Superconductivity in PdAlx hydrides

Hydrides of PdA$\text{l}$_x alloys prepared by electron beam co-evaporation and subsequent implantation of hydrogen ions are found to exhibit anomalous changes in the superconducting transition temperature with increasing A$\text{l}$ content. At large hydrogen concentrations, H/Pd ～ 1, the addition of aluminum increases T_c only slightly (from 8.1 to 8.6°K for 7 at. % A$\text{l}$), and then yields an abrupt decrease in T_c at 10% A$\text{l}$. This behavior is in sharp contrast to PdAg_xH_y alloys which exhibit an enhanced T_c ～ 16°K at relatively low silver content.     

Systematically weak temperature dependence of electric field gradients in distorted cubic metals

The temperature dependence of the electric field gradient for radiation-damaged copper and for the dilute alloy Zn$\text{Ag}$ has been measured using the perturbed angular correlation technique. A systematically weak temperature dependence is found for distorted cubic metals compared to non-cubic metals. The strong temperature dependence for the Zn$\text{Ag}$, In$\text{Ag}$ and Sn$\text{Ag}$ alloys as reported in literature has not been confirmed in the present experiment.     

A study of the (00) LEED beam intensity at normal incidence from CdS(0001), Cu(001), Cu(111), and Ni(111)

A Faraday cage apparatus is used for the measurement of the (00) LEED beam intensity, I₍₀₀₎, and the total secondary emission coefficient, δ(E_k), for angles of incidence from 0° ± 2° to 8° ± 2°, with an energy resolution of ± 0.037 of the incident beam energy, in the energy range 1 to 200 eV. The data are normalized and expressed as a fraction of the incident beam intensity. The basic principle of operation is the separation of the incident and specularly diffracted beams in a uniform magnetic field. Monolayer, or in-plane, resonances associated with the emergence of nonspecular beams, as well as beam threshold minima, are observed in I₍₀₀₎ at normal incidence from clean CdS(0001), Cu(111), and Ni(111). Some major differences are observed in the I₍₀₀₎ profiles for the clean (111) surfaces of nickel and copper. All secondary Bragg peaks, except the 2$\text{2}\text{3}$ order, have greater intensities for Ni(111) in the energy range 50–150 eV, thus indicating that the atomic scattering cross-section for electrons in this energy range is larger for nickel than for copper. For the (111) surface of nickel, the (11) resonance is missing, but the (10) resonance and all $\text{1}\text{3}$ order secondary Bragg peaks between the second and fifth orders are observed. For Cu(111) both the (10) and (11) resonances are observed, but the $\text{1}\text{3}$, $\text{2}\text{3}$, 1$\text{2}\text{3}$, and 3$\text{1}\text{3}$ order secondary Bragg peaks are missing in this energy range. These data indicate that multiple scattering with evanescent intermediate waves, or “shadowing”, is predominate on the (111) surfaces on nickel and copper for energies above 30 eV, and that below 30 eV multiple scattering with propagating intermediate waves is predominate on Cu(111). Correlation of the (00) beam intensity profiles from clean Ni(111) at 0°, 2°, and 6° with the intensity profiles of the (10). (1̄0), and (11) non-specular beams is nearly one-to-one from 30 eV to 100 eV, thus supporting the dynamical theories of LEED in which peaks in the (00) beam are expected to occur at nearly the same energies as peaks in the non-specular beams.     

Deposition of nickel from Ni(CO)4 on palladium and iron surfaces studied by X-ray photoelectron spectroscopy

The interaction of nickel carbonyl, Ni(CO)₄, with evaporated palladium and iron surfaces has been studied at 90 and 290 K by X-ray photoelectron spectroscopy. The carbonyl is weakly adsorbed in molecular form at 90 K on the metals giving a $\text{Ni 2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ peak at 854.6 eV, a C 1s at 287.2 eV and an O 1s at 533.8 eV. Some fraction of the carbonyl decomposes even at 90 K on iron to give deposited nickel atoms. In the interaction with palladium at 290 K, deposited nickel atoms $\text{(Ni 2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{= 852.9 eV)}$ and chemisorbed CO are observed. A satellite feature of the $\text{Ni 2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ peak varies depending on the quantity of the deposited nickel atoms; the main peak-satellite separation increases with increase in the quantity. The same variation is observed for evaporated nickel-palladium alloys. This can be ascribed to the difference in the electronic states of the nickel atoms. The difference is reflected in the reactivity of the atoms with O₂. With iron the deposited nickel atoms show an increase in binding energy of 0.4 eV in the $\text{Ni 2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ Peak and no satellite when the number of nickel atoms is small. The oxidation of the surface is also studied.     

