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The intermetallic phase FeGa3 belongs to the rare examples of substances with transition metals where semiconducting behavior is found. The necessary electron count of 17 ve/fu can be formally derived from eight Fe-Ga and one Fe-Fe two-center-two-electron bond. The situation is reminiscent of the well-known Fe2(CO)9 scenario, where a direct Fe-Fe two-center-two-electron bond was shown to not be present. Fe-Fe interaction in FeGa3 and its substitution variants represents the crucial point for explanation of electronic, thermal transport, and optical properties of this material. Chemical bonding analysis in position space of FeGa3 and Fe2(CO)9 on the basis of the topology of the electron localizability indicator distribution, QTAIM atoms, two- and three-center delocalization indices, domain natural orbitals, IQA analysis, and an evaluation of the Fe-Fe dissociation energy yields a complete picture of the partially compensated Fe-Fe bond, which is nevertheless strong enough to be of decisive importance. Structural reinvestigation of differently synthesized single crystals leads to the composition Fe1+ xGa3 (0 ≤ x ≤ 0.018), where the additional Fe atoms are predicted from DFT/PBE calculations to yield a magnetic moment of about 2 μB/Fe2 atom and metallic in-gap states. Accompanying magnetization and ESR measurements are consistent with this picture.

The atomic structure of a 0.2 atom % Pt-doped complex metallic alloy, monoclinic Al13Fe4, was investigated using a single crystal prepared by the Czochralski method. High-angle annular dark-field scanning transmission electron microscopy showed that the Pt atoms were dispersed as single atoms and substituted at Fe sites in Al13Fe4. Single-crystal X-ray structural analysis revealed that the Pt atoms preferentially substitute at Fe(1). Unlike those that have been reported, Pt single atoms in the surface layers showed lower activity and selectivity than those of Al2Pt and bulk Pt for propyne hydrogenation, indicating that the active state of a given single-atom Pt site is strongly dominated by the bonding to surrounding Al atoms.

Complex intermetallic compounds such as transition metal (TM) aluminides are promising alternatives to expensive Pd-based catalysts, in particular for the semi-hydrogenation of alkynes or alkadienes. Here, we compare the gas-phase butadiene hydrogenation performances of o-Al13Co4(100), m-Al13Fe4(010) and m-Al13Ru4(010) surfaces, whose bulk terminated structural models exhibit similar cluster-like arrangements. Moreover, the effect of the surface orientation is assessed through a comparison between o-Al13Co4(100) and o-Al13Co4(010). As a result, the following room-temperature activity order is determined: Al13Co4(100) < Al13Co4(010) < Al13Ru4(010) < Al13Fe4(010). Moreover, Al13Co4(010) is found to be the most active surface at 110°C, and even more selective to butene (100%) than previously investigated Al13Fe4(010). DFT calculations show that the activity and selectivity results can be rationalized through the determination of butadiene and butene adsorption energies; in contrast, hydrogen adsorption energies do not scale with the catalytic activities. Moreover, the calculation of projected densities of states provides an insight into the Al13TM4 surface electronic structure. Isolating the TM active centers within the Al matrix induces a narrowing of the TM d-band, which leads to the high catalytic performances of Al13TM4 compounds., Graphical abstract

Intermetallic compounds have proved to be interesting alternatives to heterogeneous catalysts prepared from pure noble metals or their alloys. As to study their intrinsic properties, to determine the crystalline structures of specific surfaces and finally to understand elementary processes of heterogeneous catalysis, single crystals of these intermetallics are needed. Inspired by the recent discovery of Ga–Ni catalysts for carbon dioxide reduction to methanol, we have grown for the first time cm3-size single crystals of trigonal Ga3Ni2. We report in detail on the synthesis and Czochralski growth from high-temperature solution using Ga as native solvent. Inclusion formation of Ga-rich fluid proved to be the most severe problem that was minimized by using an extremely low pulling rate down to 25 µm/h.

Intermetallic compounds in the binary Ga-Pd system have proved to be superior as catalysts in the semi-hydrogenation of acetylene. Partial substitution of Ga by Sn even increases the activity. In order to study elementary processes of catalytic reactions, well-oriented surfaces of single-crystalline slaps are needed, i.e. single crystal growth is a prerequisite. For the first time, solid/liquid equilibria including the solid solutions Ga1−xSnxPd and Ga1−xSnxPd2 were studied. Based on these findings single crystal growth of the two solid solution series succeeded using the Czochralski method from (Ga,Sn)-rich solutions. Inclusion-free single crystals of a few cm3 in size could be obtained. Due to different crystal structures of the end members, Sn substitution in Ga1−xSnxPd is restricted to x ≈ 0.15 in the cubic phase. Although a complete solid solution range for Ga1−xSnxPd2 has been reported by Armbruster et al. (2018), mixed crystals of the 1:2 phase can be grown from a native ternary melt only up to approx. x = 0.54 due to missing solid/liquid equilibria at a higher ratio of Sn substitution for Ga.

