A. Sadoc

Electronic distributions of states in crystalline and quasicrystalline Al-Cu-Fe and Al-Cu-Fe-Cr alloys

Valence and conduction state distributions in a series of Al-Cu-Fe and Al-Cu-Fe-Cr alloys of different structural states and nominal compositions have been investigated by means of soft X-ray emission and photoabsorption spectroscopies. A complete description of the valence band is obtained. The DOS at EF is observed to be very low which is consistent with the high resistivity values found for the icosahedral quasicrystalline phase. The existence of a wide pseudo-gap at EF is evidenced, which is higher in the quasicrystal than in be related crystalline counterparts.

Study of the local structure of CuZr amorphous alloys by exafs. Effect of a structural relaxation

Abstract A structural investigation of Cu x Zr 1− x amorphous alloys has been made using EXAFS ( x = 0.33, 0.46, 0.60). The partial radial distribution functions of the first shell of neighbours were derived from the study of the Cu and Zr K absorption edges. The distribution of the CuCu pairs remains almost constant in the whole range of concentration while the distribution of the unlike atom pairs varies drastically, becoming more asymmetric with increasing Cu content. In the meantime, the chemical ordering increases. A study of structural relaxation has been done for the Cu 60 Zr 40 and Cu 46 Zr 54 glasses by DTA and EXAFS. An additional endothermic phenomenon appears at T g on DTA curves after annealing. No drastic change was observed by EXAFS. A slight increase of the spectrum was however obtained on the Cu edge for Cu 60 Zr 40 .

New lanthanide-doped fluoride-based vitreous materials for laser applications☆

Abstract By careful control of the immediate environment of rare-earth (RE) ions as verified by site-selective photoluminescence, extended X-ray absorption fine structure and transmission electron microscopy (image, microdiffraction and microanalysis), a new class of laser material has been devised: the composition of starting materials and the thermal annealing conditions have been carefully controlled in such a way that the local crystal field of the RE ion is the same as in a polycrystalline powder. This allows a vitreous material with a much reduced inhomogeneous linewidth to be obtained. Because of this reduction, enlarged laser cross-sections have been obtained with ratios ranging from 1.3 to 1.5 with respect to ZrF based glasses.

Electronic distributions of Al-Mn and Al-Mn-Si alloys

Electronic distributions of various symmetries have been investigated for both valence and conduction bands in a series of Al-Mn and Al-Mn-Si alloys of different structural states (amorphous, quasi-crystalline or crystalline) and Mn concentrations ranging from 14 to 22 at.%. Experiments have been carried out by means of soft-X-ray emission and photoabsorption spectroscopies. This allowed the authors to provide a complete description of the valence band states and to confirm the progressive opening of a small gap in the Al-sp DOS at the Fermi level with increasing Mn concentration in the alloys. At a given Mn concentration the gap is wider for the quasi-crystalline phase than for its crystalline counterpart.

Stability of icosahedral Al-Cu-Fe and two approximant phases under high pressure up to 35 GPa

Abstract A high-pressure X-ray diffraction study has been performed on a perfect icosahedral Al62Cu25·5Fe12·5 (i-(Al—Cu—Fe)) phase at room temperature up to 33·5 GPa using a diamond-anvil cell to determine the equation of state. X-ray diffraction was undertaken in energy-dispersive mode on a synchrotron beam line. The remarkable result is that i-(Al—Cu—Fe) remains icosahedral over this pressure range. The relative volume change V/V o as a function of pressure is least squares fitted to a Murnaghan-type equation of state so that the zero-pressure bulk modulus B 0 is 155 ± 10GPa, maintaining its pressure derivative B r 0 = 2. Two approximant phases with very similar chemical compositions but with different structures, a rhombohedral phase (Al62·B Cu26Fe11·2) and a pentagonal phase (Al64 Cu24Fe12), have been studied under the same conditions. No drastic modification of the diffraction spectra appears up to 33 GPa and both phases remain unchanged after release of the pressure. The relative volume change V/V 0...

