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The intermetallic compound InPd (CsCl type of crystal structure with a broad compositional range) is considered as a candidate catalyst for the steam reforming of methanol. Single crystals of this phase have been grown to study the structure of its three low-index surfaces under ultra-high vacuum conditions, using low energy electron diffraction (LEED), X-ray photoemission spectroscopy (XPS), and scanning tunneling microscopy (STM). During surface preparation, preferential sputtering leads to a depletion of In within the top few layers for all three surfaces. The near-surface regions remain slightly Pd-rich until annealing to ~580 K. A transition occurs between 580 and 660 K where In segregates towards the surface and the near-surface regions become slightly In-rich above ~660 K. This transition is accompanied by a sharpening of LEED patterns and formation of flat step-terrace morphology, as observed by STM. Several superstructures have been identified for the different surfaces associated with this process. Annealing to higher temperatures (=750 K) leads to faceting via thermal etching as shown for the (110) surface, with a bulk In composition close to the In-rich limit of the existence domain of the cubic phase. The Pd-rich InPd(111) is found to be consistent with a Pd-terminated bulk truncation model as shown by dynamical LEED analysis while, after annealing at higher temperature, the In-rich InPd(111) is consistent with an In-terminated bulk truncation, in agreement with density functional theory (DFT) calculations of the relative surface energies. More complex surface structures are observed for the (100) surface. Additionally, individual grains of a polycrystalline sample are characterized by micro-spot XPS and LEED as well as low-energy electron microscopy. Results from both individual grains and "global" measurements are interpreted based on comparison to our single crystals findings, DFT calculations and previous literature. [ABSTRACT FROM AUTHOR]

A combination of experimental methods was used to study the structure of In thin films deposited on the Pd(111) surface and the alloying behavior. X-ray photoelectron spectroscopy (XPS), lowenergy electron diffraction (LEED), and scanning tunneling microscopy results indicate that surface alloying takes place at room temperature. Below 2 monolayer equivalents (MLEs), the LEED patterns show the formation of three rotational domains of InPd(110) of poor structural quality on top of the Pd(111) substrate. Both core-levels and valence band XPS spectra show that the surface alloy does not yet exhibit the electronic structure characteristic of the 1:1 intermetallic compound under these conditions. Annealing the 1 MLE thin film up to 690 K yields to a transition from a multilayer InPd near-surface intermetallic phase to a monolayer-like surface alloy exhibiting a well ordered (√3 × √3) R30° superstructure and an estimated composition close to In2Pd3. Annealing above 690 K leads to further In depletion and a (1 x 1) pattern is recovered. The (√3 × √3) R30° superstructure is not observed for thicker films. Successive annealing of the 2 MLE thin film leads the progressive disappearance of the InPd diffraction spots till a sharp (1 x 1) pattern is recovered above 690 K. In the high coverage regime (from 4 to 35 MLE), the formation of three rotational domains of a bee-In7Pd3 compound with (110) orientation is observed. This In-rich phase probably grows on top of interfacial InPd(110) domains and is metastable. It transforms into a pure InPd(110) near-surface intermetallic phase in a temperature range between 500 and 600 K depending on the initial coverage. At this stage, the surface alloy exhibits core-level chemical shifts and valence band (VB) spectra identical to those of the 1:1 InPd intermetallic compound and resembling Cu-like density of states. Annealing at higher temperatures yields to a decrease of the In concentration in the near-surface region to about 20 at.% and a (1 x 1) LEED pattern is recovered. [ABSTRACT FROM AUTHOR]

We report the adsorption of lead adatoms on the pseudo-10-fold Al 13 Fe 4 (0 1 0) surface using low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). For the submonolayer regime, Pb adatoms remain highly mobile across the surface at 300 K. STM analysis indicates the formation of irregularly shaped islands of monoatomic height. The latter do not coalesce with increasing coverage. At 0.95 MLE coverage, the LEED patterns are consistent with a pseudomorphic growth of the adatoms. This is confirmed by STM measurements which reveal local motifs qualitatively similar to those observed on the clean Al 13 Fe 4 (0 1 0) surface, i.e. prior to dosing. Apart from the absence of plasmons, the XPS measurements of Pb 4 f and Al 2 s core levels are comparable to those observed for the Pb/Al(1 1 1) system. [ABSTRACT FROM AUTHOR]