Background In a routine handling of a catalyst material, exposure to air can usually not be avoided. For noble metal catalysts that are resistant to oxidation, this is not an issue, but becomes important for intermetallic catalysts composed of two or more non-noble chemical elements that possess much different standard enthalpies of the oxide formation. The element with higher affinity to oxygen concentrates on the surface in the oxide form, whereas the element with lower affinity sinks into the subsurface region. This changes the number of active sites and the catalytic performance of the catalyst. We have investigated the instability of the surface composition to oxidation of the Ga3Ni2 noble metal-free intermetallic compound, a new catalyst for the CO2 reduction to CO, CH4 and methanol. Methods The instability of the oxidized Ga3Ni2 surface composition to different heating–annealing conditions was studied by X-ray photoelectron spectroscopy (XPS), used to determine the elemental composition and the chemical bonding in the near-surface region. The dispersion of active sites available for the chemisorption of H2 and CO on the Ga3Ni2 catalyst surface was determined by H2 and CO temperature-programmed desorption. CO2 conversion experiments were performed by using the catalyst material reduced in hydrogen at temperatures of 300 and 600 °C. Results XPS study of the Ga3Ni2 surface subjected to different heating–annealing conditions has revealed that the concentration of Ga at the oxidized surface is strongly enhanced and the concentration of Ni is strongly depleted with respect to the values in the bulk. By annealing the surface at 600 °C in ultra-high vacuum, the oxides have evaporated and thermal diffusion of atoms near the surface has partially reconstructed the surface composition towards the energetically more favorable bulk value, whereas annealing at a lower temperature of 300 °C was ineffective to change the surface composition. Catalytic tests were in agreement with the XPS results, where an increased CO2 conversion for the catalyst reduced with hydrogen at a higher temperature followed an increased Ni/Ga surface concentration ratio. Conclusions The instability of the active surface chemical composition to oxidation in air must be taken into account when considering noble metal-free intermetallic catalysts as alternatives to the conventional catalysts based on noble metals. Ga3Ni2 and other Ga–Ni intermetallic compounds are good examples of binary intermetallic catalysts, whose catalytic performance is strongly affected by exposure to the air.

Intermetallic compound GaPd 2 is a highly selective catalyst material for the semi-hydrogenation of acetylene. We have determined anisotropic electronic, thermal and magnetic properties of a GaPd 2 monocrystal along three orthogonal orthorhombic directions of the structure. By using 69 Ga and 71 Ga NMR spectroscopy, we have determined the electric-field-gradient tensor at the Ga site in the unit cell and the Knight shift, which yields the electronic density of states (DOS) at the Fermi energy e F . The DOS at e F was determined independently also from the specific heat. To see the change of electronic properties of the GaPd 2 phase on going from the bulk material to the nanoparticles morphology, we have synthesized GaPd 2 /SiO 2 supported nanoparticles and determined their electronic DOS at e F from the 71 Ga NMR spin-lattice relaxation rate. The electronic DOS of the GaPd 2 was also studied theoretically from first principles. All results are compared to the chemically related compound GaPd. The active–site-isolation concept for an increased catalytic selectivity is discussed in relation to the GaPd 2 and GaPd structures.

International audience; We report the adsorption of lead adatoms on the pseudo-10-fold Al13Fe4(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 300K. 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 Al13Fe4(0 1 0) surface, i.e. prior to dosing. Apart from the absence of plasmons, the XPS measurements of Pb 4f and Al 2s core levels are comparable to those observed for the Pb/Al(1 1 1) system.

Quite recently intermetallic compounds have been investigated as catalysts for heterogeneous catalysis as they can be highly active regarding a specific reaction and often show advantages in selectivity and long-term stability. The intermetallic phase InPd (CsCl structure type) is considered to be a possible catalyst for methanol steam reforming. Single crystals are needed to study the basic processes of catalysis. Using the Czochralski technique, InPd single crystals were grown from In-rich solutions as to reduce the vapor pressure of In. The crystals show some unusual rough surface morphology and gradients concerning the main components composition. By adjusting the growth parameters like growth temperature and growth rate we succeeded to get inclusion-free single crystals.

Single crystal growth of the intermetallic compounds GaPd 2 , GaPd, and Ga 7 Pd 3 is described for the first time. These phases have recently been explored as highly selective heterogeneous catalysts in the semi-hydrogenation of acetylene. Well-oriented single-crystalline surfaces are needed to perform fundamental studies on catalytic processes. The three Ga–Pd phases were grown using the Czochralski method from Ga-rich solutions. Thermodynamic properties of the Ga–Pd system determine very different growth temperatures reaching from less than 460 °C for Ga 7 Pd 3 to about 1200 °C for GaPd 2 . Avoiding mother liquid inclusion formation proved to be the key problem that could be solved by pulling rates sometimes as low as 25 µm/h and forced convection by high crystal rotation rates. In the last grown part of GaPd 2 crystals decomposition into GaPd 2 /Ga 3 Pd 5 lamellas occurred which can be explained either by spinodal decomposition or by nucleation caused by the GaPd 2 stability region that becomes narrower with decreasing temperatures.

