Anne Sadoc

Electronic structure of Al-Pd-Mn crystalline and quasicrystalline alloys

An experimental and theoretical analysis of the electronic structure of Al-Pd alloys is presented. The experimental study, based on soft x-ray emission and absorption spectroscopies, is further extended to the case of quasicrystalline Al-Pd-Mn alloys and their approximants. The electronic structure is dominated by the sp-d hybridization. The Al-sp band is strongly modified by the interaction with the Pd-d states in the middle of the valence band. Close to the Fermi level, hybridization with the Mn-d states is found to enhance the structure induced pseudo-gap in the Al partial density of states, both in the icosahedral and the decagonal quasicrystalline phases.

Local structure in hydrogenated Ti–Zr–Ni quasicrystals and approximants

Abstract We have investigated the influence of hydrogen on the local structure of Ti–Zr–Ni alloys, icosahedral quasicrystals or crystalline approximants, using extended X-ray absorption fine structure (EXAFS). With an increasing hydrogen-to-metal ratio from 0 to 1.7, a general increase of all the mean first distances was found except for the Zr–Ni (Ni–Zr) ones. The perturbation of the (quasi)lattice, induced by hydrogenation, is a maximum around the Ti and Zr atoms, which suggests that hydrogen atoms sit preferentially near titanium and zirconium atoms.

Electronic structure of hexagonal

We have investigated the electronic structure of hexagonal through theoretical as well as experimental means. Two different calculations have been carried out within the LMTO and tight-binding - LMTO methods. The experimental procedure involved soft-x-ray spectroscopy complemented by photoemission measurements. Co states have been found near the Fermi level, in interaction with Al states of hybridized p, s - d character. As a result, a pseudo-gap is generated at the Fermi level. Occupied Al states beyond 5 eV form the Fermi level are found to be almost pure s in character. The comparison between the experimental results and calculations indicates a rather good agreement with results from LMTO, consistent with the fact that this is a more elaborate band structure calculation. Agreement is only acceptable with results from the TB - LMTO calculations but these can be improved with a more careful treatment of the TB structure constants.

Electronic distributions in quasicrystalline Al-Pd-Mn alloys

We report on experimental valence and conduction electronic distributions of quasicrystalline Al-Mn-Pd alloys probed using the soft x-ray emission and absorption spectroscopy techniques. The various partial distributions are adjusted in the binding energy scale in order to investigate the electronic interactions characteristic in the material. In the valence band, interaction exists between Al states and Mn 3d states near the Fermi level and with Pd 4d states in the middle of the band. At the Fermi level the intensity of the Al 3p states is very low and a rather wide pseudo-gap is observed. In the conduction band, interaction exists also between Al p-Mn d states in the energy range of the absorption edge. Pd d-s states are found about 2 eV beyond the Fermi level. It is suggested that unlike the case of Mn, a small charge transfer may exist from Al to Pd states. Adjustment of Al 3p and Al p distributions at the same intensity at the Fermi level shows that the density of available Al p conduction states is dramatically decreased in the quasicrystal in comparison to pure Al. We propose that this could be connected to the existence of numerous narrow gaps and spikes in the densities of states as predicted theoretically elsewhere for crystal approximant phases with very large unit cells.

Theoretical and experimental investigation of the electronic structure of Ti-Zr-Ni and Ti-Zr-Ni:H alloys

Abstract We have calculated the band structure of a model 1/1 approximant of cubic Frank-Kasper type structure of the icosahedral Ti–Zr–Ni quasicrystal. We have shown that the electronic structure is dominated by the metals d bands with a complete mixing of Ti and Zr states. With respect to the pure metal, Ni d states appear to be strongly modified upon alloying. The experimental investigation of the icosahedral alloy shows good agreement with the theoretical calculations for the approximant. We have shown experimentally that dramatic modifications of both occupied and unoccupied states occur in the hydrogenated quasicrystal, due to bonding to H. We have asserted preferential Zr–H rather than Ni–H bonding. We have suggested that Ti–H bonding should also be significant. We interpret the high H/metal ratio by the fact that H bonds to each of the elements in the quasicrystal.

Evolution of the local structure with hydrogenation in Ti-Zr-Ni quasicrystals and approximants

The effect of hydrogenation on the local structure of Ti-Zr-Ni alloys has been studied by means of extended x-ray absorption fine structure. The alloys were either icosahedral quasicrystals or crystalline approximants. They were loaded to different hydrogen-to-metal ratios 1.2? H/M ?1.7. The desorption process was also investigated by studying the local order in an alloy charged to H/M = 1.2 then desorbed to H/M =0.84. The local structure was identical in the crystalline and quasicrystalline alloys with the same H/M ratio (0 or?1.2) and that of the desorbed sample was intermediate between that of the non-hydrogenated samples and that of the samples charged to?1.2. Therefore, it is possible to follow the evolution of the local structure with hydrogenation from H/M = 0 to?1.7. A general increase of all the mean first distances was found except for the Zr-Ni (Ni-Zr) one. There is a remarkable inversion of the atomic subshells of titanium and nickel in the first environment of zirconium atoms around H/M = 1. For the 1.56?hydrogenated sample, the effect of adding a small amount of lead, which stabilizes the icosahedral phase, was studied and it was demonstrated that lead atoms substitute for the nickel atoms and are, therefore, incorporated into the quasilattice.

