and dispersion of the iron phases present in oxidic and sulfided catalysts.Within the surface loading range studied, carbon‐supported iron sulfide catalysts are much more active for thiophene HDS than the alumina‐ and silica‐supported ones. The activity of sulfided iron on alumina catalysts was found to be even lower than expected on the basis of data obtained earlier for other (supported) transition metal sulfides. This is most likely caused by an incomplete sulfidation. Among the various carbon supports employed, activated carbons, despite their high degree of micro porosity are more suitable than carbon blacks and carbon black composites.Based on a comparison of the effective Debye temperature derived from Mössbauer data with dispersions also obtained from Mössbauer measurements on oxidic catalyst precursors, the interaction between the iron‐containing phase and the various carbon supports is described in a qualitative way.

Europium charge states in sputtered amorphous EuPd x ( x = 2.4 and 3.0)

151Eu Mössbauer spectroscopy measurements on two amorphous samples of EuPd x ( x = 2.4 and 3.0) prepared by sputtering, show the presence of inhomogeneous mixed valent (Eu 2+ and Eu 3+) states with sample averaged valencies 2.25 and 2.85 respectively. Magnetic ordering and hyperfine fields are visible on the Eu 2+ components. The two charge states have the same effective Debye temperature. The amorphous and recrystallised samples are compared.

Determination of surface debye temperatures by leed

The method to determine the surface Debye temperature by LEED beam intensity versus temperature measurements is critically evaluated. Simulated in( I ) versus T plots obtained with the help of the Gallon-model using a kinematic approach for the specular beam at Bragg diffraction conditions are presented. It is shown that the assumed linearity in these plots is fundamentally incorrect. With our newly developed model good agreement is obtained with experimentally determined effective Debye temperature versus electron energy plots for Pt(111), Pt(100), Rh(111) and Rh(100). However, it is shown that the method of electron energy variation is not reliable for straightforward determination of the surface Debye temperature, whereas variation of angle of incidence is better suitable. In addition, plots measured on a Pt-Rh(100) surface with a Pt surface content varying between 25 and 60 at% are compared with simulated plots for disordered and ordered alloys.

Measurement of the effective Debye temperature for the Ti(0001) surface

Measurement of the effective Debye temperature for the Ti(0001) surface

Measurement of the effective Debye temperature for the Ti(0001) surface

We have measured the surface Debye temperature for the clean Ti(0001) surface from the temperature dependence of the intensity of the specular low-energy electron diffraction (LEED) beam. Using a kinematical analysis, we find the inner potential to be approximately 12 eV and that the effective Debye temperature varies from 240 to 380 K between 50 and 320 eV beam energy. The variation with beam energy is quite linear which suggests that the diffraction peaks used in the analysis are due to events with only a single backscattering component.

Measurement of the effective debye temperature for the Ti(0001) surface

Measurement of the effective debye temperature for the Ti(0001) surface

Calculation of the Debye-Waller factor for atom-surface scattering: He on Ag(111).

By a consistent application of the distorted-wave Born approximation, the Debye-Waller exponent for the scattering of He atoms from the Ag(111) surface is directly computed. When diffraction is negligible, as in this case, the decrease in specular intensity is simply obtained by summing the scattering due to all possible one-phonon processes. Using one-phonon computed intensities that agree with experiment along high-symmetry directions of the surface Brillouin zone, we find that the computed Debye-Waller exponent agrees with experiment and corresponds to an effective Debye temperature of 241 K. All corrections to the eikonal-type formula, 2W=4${p}_{z}^{2}$〈${u}_{z}^{2}$〉, are automatically included. In particular, the contribution of phonons with high parallel momentum is sharply cut off.