Two symmetrically nonequivalent silicon carbide (SiC) substrate orientations, (0001) Si-terminated and $$(000\overline{1} )$$ C-terminated, were used in the physical vapour transport growth of bulk aluminium nitride (AlN) single crystals. The crystals grown on Si-faces always exhibit an Al-polar growth surface. AlN growth on $$(000\overline{1} )$$ C-terminated surfaces of the SiC substrates was performed to obtain N-polar growth surfaces. An abrupt interface was observed between the AlN crystal and the C-face substrate which is in contrast to the growth on Si-faces where hexagonally shaped SiC hillocks are formed. The growth on C-faces is usually dominated by multi-site nucleation. Applying similar supersaturation conditions that led to step-flow growth on Si-faces to the C-faces resulted in a spiral growth mode, even on highly off-oriented substrates. The obtained broad X-ray diffraction rocking curves of such samples (full-width at half-maximum ≈380 arcsec) indicate the presence of more misfit dislocations and significant misfit stress. In addition, polarity inversion is observed in C-face grown crystals. Though the structural properties of the crystals grown on C-face are inferior to that of the crystals grown on Si-face, the incorporation of unintentional Si impurity was found to be lower (

The atomic structure and composition of a catalyst's surface have a major influence on its performance regarding activity and selectivity. In this respect, intermetallic compounds are promising future catalyst materials, as their surfaces exhibit small and well-defined ensembles of active metal atoms. In this study, the active adsorption sites of the 3-fold-symmetric surfaces of the PdGa interrnetallic compound were investigated in a combined experimental and computational approach using CO as a test molecule. The PdGa(111) and (-1-1-1) surfaces exhibit very similar electronic structures, but have Pd sites with very different, well-defined atomic coordination and separation. They thereby serve as prototypical model systems for studying ensemble effects on bimetallic catalytic surfaces. Scanning tunneling microscopy and Fourier transform infrared spectroscopy show that the CO adsorption on both surfaces is solely associated with the topmost Pd atoms and Ga acts only as an inactive spacer. The different local configurations of these Pd atoms dictate the CO adsorption sites as a function of coverage. The experimental results are corroborated by density functional theory and illustrate the site separation and ensemble effects for molecular adsorption on intermetallic single crystalline surfaces.

The oxidation of the Al4(Cr, Fe) quasicrystal approximant has been studied using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). STM data indicate that the initial oxidation on the (1 0 0) surface proceeds in a highly ordered fashion. Oxygen preferentially adsorbs onto the surface of terraces, and step edges remain intact. The STM images show ordered stripes after initial oxidation. XPS data indicate that only Al is oxidised, with at least two oxidation states present. The Cr and Fe peaks remain unchanged. The oxidation of Al, rather than Cr or Fe, is consistent with the enthalpies of formation for each oxide. The stripes visible on the STM images are therefore identified to be the initial stages of aluminium oxide formation. XPS performed after higher O2 exposure indicates that the (1 0 0) termination shows hindered Al oxide film thickness growth rates compared to the (0 1 0) and (0 0 1) surfaces.

A comprehensive study of the BaF2–Ba3(BO3)2 phase diagram has revealed a significant difference between the two intermediate phases Ba5(BO3)3F and Ba7(BO3)4−y F2+3y . The latter exhibited (BO3)3− ↔ 3F− anionic substitution which, unusually, strongly influences the solidus temperature. A comparison of the Ba5(BO3)3F and Ba7(BO3)4−y F2+3y crystal structures, along with consideration of other compounds demonstrating (BO3)3− ↔ 3F− isomorphism, allows for the disclosure of the mechanism of (BO3)3− ↔ 3F− heterovalent anionic substitution in fluoride borates via [(BO3)F]4− tetrahedral groups being replaced by four fluoride anions. No exception to this mechanism has been discovered among all known phases with (BO3)3− ↔ 3F− substitution.

Scratching the surface: The high selectivity of PdGa catalysts towards the partial hydrogenation of acetylene was previously attributed to the separation of Pd atomic sites at the surfaces. The atomic structures of PdGa surfaces are determined by means of a combined experimental and computational approach allowing investigation of the catalytic dissociation of hydrogen. The two opposite (111) and {{\left( {\bar 1\bar 1\bar 1} \right)}} surfaces show a very different arrangement of catalytic centers.

Aluminium nitride (AlN) bulk single crystals were grown on off-oriented4H-SiC substrates by the sublimation method. High-quality crystals withabout 25 mm in diameter and up to 5 mm in thickness were obtained withan optimized growth process. The crystals show hexagonal symmetry withwell developed 510 (1) over bar16 side facets. Confocal-Ramanspectroscopy and double-crystal X-ray diffraction rocking curve(DCXRD-RC) measurements confirm the high structural quality of the growncrystals. In Raman spectroscopy, the full-width at half maximum (FWHM)of E-2(high) phonon mode decreases from 40 cm(-1) for thin layers (800mu m) to 22 cm(-1) for a 5 mm thick crystal, showing the improvement inquality with thickness. The DCXRD rocking curve FWHM obtained for a 5mmthick crystal is about 120 arcsec. A basal plane bending of 100 arcsecis observed by rocking curve measurements while scanning 17 mm of thesample’s diameter. Two kinds of growth modes namely, step flow growthand spiral growth modes were observed under similar growth conditions.This may be attributed to a slight variation of super-saturation in thegas phase. In both cases, a single active hexagonal growth centre isformed. As-grown surfaces have been found to be always Al-polar. Wetchemical etching revealed both, threading type and basal-planedislocations (BPD) on the surfaces of the samples with total etch pitdensities (EPDs) in the range of 8 x 10(5)-1 x 10(6) cm(-2). (C) 2011WILEY-VCH Verlag GmbH \& Co. KGaA, Weinheim

