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A two- and, alternatively, a four-crystal monochromator were used for simultaneous measurements of the profiles backward (h) and forward (o) diffracted by a thin Si (111) crystal plate for diffraction angles up to exactly 90 ° at DCI-LURE (Orsay). It is shown that the set-up with a four-crystal monochromator allows the characterization of the back-diffraction region for any crystal plate reflection. Asymmetry and full width at half-maximum (FWHM) of the experimental backwarddiffraction profiles are analyzed. Possible simultaneous diffractions occurring near 90 ° incidence, giving extra peaks in the forward-diffracted profiles, are studied. The good contrast of the o-beam profiles suggests that the back-diffracted o beam could be used as a highly monochromatic beam.

Short range ordering is compared in three amorphous III-V compounds: InP, GaP and GaAs. For a-InP and a-GaP, a high degree of chemical disorder is evident from an extended X-ray absorption fine structure (EXAFS) study. In the case of a-GaAs, the three partial pair distribution functions deduced from anomalous X-ray diffraction experiments indicate strong chemical ordering. A joint analysis of EXAFS and anomalous X-ray diffraction data reveals the presence of threefold coordinated As which is shown to play an important role in the relaxation of the network as demonstrated by the anomalous X-ray diffraction measurements.

The linearly polarized Cu {ital L}{sub 3}-edge x-ray-absorption near-edge structure (XANES) of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} has been measured and the spectra are interpreted by the full multiple-scattering approach in real space. The polarized spectra over a range of 20 eV can be predicted in terms of the one-electron dipole ({Delta}{ital l}=+1) transition Cu 2{ital p}{r arrow}{epsilon}{ital d}, probing the unoccupied {ital d}-like ({ital l}=2) density of states projected on the Cu site with orbital angular momentum {ital m}{sub {ital l}}=0, 1 in the {bold E}( ){ital z} spectra, and the {ital m}{sub {ital l}}=2, 1, and 0 in the {bold E}{perpendicular}{ital c} spectra. The oscillator strength for the dipole allowed transitions ({Delta}{ital l}={minus}1) Cu 2{ital p}{r arrow}{var epsilon}{ital s} is shown to be a factor of 100 weaker than the 2{ital p}{r arrow}3{ital d} transitions. The Coulomb interaction in the final state between the Cu 2{ital p} core hole and the excited Cu 3{ital d} electron is found to be 5.5 eV forming a bound state below the continuum threshold, the well-known Cu {ital L}{sub 3} white line. On the contrary, the core hole induces a nearly rigid redshift about 1 eV of the high-energy conduction bands.

The short range order around Ga and As sites in a-GaAs films has been investigated by EXAFS and anomalous X-ray scattering. The three partial pair distribution functions indicate a strong chemical ordering. The presence of three-fold coordinated As is shown to play an important role in the relaxation of the network.

The local atomic structure of an amorphous NiZr 2 alloy was investigated using the anomalous wide-angle x-ray scattering (AWAXS), differential anomalous scattering, and reverse Monte Carlo (RMC) simulations techniques. The AWAXS measurements were performed at eight different incident photon energies, including some close to the Ni and Zr K edges. From the measurements eight total structure factors S(K,E) were derived. Using the AWAXS data four differential structure factors DSF i (K,E m ,E n ) were derived, two about the Ni and Zr edges. The partial structure factors S N i - N i (K), S N i - Z r (K), and S Z r - Z r (K) were estimated by using two different methods. First, the S(K,E) and DSF i (K,E m ,E n ) factors were combined and used in a matrix inversion process. Second, three S(K,E) factors were used as input data in the RMC technique. The coordination numbers and interatomic distances for the first neighbors extracted from the partial structure factors obtained by these two methods show a good agreement. By using the three-dimensional structure derived from the RMC simulations, the bond-angle distributions were calculated and they suggest the presence of distorted triangular-faced polyhedral units in the amorphous NiZr 2 structure. We have used the Warren chemical short-range order parameter to evaluate the chemical short-range order for the amorphous NiZr 2 alloy and for the NiZr 2 compound. The calculated values show that the chemical short-range order found in these two materials is similar to that found in a solid solution.

The orbital angular momentum ml of the Cu 3d holes in high Tc superconductors has been measured by Cu L3 x-ray absorption spectroscopy. The symmetry of the itinerant states $$3d^9 \underline L$$ (1) induced by doping at the Fermi level is deduced to be a mixture of 3d holes with ml=±2 and ml=0 orbital angular momentum and oxygen O 2px,y holes in the molecular orbital combination of local bl and a1 symmetry, $$\underline L \left( {b_1 } \right)$$ (1) and $$\underline L \left( {a_1 } \right)$$ (1) . The coupling of the itinerant charges with lattice dynamics and doping induced structural changes, point toward a polaron associated with the 3d3z2-r2 electronic states.

The change of the orbital angular momentum M l of the Cu 3d holes going from the insulating to the metallic phase has been studied in several families of high Tc superconductors. The symmetry of the 3d9 L states in the metallic phase has been studied by quantitative analysis of the variation of polarized Cu L3 x-ray absorption spectra. At a doping level nh∼15% we have found 10% of Zhang-Rice singlets 3d x 2 -y 2 L (b 1 ) and 5% of 3d 3z 2 -r 2 L (a 1 ) states. Therefore the percentage of the 3d3z2−r2 L states on the total number of the 3d9 L states is about 30% i.e. much larger than the probability of single hole states 3d3z2−r2 in the insulating phase. The EXAFS and XANES studies of the Cu site structure and dynamics in Bi2Sr2Ca1−xYxCu2O8 system point toward the coupling of the charge carriers with distortions of the Cu sites driven by the M l=0 character of the Cu 3d holes that can be called a 3dz2−r2 polaron.

We discuss the results of an extended X-ray absorption fine-structure (EXAFS) study of chemical disorder in flash-evaporated nearly-stoichiometric amorphous Gap. The EXAFS spectra of the Kedges of both Ga and P are compared to similar data for crystalline Gap. The differences, which appear most clearly on the P edge, are analysed with a fitting procedure and are assigned to the presence of ‘wrong bonds’, namely bonds between like atoms, in the amorphous compound.

The local environment of P and In in amorphous InP has been investigated by EXAFS at both K edges. EXAFS spectra are fitted by a two shell model with two types of neighbours, which suggest the presence of partial chemical disorder in this material.

The electronic density of states of flash-evaporated amorphous In 1−x P x films, with x > 0.5, is investigated in detail by X-ray photoelectron spectroscopy and by combined optical and photothermal deflection measurements. The results are analyzed in terms of structural and chemical defects, in relation with the predictions of recent theoretical work.

We report a study on the disorder effects in amorphous GaP. Exafs of near stoichiometric films were examined on the two edges Ga and P, and compared to crystalline GaP. The differences mostly apparent on the P edge are evidenced by a fitting procedure to be due to wrong bonds, i.e. bonds between like atoms.