Susceptibility behaviour of Mn dissolved in liquid Cu—Al-alloys

Measurements of the magnetic susceptibility of liquid Mn_0.05—Cu—Al alloys up to 1700 K indicate Curie—Weiss like behaviour at the Cu-rich end and minima of the reciprocal Mn susceptibility values as a function of the temperature 1/_ξMn(T) at the A1-rich end of the series. The occurrence of 1/_ξMn(T) minima at temperatures, which are sensitively depending on the alloy composition, hints at a transition to localized magnetic moment behaviour even in the A1-based alloys at high temperatures. Both, the aspects of extremely increasing Kondo temperature and that of decreasing spin fluctuation times, allow to discuss the susceptibility behaviour uniformly for the whole alloy series. Thus, the results do not support a fundamental distinction (magnetic—nonmagnetic) between the alloy systems $\text{Cu}$—Mn and $\text{A1}$—Mn.     

The surface composition of the gold-palladium binary alloy system

The surface composition of the AuPd alloy system has been determined by Auger electron spectroscopy. Measurements were performed on polished polycrystalline alloy foils. After cleaning, the intensities of the 71 eV and 2024 eV gold Auger transitions, and the intensity of the 330 eV palladium transition were measured, and then converted to atom concentrations in the surface layer. The surfaces of the annealed samples were found to be significantly enriched in gold with respect to the bulk. This result disagrees with the regular solution theory prediction. After extensive sputtering of the AuPd alloys with 1.5 keV Ar⁺ ions, a slight surface enrichment with palladium was found, as predicted by the simple theoretical model for sputtering.     

AES study of nickel carburization: Electron beam-induced effects

A preliminary study of the CO/CO₂ carburization of (111) Ni at 217°C and 1 × 10^?6 Torr total pressure by Auger electron spectroscopy was undertaken. It was found that electron beam (～20 μA, ～2500 μA/cm², 3 kV) induced effects were significant for CO and CO₂ adsorbed on the nickel surface; these effects could be factored out by using delayed beam techniques. The electron-induced reactions were similar in both cases and plots of normalized carbon peak height versus time could be characterized by the equation $\text{C = A(1-}\text{e}{}^{\mathrm{<mtext>?</mtext><mtext>t</mtext><mtext>\tau </mtext>}}$) were τ_CO = 32.0 min , A_CO = 206.2, τ_CO2 = 39.8 min, A_CO2 = 176.2. Observed oxygen peaks were smaller than expected. The evidence of several researchers suggests electron beam induced dissociation of CO and CO₂ followed by electron beam induced desorption of the resulting oxygen. Reactant gas interaction with the electron gun cathode was significant with resulting beam movements causing scatter in AES peak height measurements.     

Chemical shifts in auger electron spectra from silicon in silicon nitride

The chemisorption of nitric oxide on (110) nickel has been investigated by Auger electron spectroscopy, LEED and thermal desorption. The NO adsorbed irreversibly at 300 K and a faint (2 × 3) structure was observed. At 500 K this pattern intensified, the nitrogen Auger signal increased and the oxygen signal decreased. This is interpreted as the dissociation of NO which had been bound via nitrogen to the surface. By measuring the rate of the decomposition as a function of temperature the dissociation energy is calculated at 125 kJ mol^?1. At ～860 K nitrogen desorbs. The rate of this desorption has been measured by AES and by quantitative thermal desorption. It is shown that the desorption of N₂ is first order and that the binding energy is 213 kJ mol^?1. The small increase in desorption temperature with increasing coverage is interpreted as due to an attractive interaction between adsorbed molecules of ～14 kJ mol^?1 for a monolayer. The (2 × 3) LEED pattern which persists from 500–800 K is shown to be associated with nitrogen only. The same pattern is obtained on a carbon contaminated crystal from which oxygen has desorbed as CO and CO₂. The (2 × 3) pattern has spots split along the (0.1) direction as $\text{(m,}\text{n}\text{3}\text{)}$ and $\text{(}\text{m}\text{2, n}\text{)}$. This is interpreted as domains of (2 × 3) structures separated by boundaries which give phase differences of $\text{2π}\text{3}$ and π. The split spots coalesce as the nitrogen starts to desorb. A (2 × 1) pattern due to adsorbed oxygen was then observed to 1100 K when the oxygen dissolved in the crystal leaving the nickel (110) pattern.     