The intermetallic PdGa is a highly selective and potent catalyst in the semihydrogenation of acetylene, which is attributed to the surface stability and isolated Pd atom ensembles. In this context PdGa single crystals of form B with (111) orientation were investigated by means of X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), scanning tunneling microscopy (STM), X-ray photoelectron diffraction (XPD), and low-energy electron diffraction (LEED) to study the electronic and geometric properties of this surface. UPS and thermal desorption spectroscopy (TDS) were used to probe the chemisorption behavior of CO. The PdGa(111) surface exhibits a (1 × 1) LEED and a pronounced XPD pattern indicating an unreconstructed bulk-truncated surface. Low-temperature STM reveals a smooth surface with a (1 × 1) unit cell. No segregation occurs, and no impurities are detected by XPS. The electronic structure and the CO adsorption properties reveal PdGa(111) to be a bulk-truncated intermetallic compound with Pd-Ga partial covalent bonding.

1-inch diameter, (0001)-plane AlN single crystals were grown on 6H-SiC substrates by the sublimation method. With the optimised growth conditions, 1-3 mm thick, yellow-coloured crystals with smooth c -facet and good structural properties were obtained. X-ray diffraction rocking curves show a full width at half maximum (FWHM) value of about 100 arcsec. The crystal surface exhibits 3D-microstructures with different morphologies as observed by laser scanning microscopy. All these structures reflect the hexagonal symmetry. The origin of these structures was further investigated by atomic force microscopy and an overlapping double growth spiral was observed in the middle of these structures that is attributed to a dislocation core. Green colouration observed in some of the grown crystals was analysed with the electron probe microanalysis and the results infer that more incorporation of carbon may be the cause. Further a high silicon content of up to 5.5 wt\% was observed at the top layer of 3 mm thick green-coloured crystals whereas 3 wt\% were measured for the yellow-coloured ones. The distribution of Si is uniform over the entire diameter of the crystals. Yellowish crystals show improved structural properties compared to the greenish ones. The FWHM value and the peak positions of Raman phonon modes represent a high crystalline quality and also confirm that the crystals are relaxed from stress. Raman micro-mapping analysis indicates the presence of small, localized tensile stress on the growth spiral region, whereas this is not observed at smooth areas of the crystal surface without spirals. Uniform E 2(high) FWHM and phonon intensity over the entire mapped area further prove that the grown crystals are structurally very homogeneous.

Complex metallic phases in the Al-Cr-Fe system are considered to beinteresting because of their enhanced resistance against corrosion.Single crystal growth of the Al(4)(Cr,Fe) and Al(13)(Fe,Cr)(4) phases,which is a prerequisite for detailed studies is presented herein for thefirst time. Along with their binary end members Al(4)Cr and Al(13)Fe(4)growth of cm(3)-size crystals was achieved by using the Czochralskimethod from Al-rich solutions at temperatures of approximately 1000degrees C. Special emphasis is put on the refinement of the Al-richcorner of the ternary phase diagram in determining the 1000 degrees Cequilibria between the incongruent melts and the corresponding ternarysolid solutions. Our findings confirm earlier data on the existenceregion of Al(13)(Fe,Cr)(4), but significantly differ from those withrespect to the so called Al(4)(Cr, Fe) phase. It is shown that theexistence region of Al(4)(Cr,Fe) decomposes into four regions ofstructurally different phases depending on the Cr/Fe ratio. Binarymu-Al(4)Cr crystallizes in the hexagonal space group P6(3)/mmc and candissolve only up to about 1 At-\% iron. More Fe-rich alloys crystallizein the orthorhombic space group Cmcm (about 2 At-\% iron) or Immm(containing between 3 and about 6 At-\% iron). For crystals containingabout 7 At-\% iron the structure belongs to the space group R (3) overbar. The grown single crystals were characterized by electron probemicroanalysis revealing only weak segregation effects. The structuralperfection of the orthorhombic phase was studied using X-ray topographywith the Lang technique. Significant anisotropic properties of thisphase allow to discuss structural similarities with decagonalquasicrystals.

Al-based complex metallic alloys (CMAs) are crystalline intermetallic phases characterised by very complex crystal lattices, showing intriguing surface properties such as low wetting, low friction and hardness which include, for the Al–Cr–Fe system, high corrosion resistance. However, their passivation mechanisms (nm-thick oxide layer formation) still need to be understood in relation with localised corrosion susceptibility. New single phases were therefore characterised by electrochemical Microcell technique. The chromium cations in the oxide proved to play a significant role in stabilising the passive layer at low pH. Corrosion resistance of single- and polycrystalline samples was first compared by means of inductively coupled plasma mass spectrometer (ICPMS) analysis during immersion. Higher dissolution rates have been measured for the low Cr containing (12.5 at%) polycrystals compared to the single crystals. The resistance to localised corrosion attack of the single crystals in an aggressive 0.01 M HCl + 1 M NaCl solution at pH 2 was high, with an anisotropic behaviour of the anodic breakdown potential related to the grain orientation. The orthorhombic (0 0 1) termination with higher Cr/Al atoms ratio formed a passive layer with increased resistance against pitting. Ageing of the samples and the formation of surface Al oxy-hydroxide appeared to decrease the pitting corrosion resistance.

