J.-L. Hodeau

Electron- and powder neutron-diffraction studies of YBa2(Cu1−yCoy)3O6+x with 0.05<y<0.33

Abstract Power samples of YBa2(Cu1-y)Coy)3O6+x compounds with y >0.05, in the oxidized and reduced state, have been characterized by thermogravimetry, and X-ray, neutron-, and electron-diffraction methods. Neutron experiments showed that (1) the Co atoms substitute exclusively for the Cu(1) atoms up to y∼0.2; (2) extra oxygen atoms are needed to give the Co atoms the appropriate coordination number which is in average equal to 5; (3) the Cu-O distances were found to be a function of the Co concentration and the oxygen stoichiometry. Co-doping yields a lengthening of the distance between the plane Cu and the apical oxygen and a corresponding shortening of the distance between the chain Cu and the same oxygen. For 0.025⩽y⩽0.10 the electron- diffraction experiments showed the presence of diffuse scattering indicating the existence of two superposed fields of transverse displacements modulated with long periodicities (4-5)( a+b ) along the |110| directions. For higher Co concentration diffuse scattering still exists, but indicates shorter modulation periodicities along the same directions. This diffuse scattering is due to local orthorhombic-like distortions induced by the Co insertion and distribution. The Co atoms tend to form dimers or small clusters which favor micro-twinning. The bond-distance analysis showed that the formal valence of the plane Cu atoms is the important factor in determining the value of Tc, provided that only the chain Cu atoms are substituted. Our results are in agreement with those obtained by other authors for the Fe-doped 123 compounds.

Structural and magnetic properties of CoPt mixed clusters

In this present work, we report a structural and magnetic study of mixed Co58Pt42 clusters. MgO, Nb and Si matrix can be used to embed clusters, avoiding any magnetic interactions between particles. Transmission Electron Microscopy (TEM) observations show that Co58Pt42 supported isolated clusters are about 2nm in diameter and crystallized in the A1 fcc chemically disordered phase. Grazing Incidence Small Angle X-ray Scattering (GISAXS) and Grazing Incidence Wide Angle X-ray Scattering (GIWAXS) reveal that buried clusters conserve these properties, interaction with matrix atoms being limited to their first atomic layers. Considering that 60% of particle atoms are located at surface, this interactions leads to a drastic change in magnetic properties which were investigated with conventional magnetometry and X-Ray Magnetic Circular Dichroism (XMCD). Magnetization and blocking temperature are weaker for clusters embedded in Nb than in MgO, and totally vanish in silicon as silicides are formed. Magnetic volume of clusters embedded in MgO is close to the crystallized volume determined by GIWAXS experiments. Cluster can be seen as a pure ferromagnetic CoPt crystallized core surrounded by a cluster-matrix mixed shell. The outer shell plays a predominant role in magnetic properties, especially for clusters embedded in niobium which have a blocking temperature 3 times smaller than clusters embedded in MgO.

Structural deformation in oxidized and reduced YBa2(Cu1−yCoy)3O6+x compounds (0<y≤0.05)

We have investigated in detail the structural properties of the system YBa2 CuPin1−y Coy)3O6+x with different values of x and y. X-ray diffraction shows that the Co doping induces an orthorhombic-to-tetragonal transition at y≈0.025. Electron diffraction and electron microscopy indicate that for y<0.025, displacement fields oriented along the 〈110〉 directions exist throughout the crystallite. For y0.025 electron diffraction shows the presence of intense streaks of diffuse scattering located at the tails of the Bragg reflections in the (001) plane of the reciprocal space, and oriented along the 〈110〉 directions. For the same oxidation treatment the size of the diffuse streaks increases with increasing dopant concentration. They disappear when the sample undergoes a reduction. Two-beam images show a contrasted cross-hatched pattern whose hatchings are also oriented along the 〈110〉 directions. These features are displacive in origin and correspond to the superposition of transverse displacement fields whose propagation vectors are oriented along the 〈110〉 directions. X-ray diffraction experiments with a conventional source as well as with synchroton radiation at LURE were carried out on a well oxidized single crystal (y=0.04). The diffuse scattering is of the same type as that seen with electron diffractiion. These X-ray diffraction experiments confirms that the diffuse scattering is displacive in origin and shows that the displacements concern all atoms. The displacements are linked to the deformations created by the specific arrangement of the oxygen atoms forming the -Cu1-O4 microchains. One way to relax the internal constrains due to the microchains is to form microtwins. The lattice is distorted at each twin wall and this distortion induces transverse zig-zag displacements of all atoms, perpendicular to the twin walls.

Polymerized Fullerite Structures: Experiment and Theory

The C60 molecule discovered by Kroto et al. [1] is such an extremely stable closed-cage cluster that it can be viewed in some respects as a super-atom or new chemical species [2]. Considerable research involving chemical and physical properties has been done since the discovery of a method allowing its production on a sufficiently large scale [3].

Anisotropic stress imprinting on 3D C60 polymers

We present synchrotron X-ray diffraction measurements performed on C60 samples quenched from 13 GPa and 820 K. The analysis of all the reciprocal space of a quenched sample shows that it consists of a 3-D arrangement of polymerised fullerene molecules. Moreover, it is shown that the intensity distribution in reciprocal space occurs in ellipsoidal surfaces instead of the normal spherical surfaces characteristic of a powder sample. This implies that the applied anisotropic stress is imprinted in the quenched transformed samples.