Electron density and electron redistribution in alloys—II: Electron density in alloys and intermetallic phases

Electron density for alloys which have close-packed metallic structures is calculated by assigning valence electrons to octahedral and tetrahedral interstices, a method which has been previously used for elemental metals. Some localization of electron density is proposed for β -phases when there is considerable difference in ion core sizes. This method of characterizing electron density in alloys can be used to derive structures with the amount of electron transfer if an assumption is made for the volume fraction occupied by each component of the alloy. In general, the electronic structure of intermetallic phases appears to be dominated by the correspondence of a definite number of valence electrons with the number of interstices in the metallic structure (the Hume-Rothery $\text{e}\text{a}$ ratios). The model used can also accommodate electron distributions which include both ionic and covalent components of electron density. This is the case for Laves phases and the metallic A-15 compounds. There is a preponderance of intermetallic phases where one component is a d-shell metal. Evidence is presented that in several such alloys there is a change in d-shell configuration of the elemental metal which serves to minimize size differences of the ion cores of the alloy.     

Analysis profiles of oxide films on chrome steel by auger emission and x-ray photoelectron spectroscopies

Samples of steel from two different sources were examined. The materials had nominally the same bulk composition but different samples from each batch had been hardened under two slightly different conditions.The surface oxide films were analysed by both Auger emission spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) after a number of ion bombardments, and the variation in composition with depth was established. In general the outermost oxide layers were chromium deficient and XPS results suggested the presence of Fe₂O₃ at the surface, together with atmospheric contamination. After ion bombardment the proportion of chromium oxide increased and both iron metal and oxide (Fe₃O₄) were present. The amount of chromium oxide reached a maximum at the steel oxide interface and on further bombardment was replaced by chromium metal. AES and XPS results were in agreement qualitatively and also quantitatively after measurement with a curve analyser of the areas under the peaks of certain elements.The present investigation has shown that AES and XPS can give very similar analyses provided some simple corrections are applied. However, the use of the higher resolution of XPS can provide additional information which cannot be obtained by AES. Thus, using expanded XPS scans, together with suitable curve analysis techniques, it is relatively simple to separate signals from oxide and from free metal regions of the sample, and to study in detail the changes in composition of the oxide from the oxide-air to the oxide-metal interfaces.The study of the four steel samples in this manner has shown that there is a thin region rich in chromium oxide adjacent to the metal. The high level of chromium falls off fairly rapidly nearer the surface and appears to stabilize at a very low level. This is particularly obvious in the thicker oxides (PH), where there is a thick surface layer which is principally iron oxide. It would appear that under the conditions of the heat treatment given to the present samples the thickening of the oxide is due almost entirely to iron oxide. Other work has shown that the low temperature air-formed oxide on these steels is chromium rich and it is suggested that the thickening is due to diffusion of iron through the original film with little movement of the chromium.In view of the high chromium deficiency in the outer layers of the oxides examined it is apparent that the supposedly protective chromium oxide film on chrome steel is situated at the oxide-metal interface and not on the outer surface.The present investigation has shown the existence of two unreported iron satellites. One would appear to be associated with the mixed valence oxide Fe₃O₄ and another with the purely trivalent oxide Fe₂O₃. The satellites could be associated with a strong plasma loss mechanism, or be due to shake-up phenomena as in the case of copper and nickel oxides¹⁴As might be expected from the normal oxidation behaviour of iron, the present oxide films appear to consist of a thin layer of Fe₂O₃ overlying the main Fe₃O₄ oxide. The removal of this outermost Fe₂O₃ layer is rapid and is accompanied by the changes in the satellites mentioned above, together with changes in the position of the Fe 2p_{$\text{3}\text{2}$} oxide peak.In conclusion, it is clear that a detailed XPS examination can not only provide information on overall compositional changes, as can be obtained by AES, but can also provide a comprehensive picture of changes in oxide composition, including those due to oxides of one metal in different valency states.     