We compare the growth of a Bi thin film on the ten-fold surface of the decagonal Al-Ni-Co quasicrystal at different substrate temperatures and fluxes using scanning tunneling microscopy (STM) and l...

Single crystal growth of PdGa which is a highly selective catalyst for the semi-hydrogenation of acetylene is reported for the first time. Bridgman as well as Czochralski methods were used starting from off-stoichiometric or congruent melting compositions. While Bridgman growth always resulted in polycrystalline ingots with individual grains restricted to a few millimeters in size, using the Czochralski technique from Ga-rich solutions allowed the growth of PdGa single crystals of several cubic centimeters.

We present an experimental and theoretical study of the anisotropic Hall coefficient R H of the Y–Al–Ni–Co periodic approximant to the decagonal phase with composition Al76Co22Ni2. Performing ab-initio calculation of R H for the original Y–Al–Ni–Co structural model [B. Zhang, V. Gramlich, W. Steurer, Z. Kristallogr. 210 (1995) 498] and its relaxed version, we reproduced the experimentally observed anisotropy for all combinations of crystalline directions of the electric current and magnetic field, where the relaxed model yielded better quantitative matching to the experiment. The origin of the anisotropic Hall coefficient is the anisotropic Fermi surface, the anisotropy of which originates from the specific stacked-layer structure of the Y–Al–Ni–Co compound and the chemical decoration of the lattice. Due to the structural similarity of Y–Al–Ni–Co to the d -Al–Ni–Co-type decagonal quasicrystals, the same physical picture explains the universal R H anisotropy of this family of quasicrystals, where R H changes sign along different crystalline directions.

Growth rate anomalies in aqueous solution growth of NaCl0 3 and KCl0 3 were first revealed by V. V. Sipyagin in 1967 and attributed to \"kinetic anomalies\". These phenomena were confirmed in a series of other studies and for a variety of substances. Therefore, some universality of these anomalies was supposed that should be due to structural transformations in the liquid solutions. In this paper, we present for the first time results on growth rate oscillations found in mixed crystal growth and their impact on the isomorphic composition of solid solutions. Thick (100) Na(Cl, Br)0 3 layers were grown at fixed supercooling conditions onto NaCl0 3 seeds in a microcrystallization cell equipped with an optical microscope. In the temperature range of 33-41 °C, four kinetic anomalies were found in solutions of a chlorate : bromate molar ratio of 20 : 1 at supercoolings of 2.0 K and 3.0 K. Lattice parameter measurements of the grown Na(Cl, Br)0 3 layers revealed a linear dependence of lattice constant vs. growth rate. By measuring Vegard’s law (lattice parameter vs. solid solution composition) we were able to determine the oscillations in mixed crystal composition that accompany the kinetic anomalies in solid solution systems.

Oxidation of the Al 9 Co 2 (0 0 1) and Al 13 Co 4 (1 0 0) surfaces has been performed in a wide range of temperatures at 2 × 10 −8 or 1 × 10 −7 mbar oxygen pressure. Only Al O bonding is observed. The oxidation kinetics are controlled by the quantity of oxygen sticking on the surface. Oxidation results from a competition between several effects: formation of an oxide film, dissolution of the film, oxygen desorption and oxygen dissolution into the bulk. For temperatures lower than 710 K for the Al 9 Co 2 (0 0 1) surface and 925 K for the Al 13 Co 4 (1 0 0) surface, a ∼5 A thick oxide film is formed which does not show any long-range order and desorbs upon annealing. When oxidation is performed at higher temperatures, oxygen diffusion into the bulk is observed. A poorly ordered oxide film having a sixton structure is formed on the Al 9 Co 2 (0 0 1) surface when oxidation is performed at 775 K, which is dissolved when annealing at higher temperatures. On the Al 13 Co 4 (1 0 0) surface, only a weak streaky polar circle is observed following annealing at 925 K the film formed at room temperature, which corresponds to an hexagonal network of O atoms into small ultrathin oxide layers domains. The oxidation behaviour of the Al 9 Co 2 (0 0 1) and Al 13 Co 4 (1 0 0) surfaces has been ascribed to the strong covalent character of bonds present in these Al Co phases, which prevents aluminium diffusion. C 2 H n molecules ( n = 2, 4, 6) do not adsorb on the Al 13 Co 4 (1 0 0) surface in the experimental conditions used in this study, thus suggesting that this surface might not be the active one in the semi-hydrogenation of acetylene.