Magnetic hyperfine fields of119Sn in vacancy-associated defect structures in 3d ferromagnets

Defect structures associated with Sb or In impurity atoms in iron, cobalt, and nickel hosts have been detected by Sn-119 Mossbauer spectroscopy. With substitutional values as reference, the structures found are uniquely characterized by large, positive shifts in the Sn-119 magnetic hyperfine field and correlated increases in isomer shift. Further characterization are lower effective Debye temperatures. Preferably, the defects form and anneal in the resistivity stage III; the specific behaviou depending on whether Sb or In make up their kernel. They are interpreted as unique vacancy-type, relaxed structures SbVn or InVn. The vacancy trapping leads to order-of-magnitude field enhancements in fcc Co and Ni hosts, uniquely reflecting that here the substitutinal Sn fields happen to be small due to almost cancellation of large positive and negative contributions.

Effective Debye Temperature in Molten Salts and their Binaries

Effective Debye Temperature in Molten Salts and their Binaries

Temperature dependence of x-ray photoelectron diffraction from copper: Surface and bulk effects.

The temperature dependence of x-ray photoelectron diffraction (XPD) effects in azimuthal distributions of Cu 2${p}_{3/2}$ and Cu 3p core-level intensities from a clean copper (001) surface has been measured in the range from 298 to 1010 K and for two polar angles of emission relative to the surface of theta=7\ifmmode^\circ\else\textdegree\fi{} (highly surface sensitive) and theta=45\ifmmode^\circ\else\textdegree\fi{} (more bulk sensitive). Measurements have been performed with typical angular resolutions of \ensuremath{\sim}\ifmmode\pm\else\textpm\fi{}4\ifmmode^\circ\else\textdegree\fi{}--6\ifmmode^\circ\else\textdegree\fi{}, although some data obtained with a much higher resolution of \ensuremath{\sim}\ifmmode\pm\else\textpm\fi{}0.6\ifmmode^\circ\else\textdegree\fi{} are also discussed. Very-high-angular-resolution azimuthal scans of Cu 2${p}_{3/2}$ intensity for theta values near 45\ifmmode^\circ\else\textdegree\fi{} show much more fine structure and sensitivity to polar-angle variation than analogous results obtained at lower angular resolution.The diffraction-produced anisotropies as measured by \ensuremath{\Delta}I/${I}_{\mathrm{max}}$=(${I}_{\mathrm{max}}$-${I}_{\mathrm{min}}$)/ $_{\mathrm{max}}$ undergo a much greater relative decrease with increasing temperatures at theta=7\ifmmode^\circ\else\textdegree\fi{} than at theta=45\ifmmode^\circ\else\textdegree\fi{}. Single-scattering cluster (SSC) calculations including Debye-Waller factors predict very well the observed diffraction patterns and their qualitative temperature dependence, as well as the variation of anisotropy with temperature at theta=7\ifmmode^\circ\else\textdegree\fi{}. At theta=45\ifmmode^\circ\else\textdegree\fi{}, theory significantly underestimates the decrease in anisotropy with temperature, probably due to multiple scattering effects occurring for emission along low-index directions with a high density of scattering atoms. Double scattering calculations were performed to estimate these effects and they are compared to experiment and the results of SSC calculations. The effective surface Debye temperatures as deduced from theory and experiment at theta=7\ifmmode^\circ\else\textdegree\fi{} are 202 and 240 K for Cu 2${p}_{3/2}$ and Cu 3p emission, respectively, in good agreement with prior determinations. Finally, the Cu 2${p}_{3/2}$ XPD data for theta=7\ifmmode^\circ\else\textdegree\fi{} gave no evidence for short-range surface melting effects on Cu(001) at temperatures up to T/${T}_{\mathrm{melting}=0.74}$.

Temperature dependence of photoelectron scattering from a monolayer.

Monolayers of lead on Ge(111) were investigated using angle-resolved photoelectron spectroscopy with synchrotron radiation. A narrow state close to the valence-band maximum is found to have the characteristics of a lead-induced surface resonance. This surface resonance exhibits a strong intensity dependence on substrate temperature. The temperature dependence is found to be consistent with a Debye-Waller-factor model that incorporates atomic correlations only between overlayer atoms, and neglects multiple scattering. The effective surface Debye temperature is reduced from that of a Pb(111) surface.