Low energy electron spectroscopy of Pt (100) and Pt (100)/CO

Fine structure in the n_vi, _VIIVV spectrum of clean Pt (100) has been observed, and interpreted as “band like” in origin rather than quasi-atomic. Differences in the dependence of the Auger yield on primary beam energy are observed between the N_VI, _VIIVV and O_IIIVV peaks, and are associated with anomalies in the dependence of the inner shell ionization crossection of the 4f level. Low energy electron loss spectra on the clean surface have been investigated at primary energies in the range 71–774 eV and at angles of incidence of the beam 0–60°. The results are related to high energy loss and optical data, and assignments are given for inter-band and plasmon losses. With approximately $\text{3}\text{4}$ of a monolayer of CO on the surface there is a prominent additional loss at around 13.5 eV, which is interpreted as a one electron transition from a σ state below the d band to available states several electron volts above the Fermi level.     

The growth and alloy formation of copper on the platinum (111) and stepped (553) crystal surfaces; characterization by Leed,AES, and CO thermal desorption

Epitaxial layers of copper were formed on Pt(111) and Pt(553) single crystal surfaces by condensation of copper atoms from the vapor. Surface alloys were formed by diffusing the copper atoms into the platinum substrate at temperatures above 550 K. The activation energy for this process was found to be ~ 120 $\text{kJ}\text{mol}$. These Pt/Cu surfaces were characterized by LEED, AES, and TDS of CO. The copper grows in islands on the Pt(111) surface and one monolayer is completed before another begins. There is an apparent repulsive interaction between the copper atoms and the step sites of the Pt(553) surface which causes a second layer of copper to begin forming before the first layer is complete. Epitaxial copper atoms block CO adsorption sites on the platinum surface without affecting the CO desorption energy. When the copper is alloyed with the platinum however, the energy of desorption of CO from the platinum was reduced by as much as 20 $\text{kJ}\text{mol}$. This reduction in the desorption energy suggests an electronic modification that weakens the Pt-CO bond.     

Adsorption of copper on single crystal planes of tungsten

Properties of copper condensed on selected areas of a thermally cleaned tungsten surface have been studied by probe hole field emission microscopy. Interpretation of the φ_hkl?$\text{}\text{?}$gq relationship observed on (211), (111) and (310) for adsorption at T > 300 K is based on hardsphere models of the plane surfaces upon which the first monolayer of copper raises φ_ithkl, the second reduces it to a minimum value, and the third achieves $\text{}\text{?}$gf_sat on each plane. At T > 300 K the relatively low binding energy of copper on (110) prevents is population below ～2$\text{}\text{?}$gq as previously observed for lead, and the plateaux in the φ₁₁₀?$\text{}\text{?}$gq curves are thought to result from the difficulty of nucleating the first and second monolayers of copper on (110). Comparison of our observations with those made by LEED/Auger techniques emphasise significant differences between the substrates used, in that, on field emitters (110) is step-free and surrounded by a sink/ source of adatoms, while the LEED specimen is stepped but has no comparable local sink/ source. The initial changes in φ are ascribed to formation of an adsorbate-substrate dipole whose sign and magnitude is controlled by electron equilibration between the substrate metal and a broadened and partly-filled resonance level lying approximately 5.2 eV below the vacuum level. Measurement of the total energy distribution of electrons field emitted from (110) and (132) supports this picture which contrasts with that of Polanski and Sidorski who consider the dipole sign and strength to be controlled principally by adsite geometry. Low activation energies characterise surface transport which is controlled by one-plane processes, and in some cases transport across the probed area is controlled by processes of relatively high activation energy which take place outside the examined plane.     

Bulk-to-surface precipitation and surface diffusion of carbon on polycrystalline nickel

The time evolution of the KLL Auger spectrum of carbon as a function of temperature is used to derive the kinetics of the surface diffusion and bulk-to-surface precipitation of carbon on polycrystalline nickel. The results show that the activation energy for the surface diffusion of carbon atoms on polycrystalline nickel is 6.9 ± 0.6 $\text{kcal}\text{mole}$, and the activation energy for bulk-to-surface precipitation is 9.4 ± 0.6 $\text{kcal}\text{mole}$. The dependence on the surface diffusion coefficient D_s (cm²s^?1), on the absolute temperature T can be represented, over the experimental temperature range, 350–425° C, by: $\text{ln}\text{D}{}_{\mathrm{s}}\text{= 10.27 ?}\text{3568}\text{T}$.     