Decagonal Al–Ni–Co quasicrystals were cleaved in ultrahigh-vacuum and the resulting surfaces were investigated by dynamic scanning force microscopy. The samples were cleaved perpendicular to the tenfold axis and perpendicular to one of the twofold axes. Both surfaces show a rough structure with lateral features on the nanometer scale and height differences of angstroms to nanometers. While the corrugation of the tenfold surface does not show any correlation to the quasiperiodic bulk structure, the twofold surface exhibits row-like corrugations, which indicate the existence of columnar structure motifs along the direction of the tenfold axis as expected from structure models. Images from surface regions tilted with respect to the twofold plane strongly indicate the existence of inclined netplanes. In addition, we studied surfaces of an n 0 -(Al–Pd–Mn) quasicrystal approximant, which was cleaved perpendicular to the pseudo-tenfold b-axis. These surfaces show a corrugated structure as well, similar to the results obtained from the decagonal Al–Ni–Co surfaces. There is no indication of a correlation to the periodicity or other structural features of this orthorhombic, crystalline material. � 2004 Elsevier B.V. All rights reserved.

The solubility of gallium orthophosphate (GaPO 4 ) in 15 M phosphoric acid has been studied in the temperature range between 150°C and 250°C. The data were determined using an experimental set-up, which was especially designed for the purpose of dissolution as well as crystallization experiments. The measured values are discussed with respect to the question whether or not a considerable region of metastability exists. The temperature coefficient of the solubility has been found negative for temperatures up to about 250°C.

Powder samples of decagonal AL 70.7 Co 13.3 Ni 16.0 were investigated by X-ray diffraction (XRD) in the temperature range 15–300 K. We evaluated the temperature dependence of lattice parameters a and c , along and perpendicular to the quasiperiodic plane, and observed indications of a phase transition. Temperature dependences were modelled best by second order polynomials leading to linear thermal coefficients whose anisotropy ratio γ ( T )/ α ( T ) was found to decrease with increasing temperature from 1.60 (at 15 K) to 1.05 (at 300 K). At T =150 K the occurrence of an intermediate phase is indicated by at least three additional reflections, which cannot be indexed using integers of reflections of the decagonal lattice. Furthermore, near this temperature the lattice parameters of the decagonal phase showed deviations from a smooth polynomial fit. We will solely describe the experimental facts as they are and make the suggestion that this transition is due to a periodic superorder of the decagonal lattice.

The two-fold decagonal-Al71.8Ni14.8Co13.4(0 0 1 1 0) surface is investigated by highly surface sensitive He atom scattering. The diffraction pattern shows that the topmost surface layer preserves a perfect quasi-crystalline ordering expected from the bulk. The surface is found to develop facets with (1 0 0 0 0)-equivalent orientation, inclined by 18° to the original surface. The facets are of sufficient size to observe diffraction from them. The facet planes are identical to those identified in an as grown sample.

Single crystals of gallium orthophosphate (GaPO 4 ) being isostructural to low-quartz are usually grown from strong acidic solutions. To maintain a good structural perfection of the crystals, the growth rate of the respective growing interface has to be controlled. It is shown that striations are formed due to growth temperature changes, which are marked by a higher density of mother liquid inclusions. These induced striations have been used to determine the individual growth rates of the pinacoid {001} and the rhombohedral faces {011} at GaPO 4 single crystals.

AlN single crystals with 25 mm diameter and 3 mm thickness were grown on on-axis, Si-terminated, (001) 6H-SiC substrates with a growth rate of 20-25 μm/h by the sublimation method. The growth conditions were optimised and the grown crystals exhibit good crystalline quality as analysed by double-crystal x-ray diffraction measurements. The FWHM value of the rocking curves in ω-scan is about 100 arcsec for 3 mm thick crystals. The kind of AlN/SiC interface formation influences the structural quality of the grown crystals by forming low-angle grain boundaries. Optical and scanning electron microscopic analyses performed on cross-cut samples demonstrate the existence of an array of hillocks and voids between them at the bottom of the grown AlN layer. These hillocks are confirmed by electron probe micro analysis as SiC etch hillocks and their origin was investigated. The possible mechanism for the formation of low-angle grain boundaries is discussed and a simple physical model is proposed. Optimisation of the growth conditions was carried out in order to obtain crystals with low silicon carbide incorporation.

International audience; Isothermal oxidation of Al65Cr27Fe8 and Al80Cr15Fe5 was studied in the 600-1080 degrees C range. Formation of transient alumina layers is obtained up to 900 degrees C. On Al65Cr27Fe8 transient to alpha-phase transformations occur when performing oxidation at 1000 degrees C, together with the possible appearance of (Al0.9Cr0.1)(2)O-3. At 1080 degrees C, direct formation of alpha-alumina is obtained. On Al80Cr15Fe5, spallation of the oxide layer during the cooling stage is observed following oxidation at 800 and 900 degrees C, revealing thermal etching of the underneath alloy surface. At 1050 degrees C the alpha-Al2O3 scale is directly formed but plastic deformation and recrystallization of the underneath alloy into several intermetallic phases is observed.

Etch figures on gallium orthophosphate (GaPO4) crystals are presented for the {001}, {010},f2 10 g,f210 g and {101} faces. Primarily they are used in detecting possibly occurring domains of twinning. Generally, the etch figures of GaPO4 show similarities as well as dierences compared to those of low-quartz modifications of SiO2 and AlPO4. # 2000 Elsevier Science B.V. All rights reserved. PACS: 77.84; 61.72; 61.72.F; 61.72.M

We present the results of 27 Al nuclear magnetic resonance (NMR) experiments on the phase Co 4 Al 13 . These results are compared to a recent structure model [1], which demonstrates a unique bonding for Al atoms in the Co–Al–Co molecular groups. In our measurement, two 27 Al signals were identified. The first one originates from Al atoms forming cages. The second signal corresponds to Al sites with exceptionally large almost axially symmetric quadrupole coupling. This finding is in perfect agreement with isolated Co–Al–Co molecular groups in accordance to Ref. [1].

