Surface diffusion of Pd and Au on W single crystal planes: II. Anisotropy of Pd surface diffusion due to the influence of substrate structure

Large anisotropy effects have been observed for the spreading of Pd on W single crystal planes exhibiting regular step structures. The spreading rate was studied as function of step density and step direction at temperatures around 1000 K. The terraces had always (110) orientation. Step densities as low as 3.3 × 10⁴cm^?1 corresponding to an average terrace width of 3000 Å already cause enhanced spreading rates parallel to the step direction. Steps parallel to the [001] direction showed the largest promoting action. The rate perpendicular to steps does not depend markedly on step density. The diffusion behaviour on the “step free” (110) plane turned out to be isotropic. A model is proposed which assumes the mass transport to occur in the second Pd layer. The increase of the diffusion rate along steps is correlated with the much larger density of diffusing atoms in step sites as compared to terrace sites due to the difference in binding energy.     

Study of cation tracer diffusion in NaF

Cation tracer diffusion coefficients were measured in pure NaF crystals in the intrinsic ionic conductivity range (876–970 °C). The results can be rationalized satisfactorily in terms of contributions to the observed Na tracer diffusivities arising from both free vacancies and neutral vacancy pairs, the latter contribution amounting to about 53 per cent of the total Na diffusion at the highest measuring temperature. The best-fit defect parameters derived in an earlier conductivity study [21] from this laboratory on similar NaF crystals give for the free vacancy contribution D_v^*(Na) = 4·25 exp (?2·21 eV/kT) cm²s^?1. A combination of these D_v^*(Na) values with the present diffusion data yields for the vacancy-pair contribution D_p^*(Na) = 1·15 × 10⁸exp (?4·04 eV/kT) cm²s^?1. Comparison of the present findings with published values of the anion tracer diffusion coefficient in NaF showed that D_p^* (F) is 2·3 to 4·4 times larger than D_p^*(Na) over the temperature range of our observations, the difference between the two contributions increasing with decreasing temperature. When approximate account is taken of the temperature-dependence of the two pair correlation factors, this last result indicates that the anion jumps into the vacancy pair occur with a higher frequency, and increasingly so at lower temperatures, than do those involving the cations.     

Effects of anisotropy,order parameter,and viscosity on translational self-diffusion in nematic liquid crystals

A theory developed for the translational diffusion in nematic liquids crystals shows a dependence on the viscosities, order parameter, and molecular structure. Theoretical results for self-diffusion in p-azoxyanisole at 125°C are D^{∥ = 4.3 × 10?6, D⊥ = 3.1 × 10?6 c2/s}, andD^∥/D^⊥ = 1.4.     

Chemisorption of H2 on W(100): Absolute sticking probability,coverage, and electron stimulated desorption

Measurements of both the absolute sticking probability near normal incidence and the coverage of H₂ adsorbed on W(100) at ~ 300K have been made using a precision gas dosing system; a known fraction of the molecules entering the vacuum chamber struck the sample crystal before reaching a mass spectrometer detector. The initial sticking probability S₀ for H₂/W(100) is 0.51 ± 0.03; the hydrogen coverage extrapolated to S = 0 is 2.0 × 10¹⁵ atoms cm^?2. The initial sticking probability S₀ for D₂/W(100) is 0.57 ± 0.03; the isotope effect for sticking probability is smaller than previously reported. Electron stimulated desorption (ESD) studies reveal that the low coverage β₂ hydrogen state on W(100) yields H⁺ ions upon bombardment by 100 eV electrons; the ion desorption cross section is ~ 1.8 × 10^?23 cm². The H⁺ ion cross section at saturation hydrogen coverage when the β₁ state is fully populated is ? 10^?25 cm². An isotope effect in electron stimulated desorption of H⁺ and D⁺ has been found. The H⁺ ion yield is ? 100 × greater than the D⁺ ion yield, in agreement with theory.     