Elastic constants of single crystals of the three phases of In---Pb alloys

In a study of the elastic behaviour of the InPb alloys, the elastic stiffness tensor components of crystals of each of the three phases (fct, $\text{c}\text{a}\text{> 1}$; fct, $\text{c}\text{a}\text{> 1}$; fcc) have been obtained as a function of temperature from pulse superposition measurements of ultrasonic wave velocities. Comparison of the elastic stiffness constants obtained for a fct ($\text{c}\text{a}\text{> 1}$) 5 atm.% Pb alloy with those of In itself and those of InTl and InCd alloys, establishes for this phase that alloying with Pb, as with TI and Cd, enhances the softening of the acoustic [110] phonon mode, polarization [1$\text{1}$0] near the Brillouin zone centre. The elastic properties of a 17 atm.% Pb crystal, which is in the fct ($\text{c}\text{a}\text{> 1}$) phase, are quite different from those shown by In alloys in the fct ($\text{c}\text{a}\text{> 1}$ phase; in particular the response to a shear stress is remarkably isotropie: there is no phonon mode softening in this alloy. Neither is there softening of this mode (which corresponds at the zone centre to the shear stiffness $\text{1}\text{2}$(C₁₁;C₁₂)) in crystals of the fee phase — in complete contrast to the dominating influence of the softening of $\text{1}\text{2}$(C₁₁;C₁₂) in the InTl and InCd fee alloys. In fact for a fcc In-75 atm.% Pb alloy the anistropy ratio for shear $\text{2C}{}_{44}\text{(C}{}_{11}\text{C}{}_{12}\text{)}$ is close to unity. The transitions between the three phases of the InPb alloys are markedly first order and acoustic mode softening has a much smaller influence on the elastic behaviour of the fct ($\text{c}\text{a}\text{< 1}$) and fcc InPb alloys than it has on the fct ($\text{c}\text{a}\text{< 1}$) InPb, InCd and InTl alloys.     

Segregation studies in Pd-Ag

Auger studies of the surface composition of a Pd/Ag alloy ($\text{60}\text{40 wt\%)}$ display reversible, temperature dependent enhancement of silver in the absence of sulfur. Co-segregation of sulfur effects a strong depletion of silver in the detected volume. No evidence could be found for the surface formation of a bulk palladium sulfide. In the absence of compound formation, the silver depletion can be qualitatively understood in terms of a recent ternary solution model which incorporates solute—solute interactions.     

The role of impurities in the stability of ZnO surfaces

Both “as-grown” and “real” etched prism and (000$\text{1}$) oxygen surfaces have been studied by LEED and Auger electron spectroscopy. Heat treatment up to 800 K was sufficient to remove impurities other than calcium on all surfaces and potassium on the polar “real” surface. These could only be removed by ion bombardment. The Ca was associated with a (3 × 1) superstructure on the prism surface and a $\text{(}\text{3}\text{×}\text{3}\text{)}$ on the polar surface. On the “as-grown” polar surface it was also possible to see (3 × 3) structure associated with reduced amounts of Ca. The especially strong binding of the electropositive elements on the negative oxygen polar surface is due to charge transfer, i.e. impurity stabilisation, this in turn can lead to chemical shifts in some of the Zn Auger transitions and to changes in the oxygen peak shape.     

Magnetic hyperfine field for zinc in iron

The magnetic hyperfine field for zinc in iron has been measured by the DPAD method following the reaction ⁶⁴Ni(α, n)⁶⁷Zn^m. Measurements performed with a sandwich target and with ⁶⁴Ni($\text{Fe}$) alloy targets containing 3% and 6% nickel yielded similar results: H_hf(0) = -191 ± 3 kG.     

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.     

Interpretation of a new feature in photoemission from Cu based alloys with polyvalent solutes

In our recent angle resolved photoemission studies of oriented single crystal surface of $\text{Cu}$Al and $\text{Cu}$Ge solid solutions we had found a new structure lying between 4 and 5 eV below the Fermi energy, which could not be related to either the bulk or the surface states in the random alloy. To understand its nature and origin we report and discuss photoemission measurements from Cu films, submonolayer to a monolayer thick, deposited on the Al(111) and $\text{Cu}\text{Al(111)(}\text{3}\text{×}\text{3}\text{R}\text{30°)}$ alloy surfaces and from Al and Zn alloys containing dilute Cu impurities. Some experiments on the (100) surfaces in the various cases were also carried out. On the basis of these results, we suggest that the aforementioned new photoemission feature is characteristic of Cu clusters and isolated Cu impurities in polyvalent impurity-rich environment. These clusters probably lie at the topmost layer of the $\text{Cu}$Al and $\text{Cu}$Ge alloy surfaces.