A single crystal of Al4(Cr,Fe) with composition Al78Cr19Fe3 grown by the Czochralski method was studied by neutron diffraction for the first time. As a preliminary result the neutron diffraction experiment suggests a body-centered orthorhombic space group Immm with a = 1.24985(3) nm, b = 1.25500(3) nm and c = 3.0512(2) nm as found by Deng et al. (Pearson symbol: oI366-59.56) [1] using single crystal X-ray diffraction. Additionally, more or less diffuse maxima at positions with k = n/2 (n = odd) are clearly observable.

Phase transitions of gallium orthophosphate (GaPO4) have been studied by thermal analysis between ambient temperature and 1100°C. In situ X-ray powder diffraction at selected temperatures confirmed the three known polymorphic structure types of low-quartz, low-cristobalite and high-cristobalite GaPO4, respectively. Whereas the as-synthesized powder retained its low-quartz structure up to 970°C, powders cycled up to 1100°C exhibited three phase transitions (low-cristobalite → (≈620°C) high-cristobalite → (≈800°C) low-quartz →(≈970°C) high-cristobalite) while heating up and one phase transition (high-cristobalite → (≈590°C) low-cristobalite) while cooling down. However, the high-cristobalite/low-quartz heating transformation was found to be kinetically hindered. Isothermal in situ X-ray powder diffraction revealed a remarkable temperature dependence of this reaction kinetics.

Gallium orthophosphate (GAPO4) single crystals have been grown from phosphoric acid solutions under hydrothermal conditions. The crystals have been studied in terms of twinning because of the strong effect of this structural defect on the piezoelectric properties. The growth rates of individual faces have been compared to each other by considering the dipyramidal habit of the grown crystals.

International audience; Atomic oxygen adsorption on the pure aluminium terminated Al9Co2 (001) surface is studied by first principle calculations coupled with STM measurements. Relative adsorption energy of oxygen atoms have been calculated on different surface sites along with the associated STM images. The local electronic structure of the most favourable adsorption site is described. The preferential adsorption site is identified as a "bridge" type site between the cluster entities exposed at the (001) surface termination. The Al-O bonding between the adsorbate and the substrate presents a covalent character, with s − p hybridization occuring between the states of the adsorbed oxygen atom and the aluminium atoms of the surface. The simulated STM image of the preferential adsorption site is in agreement with experimental observations. This work shows that oxygen adsorption generates important atomic relaxations of the topmost surface layer and that sub-surface cobalt atoms strongly influence the values of the adsorption energies. The calculated Al-O distances are in agreement with those reported in Al2O and Al2O3 oxides and for oxygen adsorption on Al(111).

Experimental data of mercury vacancy equilibria in Hg1−xCdxTe for the compositions x=0.2 and x=0.3 are presented. In addition to common Hall‐effect measurements, the composition x=0.2 is also studied by the positron annihilation technique. The results obtained are in good agreement with data published in the early eighties. A systematic discussion of the change of vacancy equilibria as well as conduction type for the composition interval x=0.2 to x=0.4 is given.

Defects resulting from the crystallization of mother liquid inclusions are a common occurrence in crystals grown by the travelling heater method. The need to distinguish these defects from defects formed by solid-state precipitation is stressed. This paper reports on an investigation of Te inclusions in Cd 1− x Zn x Te crystals which were grown from Te-rich solution zones and it gives an explanation of the mechanism of their formation by interpreting the composition changes of the matrix surrounding them.

Growth of Hg1-xMnxTe (x approximately=0.10) crystals by the travelling heater method (THM) is reported using a source material preparation process that has been previously developed for Hg1-xCdxTe. Feed ingots, as well as THM single crystals, were characterized with special emphasis on the compositional homogeneity that proved to be superior to the usual results with crystals grown by the Bridgman method so far.

Using the submerged heater method for crystal growth from the melt, steady-state longitudinal segregation may be reached. The submerged heater suppresses convective motion ahead of the phase boundary. However, it also restricts the extension of the diffusion profile in the melt, thus superimposing diffusion-controlled steady-state conditions and a zone configuration. It is similar to the same situation described a long time ago as “zone melting in a diffusion-controlled regime”. Consequences for the growth of variously doped crystals by the submerged heater method are discussed and proved by experiments varying the crystallization rate for Te-doped InSb. The mentioned distribution function is shown to describe the general case of axial segregation with the submerged heater method. Previous interpretations, in terms of diffusion-controlled conditions only, are shown to be limiting cases for high crystallization rates.

“Delta” crystals are solid solutions of miscible materials with large lattice parameter differences which contain high concentration gradients in one direction (parallel to a lattice plane strongly diffracting X-rays). The system GaSb-InSb has been chosen as suitable for study. By means of a “gradient projection method”, the growth of nearly linear composition profiles with relatively steep slopes of the lattice parameter (up to (Δa/ā) / Δz = 8.3% cm-1), adjustable by the temperature gradient, have been performed. However, the grown ingots were not monocrystalline due to the use of too high a growth rate.