Reactive scattering of small molecules from platinum crystal surfaces: D2CO,CH3OH,HCOOH, and the nonanomalous kinetics of hydrogen atom recombination

The decomposition of D₂CO, CH₃OD and HCOOH on Pt(110) and of D₂CO on Pt(S)-[9(111) × (100)] was studied by molecular beam relaxation spectroscopy. D₂CO and CH₃OD evolved CO and H₂ via a desorption limited sequence of elementary steps. The rate constant for CO desorption from Pt(110) was 6 × 10¹⁴exp(? 35.5 $\text{kcal}\text{gmol}$ · RT) s^?1, and from Pt(S)-[9(111) × (100)] it was 1 × 10¹⁵ exp(?36.2 $\text{kcal}\text{gmol}\text{·RT}$) s^?1. On Pt(110) the rate constant for hydrogen formation was 10^{0 ± 1}exp(?24 $\text{kcal}\text{gmol}\text{·RT}$) $\text{m}\text{?}{}^2\text{atom}$ · s. On Pt(S)-[9(111) × (100)] two pathways for H₂ formation existed with rate constants of 8.7 × 10^?2exp( ?24.9 $\text{kcal}\text{gmol}\text{· RT}$) $\text{cm}{}^2\text{atom}$· s and 3.2 × 10^?3 exp(?19.5 $\text{kcal}\text{gmol}\text{·RT}$) $\text{cm}{}^2\text{atom}\text{·}$ s. These pre-exponential factors are in order of magnitude agreement with values typical of hydrogen recombination on other metals. When a small amount of sulfur ( ~ 0.1 ML) was adsorbed on the stepped Pt surface, only one pathway for H₂ formation existed due to blockage of stepped sites. A similar result was obtained when a beam of CO was impinged on the surface. Formic acid decomposed via a branched process to form primarily CO₂ and H₂.     

Doping mechanism of a conducting polymer: Iodine-doped polyacetylene

We have achieved a technique for determining the diffusion profiles of impurities in the polymers by using a radio-tracer and a microslicing method. We describe the diffusion profiles of iodine in polyacetylene in the temperature range ?60 +20°C. The results show that: (1) iodine penetrates through the interfibril spaces; (2) there is a simultaneous chemical reaction of the first order on the fibrils between polyacetylene and iodine. When the dopant is in solution in pentane, the liquid state interfibril diffusion obeys the Arrhenius law: D = 1.73 × 10^?3 exp[$\text{?0.122(eV)}\text{kT}$] cm² · sec^?1. The kinetic constant of the reaction obeys the similar equation: k = 0.754 exp[$\text{?0.0867(eV)}\text{kT}$] sec^?1.     

Surface diffusion of lead on tungsten

Field electron microscopy is used to study the surface diffusion of lead on tungsten. A simple method to measure rough values of the diffusion coefficient and its dependence on sub-monolayer coverage is described and tested. In the region around (001) the displacement energy found is about 1.30 eV/atom up to 10¹⁵ atoms/cm² where it decreases to 0.78 eV/atom. In the residual region except (110) this energy at 1.5×10¹⁴ atoms/cm² is 1.22 eV/atom, it decreases at 4 × 10¹⁴ atoms/cm² to 0.61 eV/atom and increases at 10¹⁵ atoms/cm² to 0.78 eV/atom. Corresponding values of the diffusion coefficient D and of the preexponential D₀ are given. The dependence of D on submonolayer coverage is discussed.     

Optical absorption edge in SnS

Optical absorption in single crystals of tin sulfide has been studied at many temperatures between 100 and 300 °K, in the wavelength range 2·2–0·8 μ. From the interference fringe patterns the absorption coefficient, reflection coefficient and index of refraction as a function of wavelength were determined for two light polarizations (ε∥a and ε∥b). From an analysis of the data, indirect band gaps of 1·142 and 1·095 eV were found for the two directions of polarization. Also it was found that the phonon assisted transitions required the participation of two phonons at different energy thresholds with energies 0·033 or 0·038 eV and 0·082 or 0·113 eV, with reference to the two axis. The temperature dependence of the indirect band gap for each direction of light polarization is linear with a slope ?4·05 × 10^?3eV and ?4·37 × 10^?3 eV respectively.     