The electronic structure of orthorhombic

We present the results of an investigation, both theoretical and experimental, of partially occupied and unoccupied electronic distributions in orthorhombic . The occupied Al states near are p - d hybridized in interaction with Ru 4d states, whereas they are almost purely s-like in character beyond 6 eV from . The Ru 4d states are found close to ; their interaction with the Al states produces an important depletion of the Al 3s - d and Al 3p distributions. As a result, the Al edges are somewhat pushed back from . Thus a wide range of energy with almost no states is present in the occupied band side. lies close to the bottom of the unoccupied band where both unoccupied Al states and unoccupied Ru states are present. These results are compared to previous data for icosahedral quasicrystalline .

High-pressure synchrotron radiation diffraction studies of icosahedral Ti-Zr-Ni and hydrogenated Ti-Zr-Ni quasicrystals

We present results from inxa0situ x-ray diffraction studies of hydrogenated icosahedral Ti-Zr-Ni quasicrystals carried out under high pressure. The icosahedral Ti53Zr27Ni20 phase and hydrogenated icosahedral Ti45Zr38Ni17 samples that contain 0.32 and 1.45 hydrogen atoms for each metal atom were studied. The icosahedral quasicrystal structure is retained up to the highest pressures investigated. The six-dimensional lattice parameter was obtained as a function of pressure for the different samples. The zero-pressure bulk modulus and its pressure derivative were determined by fitting a Murnaghan-type equation of state to the relative volume change V/V0. The zero-pressure bulk modulus is respectively obtained as B0 = 130±10, 105±10, and 110±20xa0GPa for H/M equal to 0, 0.32, and 1.45 using a constant value for the first derivative B0: 5.5±1. It seems, therefore, that the icosahedral Ti-Zr-Ni phase may be more compressible after hydrogenation, but there is no clear difference for different H/M contents. These results are tentatively related to the local structure of thesexa0materials.

Atomic and electronic structure of quasiperiodic and crystalline Mg-61 at.% Al alloys

Mg-Al alloys with composition Mg-61 at.% Al display two types of structure: a stable crystalline state (the so-called β-Mg2Al3) and a metastable one showing both quasiperiodicity and inflation symmetry. The atomic as well as electronic structures of stable crystalline β-Mg2Al3 and a metastable Mg39Al61 phase that exhibits quasiperiodicity and inflation symmetry have been studied thanks to x-ray spectroscopy techniques. Extended x-ray absorption fine-structure experiments performed above the Al and Mg K absorption edges have probed the local atomic order around Mg as well as Al atoms. X-ray emission and absorption spectroscopies have investigated occupied and unoccupied electronic states around Mg and Al. The local order has been found to be the same around Al atoms in both alloys, whereas around Mg atoms differences are seen for higher shells than first neighbours. This suggests that the same clusters must be involved in both phases and that the quasiperiodicity is connected with modification of the Mg atom environment forming the linkage between large clusters. On the other hand, the electronic distributions show differences from β-Mg2Al3 to the quasiperiodic phase that are consistent with the local order studies. A pseudogap is observed in the Mg and Al electronic structures which is more marked in the quasiperiodic phase than the crystalline alloy. By analogy to quasicrystals, we suggest the enhancement observed is due to the occurrence of an inflation mechanism in the arrangement of the clusters in the quasiperiodic alloy with respect to the β-Mg2Al3 crystal.

Local structure in Ti–Hf–Ni metallic glasses and its evolution with hydrogenation

The effect of hydrogenation on the local structure of a Ti25Hf50Ni25 amorphous alloy was studied by extended x-ray absorption fine structure (EXAFS) measurements. The samples were loaded to different hydrogen-to-metal ratios, H/M, 1.2 and 1.4. For the non-hydrogenated alloy, the local structure was found to be different from that in the crystalline Ti–Hf–Ni phase. A new icosahedral ordering, similar to that in the Ti2Ni crystal, is identified around the nickel atoms. Upon hydrogenation, little change is observed in the environment of the nickel atoms. As in the 3/2 phase and in the Ti–Zr–Ni alloys, the perturbation of the local structure with hydrogenation concerns mainly the environments of hafnium (zirconium) and titanium atoms. This indicates that the hydrogen atoms sit preferentially near hafnium (zirconium) and titanium neighbours. Moreover, a drastic decrease of the disorder parameter for the Hf–Ti correlation could be explained by a stiffening of the structure of the glass, possibly due to the formation of a hydrogen network.