Hg1 - xCdxTe and Cd1 - xZnxTe single crystals were grown by the tracwelling heater method (THM), applying two different techniques of artificially stirring the solution zone. Accelerated crucible rotation (ACRT) was used in a vertical growth arrangement and compared a technique with constant rotation around the horizontal axis of the ampoule. The dominant hydrodynamic mechanisms of noth methods are discribed by the rotating disc model and are suggested to be almost identical with respect to the growth conditions at the interface. Convective flow is effectively enhanced adhacent to the growing crystal, where the matter transport is regarded as the rate-limiting step of solution growth. Inclusion density analysis by IR microscopy was used to characterise the crystals of Cd1 - xZnxTe grown at different rates. It was shown that forced convection allows an increase in the crystal growth rate from a few mm day-1 with ACRT or horizontally rotating THM.

Proucavan je Hallov efekt u monokristalima monoklinskog Y-Al-Ni-Co [1], ortorompskog o-Al13Co4 [2] i monoklinskih m-Al13Fe4 i m-Al13(Fe, Ni)4 [3] aproksimanata dekagonalnih kvazikristala za sve kombinacije smjerova elektricne struje i magnetskog polja duž glavnih kristalnih osi u temperaturnom intervalu od 90 do 370 K. U sva cetiri spomenuta intermetalna spoja, koji pripadaju Al13TM4 (TM = prijelazni metal) klasi aproksimanata dekagonalnih (d) kvazikristala, Hallov koeficijent RH pokazuje dobro definiranu anizotropiju. RH je pozitivan (supljinskog karaktera) ili nula za magnetsko polje paralelno s ravninom koja odgovara kvaziperiodicnoj ravnini kod dekagonalnih kvazikristala, a negativan (elektronskog karaktera) ili nula za magnetsko polje okomito na tu ravninu. Jedini izuzetak je RH kod Al13Fe4, za polje usmjereno okomito na ravninu, gdje RH mijenja predznak od pozitivnog u negativni s porastom temperature. Rezultati anizotropije za RH korelirani su s anizotropijom RH u d-Al-Ni-Co [4] kvazikristalu i dan je kratak pregled teorijskih rezultata. Navedena istraživanja provode se u okviru European Integrated Centre for the Development of New Metallic Alloys and Compounds (C-MAC) i MZOS projekata broj 035-0352826-2848 i 286-0000000-3212. [1] A. Smontara, et al., Phys. Rev. B 78 (2008) 104204. [2] J. Dolinsek, et al., Phys. Rev. B 79 (2009) 184201. [3] P. Popcevic, et al., Phys. Rev. B 81 (2010) 184203. [4] P. Popcevic, et al., rad u pripremi ; D. Cmrk et al., poster na ovom ZS.

We present a detailed study of the (001) surface of the Al9Co2 crystalusing both experimental methods and first-principles calculations basedon the density functional theory. Experimentally, the (1 x 1) low-energyelectron diffraction pattern combined with the composition determinedusing X-ray photoemission spectroscopy is consistent with a bulkterminated surface. In addition, scanning tunneling microscopy (STM)measurements highlights the presence of only one type of surfacetermination. Combining experimental results with electronic structurecalculations provides many arguments to correlate the observed surfacetermination with the dense pure aluminum layer present in the Al9Co2bulk structure: (i) calculations show that this termination presents alower surface energy compared to another conceivable terminationobtained also from bulk truncation, (ii) step height measurements areconsistent with calculated interlayer spacings, (iii) calculatedbroadened densities of states and simulated STM images are qualitativelyin agreement with the experimental photoelectron spectra and theexperimental STM images.

The accelerated crucible rotation technique (ACRT) has been applied to the THM growth of Hg1−xCdxTe crystals to grow the crystals at a higher rate. These higher growth rates, which should be achieved by extending the regions of conventional stirring towards the interfaces, have been used in an attempt to explain the results in terms of simple constitutional supercooling arguments. Some different ACRT cycles which fulfil simple hydrodynamic and geometric criteria have been studied. The grown crystals were investigated with respect to their metallurgical homogeneity and their structural perfection. These properties have not been degraded by increasing the growth rate from 1.5 to 8.5 mm per day.

A horizontal travelling heater method (THM) for growing cylindrical cyrstals from a partially filled solution zone has been investigated for the first time. By applying ampoule rotation, the whole cross section of the crystal is successively brought into contact with the liquid solution, which is effectively stirred by forced convection. This approach was used to grow single-crystalline Hg1−xCdxTe ingots from a Te-rich solution zone. The structural perfection and metallurgical homogeneity are equivalent to vertically-grown THM material.

The magnetotransmission of a p-type Hg0.766Cd0.234Te bicrystal containing a single grain boundary with an inversion layer has been investigated in the submillimetre wavelength range. For the first time the cyclotron resonance lines belonging to the various electric subbands of a quasi-two-dimensional carrier system at a grain boundary could be detected. The measured cyclotron masses and the subband densities determined from Shubnikov-de Haas experiments are compared with theoretical predictions and it is found that the data can be explained very well within the framework of a triangular well approximation model which allows for non-parabolic effects.