Diffusion of 55Fe in Fe2O3 single crystals

The diffusion of ⁵⁵Fe has been measured parallel to the c axis of Fe₂O₃ single crystals at temperatures in the range 708–1303°C and at an oxygen activity of unity. The tracer penetration profiles were determined using sectioning techniques. For temperatures above 900°C the tracer diffusion coefficient is given byD¹(Fe) = 1.6 × 10⁹ exp[?6.0 (eV)/kT] cm² s^?1 and below 900°C by 2.8 × 10^?9 exp[?1.8 (eV)kT]. The high-temperature behaviour is probably characteristic of pure Fe₂O₃, whereas diffusion at lower temperatures may be influenced by impurities. The most likely defects responsible for diffusion of Fe are iron interstitials and, for oxygen, oxygen vacancies, and the observed activation energies are discussed in terms of the properties of these defects. The diffusion data and defect models have been used to predict the rate of growth of Fe₂O₃ and indicate that outward Fe diffusion is the dominant transport process. Previously published data for Fe₂O₃ growth in a variety of experimental situations have been corrected to a single rate constant using a model for multilayer growth. The corrected data are all in good agreement but are approximately two orders of magnitude greater than predicted from diffusion data, which suggests that grain boundary diffusion controls the growth of Fe₂O₃ in practice.     

Trapping probabilities and desorption energies of alkali atoms on a clean and an oxygen covered tungsten (110) surface

Alkali atoms were scattered with hyperthermal energies from a clean and an oxygen covered (θ ≈ 0.5 ML) W(110) surface. The trapping probability of K and Na atoms on oxygen covered W(110) has been measured as a function of incoming energy (0–30 eV) and incident angle. A considerable enhancement of trapping on the oxygen covered surface compared to a clean surface was observed. At energies above 25 eV there are still K and Na atoms being trapped by the oxygen covered surface. From the temperature dependence of the mean residence time τ of the initially trapped atoms the pre-exponential factor τ₀ and the desorption energy Q were derived using the relation: $\text{τ = τ}{}_0\text{exp}\text{(}\text{Q}\text{kT}{}_{\mathrm{<mtext>s</mtext>}}\text{)}$. On clean W(110) we obtained for Li: $\text{τ}{}_0\text{= (8 ±}\text{8}\text{4}\text{) × 10}{}^{\mathrm{?14}}\text{sec}$, Q = (2.78 ± 0.09) eV; for Na: τ₀ = (9 ± 3) × 10^?14 sec, Q = (2.55 ± 0.04) eV; and for K: τ₀ = (4 ± 1) × 10^?13 sec, Q = (2.05 ± 0.02) eV. Oxygen covered W(110) gave for Na: τ₀ = (7 ±3) × 10^?15 sec, Q = (2.88 ± 0.05) eV; and for K: $\text{τ}{}_0\text{= (1.3 ±}\text{0.9}\text{0.6}\text{) × 10}{}^{\mathrm{?14}}\text{sec}$, Q = (2.48 ±0.05) eV. The adsorption on clean W(110) has the features of a supermobile two-dimentional gas; on the oxygen covered W(110) adsorbed atoms have the partition function of a one-dimen-sional gas. The binding of the adatoms to the surface has a highly ionic character in the systems of the present experiment. An estimate is given for the screening length of the non-perfect conductor W(110):k_s^?1≈ 0.5 Å.     