Dynamics of rare-gas floating monolayers in the self-consistent phonon theory.

The dynamics of floating two-dimensional {2D) monolayers of Xe, Kr, Ar, and Ne are explored over a range of lattice spacings and temperature using the self-consistent phonon theory. Phonon dispersion curves and the zero-point energy are calculated and appropriate Debye temperatures and rms vibrational amplitudes (u2) estimated at T =0 K. The results show that floating monolayers of Ar and Xe represent their physisorbed counterparts well. They also show that Xe and Kr are effectively harmonic at T &25 K while Ne is a strongly anharmonic 2D crystal at all T. The selfconsistent harmonic frequencies in Ne at T =0 K are nearly twice the harmonic values. Ar is an intermediate case. The effective Debye temperature is ea -65 K in all cases and the Lindemann ratio y = (u ') /R varies from 3% in Xe to 9% in Ne at T =0 K, similar to 3D crystal values. The phonon frequencies depend sensitively on lattice parameter and temperature, more so than in 3D crystals.

Dynamic properties and Debye temperatures of bulk Au and Au clusters studied using extended x-ray-absorption fine-structure spectroscopy.

The ${L}_{3}$ edge of bulk Au has been studied using x-ray-absorption spectroscopy in the temperature range 16--300 K. An effective Debye temperature of 165 K is derived from the thermal behavior of the extended x-ray absorption fine-structure Debye-Waller factors. Their behavior is also calculated for finite systems by using a free-bounded-sphere density of states and a Debye approximation for the phonon spectra. An excellent agreement is found with experimental data previously reported on Au clusters with mean diameters ranging from 15 to 43 A\r{}. This confirms that the dynamic properties of Au clusters are well described by a liquid-drop model in agreement with heat-capacity measurements.

Composition and dynamical characteristics of the surface of single crystals FeNi 3 (100), (110), (111)

Surface segregation as a function of sample temperature has been investigated using AES for the (111), (100) and (110) faces of FeNi 3 single crystal. The surface layer composition is shown to suffer a non-monotonous and significant change when the crystal temperature goes over the order-disorder transition point and exceeds the magnetic ordering temperature. The role of atomic and magnetic ordering in the bulk and at the surface of ferromagnetic FeNi 3 alloy is considered. It has been found by LEED intensity measurements that the Debye-Waller plot for FeNi 3 is composed of two straight lines, the change in their slopes corresponding to the order-disorder transition temperature. In the range 400–650 K the effective Debye temperatures, Θ eff, for surface atom vibrations are 230, 250 and 190 K for the (100), (111) and (110) FeNi 3 faces; in the range 750–900 K the Θ eff values are 10–15% higher. It is supposed that both the increase in iron content in the surface layer and the alloy disordering, lead to a dynamical parameter change in the high temperature range.

Superheating of small solid-argon bubbles in aluminum.

Small solid-Ar bubbles, formed by the implantation of 50-keV Ar+ ions into Al, are observed to melt at 730 K, compared with an equilibrium bulk melting temperature of 250 K. A model based on the suppression of Ar thermal vibrations at the Al interface is used to account for the superheating of 480 K. Measurements on the temperature dependence of diffracted electron intensities from these bubbles yield Debye-Waller factors consistent with an effective Debye temperature of about 140 K, compared with a bulk value of 110 K.

Vibrational properties of the graphite (0001) surface

The temperature dependence of low-energy electron diffraction intensity-energy spectra from the graphite (0001) surface has been measured from 100 to 600 K. The perpendicular and parallel effective surface Debye temperatures and the effective mean-square vibration amplitudes as a function of electron energy have been determined from these data. The values show surface enhancement and approach the bulk values with increasing electron energy. At energies below ≈ 100 eV, however, there is little anisotropy observed between the effective Debye temperatures parallel and perpendicular to the graphite surface.