Microstructures and morphological features of a series of sintered quasicrystalline Al62.5-xCu25.3Fe12.2Bx alloys, with x ranging from 0 to 5 at.% were studied using x-ray diffraction, scanning electron microscopy, x-ray mapping, and electron probe microanalysis. Electron backscattering diffraction (EBSD) was also used to get information about the structures of some phases and identify the crystalline relationship in-between phases. Increasing x results in the change of the nature of extra phases. These secondary phases are all less than 1% in volume of the total matter except for the β phase at 5% of boron. Whatever the percentage of boron considered, boron seems to concentrate essentially in the parasite phases confirming doubts found in literature about the solubility of boron inside the face-centered-icosahedral Al-Cu-Fe phase. No special crystallographic relationship in between the tested phases could be spotted. EBSD is thus also confirmed as an excellent technique to get quasicrystalline grains orientations. [ABSTRACT FROM AUTHOR]

We report the formation of an ordered C60 monolayer on the Al9Co2(001) surface using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), x-ray and ultraviolet photoelectron spectroscopy (XPS/UPS), and ab initio calculations. Dosing fullerenes at 300 K results in a disordered overlayer. However, the adsorption of C60 with the sample held between 573-673 K leads to a [4, -2ι 1,3] phase. The growth of C60 proceeds with the formation of two domains which are mirror symmetric with respect to the [100] direction. Within each domain, the superstructure unit cell contains six molecules and this implies an area per fullerene equal to 91 Å2 . The molecules exhibit two types of contrast (bright and dim) which are bias dependent. The adsorption energies and preferred molecular configuration at several possible adsorption sites have been determined theoretically. These calculations lead to a possible scheme describing the configuration of each C60 in the observed superstructure. Several defects (vacancies, protrusions,. . . ) and domain boundaries observed in the film are also discussed. If the sample temperature is higher than 693 K when dosing, impinging C60 molecules dissociate at the surface, hence leading to the formation of a carbide film as observed by STM and LEED measurements. The formation of AI4C3 domains and the molecular dissociation are confirmed by XPS/UPS measurements acquired at different stages of the experiment. The cluster substructure present at the Al9Co2(001) surface dictates the carbide domain orientations. [ABSTRACT FROM AUTHOR]

he Al5Co2 crystal is a complex intermetallic compound, whose structure can be described by a stacking of chemically bonded atomic motifs. It is a potentially new catalytic material for heterogeneous hydrogenation. A single crystal of this phase has been grown by the Czochralski technique in order to study the influence of the three-dimensional bulk substructure on the two-dimensional surface using both experimental ultrahigh vacuum surface techniques and ab initio methods based on the density functional theory. Some bulk properties are first presented, focusing on chemical bond strengths, the determination of the Al and Co chemical potentials in Al5Co2, the vibrational properties, and the specific heat. Then, the combination of experimental and computational approaches allows the identification of the surface structure, which was found to depend on the surface preparation conditions. In all cases, the surface terminates at specific bulk layers (Al-rich puckered layers) where various fractions of specific sets of Al atoms are missing, identified as Al3 atoms left at the surface resulting from cluster truncation. Finally, electron density of states calculations and spectroscopic measurements were compared and indicate a strong sp-d hybridization of the topmost pure Al layer with subsurface Co atoms. This could influence the surface reactivity and the catalytic performances of this material. [ABSTRACT FROM AUTHOR]