Diffusion of hydrogen and deuterium on the (111) plane of tungsten

The diffusion of ¹H and ²H on the (111) plane of a W field emitter has been studied by the fluctuation method at various coverages. Both activated and unactivated diffusion is observed; the latter shows very little isotope effect, suggesting that coupling to the substrate is so strong that mass renormalization makes the effective masses of ¹H and ²H nearly identical. Values of D in the tunneling, i.e. temperature independent, regime are 10^?13?5 × 10^?14 cm²/s depending on coverage. For activated diffusion at high coverages, corresponding to population of the β₁ state E = 2.4?3.2 kcal/mol and D₀ = 2 × 10^?8 ?5 × 10^?7 cm²/s, depending on coverage. For lower coverages, corresponding to β₂ population, E = 7–9 kcal/mol, D₀ = 9 × 10^?6 ?2 × 10^?3 cm²/s, again depending on coverage. Similar values are obtained for ²H, with E and D₀ values slightly reduced. An exponentially decaying correlation signal for clean W was also seen and interpreted in terms of flip-flop of W atoms.     

Diffusionskonstante und charakteristische Absorption vor der Bandkante von Mn in ZnO-Kristallen

ZnO single crystals were doped with Mn and Co by diffusion. In the temperature range from 1400–1600 K the Mn and Co-diffusion-constants were determined:D _Mn=3.2 · 10^?3 exp (?2.87 eV/kT) cm² sec^?1 andD _Co=1·10exp(?3.98 eV/kT) cm² sec^?1. The Mn doped ZnO crystals show a characteristic colour due to an absorption near the intrinsic absorption edge. The corresponding absorption spectra were measured forE⊥c andE∥c. A discussion of different absorption mechanism shows that a charge-transfer transition is responsible for this absorption.     

Iodine as a donor in CdS

Iodine doped single crystals of CdS were grown from the vapor phase. High temperature Hall effect measurements for the crystals equilibrated with Cd and S₂ vapors at temperatures between 700 and 1000°C gave the free electron concentration as a function of p_Cd or p_S2 and temperature. The results can be explained on the basis of a model in which the CdS is saturated with iodine at low p_Cd (=high p_S2) but unsaturated at high p_Cd.The solubility of iodine in CdS is given by c_t=1·73×10²²p_S2^?1/8 exp (?1·045 eV/kT) cm^?3 atm^?1/8=4·62×10¹⁹p_Cd^1/4 exp (?0·195 eV/kT) cm^?3 atm^1/4The formation of pairs (I_SV_Cd)′ from I_S^· and V_Cd″ is governed by the equilibrium constant K_{P(I, V)}=4 exp (≤1·1 eV/kT)If Cd diffusion occurs primarily by free vacancies, the Cd^* tracer self diffusion leads to a vacancy mobility of (1·2±0·5)×10^?5 cm² sec^?1 at 900°C, in agreement with results reported by Woodbury [12], but (7±3) times larger than reported by Kumar and Kroger [10].     

A measurement of surface diffusion across steps on a curved surface (Pd on W)

The surface diffusion of palladium on the curved part of a tungsten crystal is studied by field electron microscopy. The variation of the local coverage distribution is measured by a probe-hole device on the stepped surface region around (001). The measured data allow a determination of the mass transport surface diffusion coefficient D of Pd on W across atomic steps as a function of temperature, coverage and step density. D has been found (1) to be constant for a given step density and for coverages lower than about 5 × 10¹⁴ Pd adatoms/cm², (2) to increase for higher coverages, and (3) to increase with increasing step density for a given coverage. The activation energy of the process is nearly constant (about 24 kcal/mol) for all coverages up to about 6 × 10¹⁴ adatoms/cm², while the pre-exponential factor of D increases with increasing step density. Interpretation of the results gives some information on the diffusion mechanism.     

Chemically driven diffusion of Pd in the surface region of Zn(0001): An ultraviolet photoemission investigation

The diffusion of deposited Pd through the (0001) surface region of zinc has been studied with photoemission at hν = 21.2 eV by following the time evolution of the Zn 3d and Pd 4d peaks for a Pd initial coverage of 1, 3, 10 and 15 monolayers. The time decay of the Pd 4d signal is explained with a model where the diffusion coefficient D is not constant; it is (4.6 ± 0.5) × 10^?19 cm² s^?1 for t?7000s, then decreases to (5.5 ± 1) × 10^?20 cm² s^?1 for t ? 15,000 s. The D values correlate well with the spectroscopic results on the valence state evolution during diffusion. At short times (higher D) the spectra show an electron energy gain of Pd atoms during diffusion while at higher time (lower D) this gain is negligible. The initial diffusion is chemically driven while at longer times the diffusion becomes gradually entropic.     