Short range order study in amorphous Ni64Zr36 by EXAFS

A structural investigation of the local order in amorphous Ni 64 Zr 36 alloy has been made using EXAFS. The results are consistent with data obtained by neutron diffraction using the isotopic substitution method. Extensive comparison is made with other metallic amorphous systems of similar concentration.

The effect of structural relaxation on the local structure of Cu40Zr60 amorphous alloys

Abstract Structural relaxation phenomena associated with heat treatments near the glass transition temperature range were investigated in liquid-quenched amorphous Cu40Zr60 alloys by means of differential scanning calorimetry (DSC), small angle X-ray scattering (SAXS) and EXAFS. A kinetic study was performed by DSC in order to determine the evolution of the configurational enthalpy ( ‡ H σ ) against temperature (upon continuous heating or during isothermal annealing treatments) and to define the heat treatment suitable to obtain highly relaxed samples. SAXS results show that the structure of the relaxed samples remains homogeneous at the intermediate range of order up to at least 100 A, whatever be the previous heat treatment. EXAFS detects a slight change in the local structure only for the most highly relaxed samples; this change could be interpreted by a variation of 0.5 atom in the coordination numbers (probably of the ZrZr pairs) or by a change of the disorder parameter of about 0.01 A. No phase separation is detected in the relaxed amorphous Cu40Zr60 specimens.

Structural analysis of quasicrystals and approximants in AlCu(Ru, Fe) systems

Abstract Using extended X-ray absorption fine-structure experiments performed above the Ru, Cu and Fe K absorption edges, the local order around respectively Cu and Ru, Cu and Fe has been determined in icosahedral Al65Cu20Ru15 and in rhombohedra1 Al62·8Cu26Fe11·2. Strong resemblances are observed in the environments of Cu and of Ru or Fe atoms in these alloys and in icosahedral Al63Cu24·5Fe12·5. Chemical ordering between Al and Ru or between Al and Fe atoms has been found. Correlations with the electronic structure are made.

Icosahedral Ti-Zr-Ni and hydrogenated Ti-Zr-Ni quasicrystals under high pressure

Abstract We present results from the first X-ray diffraction studies under high pressure in icosahedral Ti–Ni–Zr quasicrystals (QCs). The icosahedral Ti 53 Zr 27 Ni 20 phase and an icosahedral Ti 45 Zr 38 Ni 17 phase that also contains 0.32 hydrogen atoms for each metal atom were investigated. The icosahedral QC structure is retained up to the highest pressures used. The six-dimensional (6D) lattice parameters were obtained as a function of pressure. The zero-pressure bulk modulus and its pressure derivative were determined for the different samples by fitting the 6D parameter change to a Murnaghan-type equation of state. It is worthwhile mentioning that the hydrogenated sample appears more compressible than the one without hydrogen.

Al p Conduction States in Quasi-Crystalline Al-Cu-Fe Alloy and Related Compounds

We have examined in details the Al p conduction states as well as the Al 3p valence distributions in quasi-crystalline Al62Cu25.5Fe12.5, rhombohedral approximant R-Al62.5Cu26.5Fe11 and tetragonal ω-Al7Cu2Fe phase. A significant issue is that, in a wide energy range beyond the Fermi level, there is a noticeable decrease of the density of the Al p conduction states in the quasi-crystalline material. Conversely, the conduction Al p band is very much different in metallic aluminium, whereas the situation is found intermediate in the ω- and R-compounds. We point out that a major difference between the icosahedral quasi-crystalline alloy and its rhombohedral approximant is shown by the conduction electronic distributions in contrast with many other properties which, as is well known, present no significant difference. Such a behaviour may be ascribed in part to the progressive disappearance of the crystalline periodic order. Our result is the first experimental evidence in favour of theoretical predictions made elsewhere that the electronic band structure of approximant phase and merely of quasi-crystals consists in a succession of numerous spikes and gaps.