The growth and stability of Bi thin films on the Al13Co4(100) surface has been investigated from the submonolayer to high-coverage regime by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) for temperatures ranging from 57 to 633 K. Initially, Bi adsorption leads to the formation of a pseudomorphic monolayer, followed by the growth of islands of different heights with increasing coverage. The in-plane structure, island height, and island morphology indicate that these islands adopt either a pseudocubic (110) or hexagonal (111) orientation normal to the surface. The (HO)-oriented islands correspond to bilayer stacking (either two or four monolayers in height) while the (11 l)-oriented islands correspond to either three- or four-layer stacking. The in-plane orientation of (110) islands with respect to the substrate is random, while (111) islands adopt one of four possible orientations. In addition, the (111) islands show a moire structure. The fact that Bi islands grow with either (110) or (111) orientation simultaneously on the same substrate relates to a subtle energy balance between both orientations according to ab initio calculations, allowing both structures to coexist. The island density dependence versus both deposition temperature and flux, their most frequent structure type, reshaping effects, and chemical reactivity of the different allotropes are also discussed in this paper. [ABSTRACT FROM AUTHOR]

Structural investigations of the (210) and (100) surfaces of Al5Co2 using experimental ultrahigh vacuum techniques and ab initio computational methods are presented in this work. Both surfaces are identified and show bulk terminations where only specific atoms remain. These atoms can be seen as truncated parts of chemically bonded atomic motifs which have been identified in the bulk in a previous work [M. Meier et al., Phys. Rev. B 91, 085414 (2015)]. Whereas the (210) surface presents a single termination, the structure of the (100) topmost layers is found to be highly dependent on the preparation conditions, especially on the annealing temperature. This behavior, also observed for the (001) surface studied previously, can be partly explained when considering the bonding strength of the truncated motif parts with the subsurface. [ABSTRACT FROM AUTHOR]

By investigating the magnetism of spins on a quasiperiodic lattice, we present an experimental study of static and dynamic magnetic properties, specific heat, and magnetic entropy of the Gd3Au13Sn4 quasicrystalline approximant. The magnetic sublattice of Gd3Au13Sn4 is a periodic arrangement of nonoverlapping spin clusters of almost perfect icosahedral symmetry, where gadolinium localized f magnetic moments are distributed on equilateral triangles. The absence of disorder on the magnetic sublattice and the antiferromagnetic (AFM) interactions between the nearest-neighbor spins distributed on triangles result in geometrical frustration of spin-spin interactions. Thus, the Gd3Au13Sn4 phase can be viewed as a prototype site-ordered, geometrically frustrated spin system on icosahedral clusters. The zero-field-cooled and field-cooled magnetic susceptibilities, the alternating current susceptibility, the thermoremanent magnetization, the memory effect, the magnetic specific heat, and the magnetic entropy all show that the spin system undergoes at low temperatures a transition to a nonergodic state at the spin freezing temperature Tf ≈ 2.8 K. Below this, the ergodicity is broken on the experimental timescale, because the thermally activated correlation times for the spin reorientations become macroscopically long. The magnetic state achieved at low temperatures by continuous cooling in low magnetic fields is likely a superposition of (1) metastable states with randomly frozen spins that have no long- range order yet undergo gradual spin-freezing dynamics and (2) an AFM-like magnetically ordered state with critical slowing dynamics. The magnetic properties of the site-ordered, geometrically frustrated Gd3Au13Sn4 system are discussed in comparison to site-disordered spin glasses that contain both randomness and frustration. [ABSTRACT FROM AUTHOR]

A detailed study of the (001) surface of the Al2Cu crystal using both experimental and ab initio computational methods is presented in this work. The combination of both approaches gives many arguments to match the surface plane with a bulk truncated surface model terminated by incomplete Al planes. The missing rows of Al atoms lead to a 2√2 × √2R 45° surface reconstruction with two domains rotated by 90° from each other. Ab initio calculations demonstrate that the energetic cost associated with the removal of pairs of Al atoms is the lowest for the two nearest surface Al atoms (covalentlike interaction). They reveal that the remaining atomic rows of various widths are oriented according to the graphitelike Al 63 nets used to describe the Al2Cu bulk structure. The surface dynamics observed at 300 K at the Al2Cu surface is also presented. Finally, configurational and vibrational entropies are introduced to discuss the reduced surface plane density. [ABSTRACT FROM AUTHOR]