Diffusion behavior of tungsten clusters on the tungsten (110) planes

Diffusion of tungsten diatomic clusters on the W (110) exhibits comparable prefactor (1.6×10^-4 cm²/sec) as single W atoms, but with slightly higher activation energy. Clusters with three atoms are unstable with respect to diffusion.     

Measurement of diffusion coefficients of Ag in YBa2Cu3O7 by the EDXRF technique

The diffusion of Ag in superconducting YBa₂Cu₃O₇ ceramic was studied over the temperature range 700–850 °C by the energy‐dispersive X‐ray fluorescence (EDXRF) technique. For the excitation of silver atoms, an annular Am‐241 radioisotope source (50 mCi) emitting 59.543 keV photons was used. The temperature dependences of Ag diffusion coefficients in grains (D₁) and over the grain boundaries (D₂) are described by the equations D₁ = 1.4 × 10^?2exp[?(1.18 ± 0.10)/kT] and D₂ = 3.1 × 10^?4exp[?(0.87 ± 0.10)/kT]. Copyright © 2003 John Wiley & Sons, Ltd.     

Effect of the ionic radius on jump frequencies of alkaline earth cations in NaCl crystals

By comparing diffusion coefficientsD of bivalent cations Ba²⁺, Ca²⁺, Sr²⁺ in NaCl crystals it was shown that in the temperature range above 550 °CD (Ba²⁺)>D (Sr²⁺)>D (Ca²⁺) is valid. Temperature dependences of jump frequenciesw ₂ of these cations are described byw ₂ (Ba²⁺)=(2·15±0·55) × 10¹² × exp {?(0·817±0.007)/kT};w ₂ (Sr²⁺)=(2·9±1·1) × 10¹² × exp {?(0·84±0.02)/kT} andw ₂ (Ca²⁺)=(5·5±6·5) × 10¹⁰ × exp {?(0·51±0·07)/kT}. It was demonstrated that in NaCl crystals the activation enthalpy and the preexponential factor of the jump frequencyw ₂ increase with increasing ionic radius and mass of the bivalent alkaline earth cation.     

Diffusion constant and surface states of positrons in metals

Positron lifetime spectra and angular correlation curves for seven fine-grained powders of Fe, Co, Ni, and W are analyzed. From the lifetime data, the positron diffusion constant in metals atT=300°K was found to beD ₊=(1.0±0.5)×10^?2 cm² sec^?1. Evidence is presented that positrons are trapped in metal surface states.     

Diffusion of chromium and manganese in γ-TiAl

The γ-TiAl intermetallic compound with suitable alloying additions has shown considerable promise as a material for high-temperature applications. Diffusion studies in this alloy system are useful in assessment of their creep behaviour and structural stability in service conditions. Tracer diffusion coefficients of ⁵¹Cr and ⁵⁴Mn in a γ-TiAl intermetallic compound containing 54.1 at. % aluminium were determined in the temperature range from 1095 to 1470?K. The temperature dependence of both the diffusing species follows a linear Arrhenius behaviour and can be expressed as D _Cr?=?4.4?×?10^?3exp(?350?kJ?mol^?1/RT)?m^2?s^?1 and D _Mn?=?1.2?×?10^?3?×exp(?326?kJ?mol^?1/RT)?m^2?s^?1. The data are analysed on the basis of empirical correlations between the diffusion and melting parameters applicable for conventional mono-vacancy diffusion mechanism in metals. It is concluded that impurity diffusion in γ-TiAl occurs through the migration of thermal vacancies via nearest-neighbour or next-nearest neighbour jumps.