S. Sh. Shil'Shtein

Superpositional many-detector systems and neutron diffraction of microsamples☆

Abstract The superposition method for recording neutron diffraction patterns by means of multidetector movable systems is described. Luminosity and resolution characteritics of “small” systems (with 10–16 detectors) and the annular diffractometer DISK (with 224 detectors) are shown. Applications of the diffractometer DISK to structural research using microquantities of matter (of about 1 mg or less) are proposed and results of such experiments are demonstrated.

Investigation of the HgBa2CuO4+δ structure under external pressures up to 5 GPa by neutron powder diffraction

Abstract Neutron powder diffraction investigation of the changes in the structure of the mercury superconductor HgBa 2 CuO 4+δ has been carried out in the pressure range 0–5.07 GPa at room temperature. The compressibility values of the unit cell parameters and several bond distances in the structure have been determined. The relative reductions of the unit cell parameters are close to each other and are approximately 1.7% in the base plane and 2.3% along the c -axis. At the same time, the interatomic distance modulations are strongly inhomogeneous: approximately 4% for the apical CuO bond and only about 0.4% for the apical HgO bond.

Neutron diffraction study of the structure of deuterated and fluorinated fullerene C60

Synthesized C60F48 fluoride and C60D8, C60D18 and C60D36 deuterides of fullerene C60 are described. All the compounds have a f.c.c. structure (like the original C60) with periodsa=1.726, 1.423, 1.448, and 1.498 nm, respectively. Powder neutron diffraction diagrams are described on the basis of a double-shell model, i.e., an internal carbon sphere with a radius of 0.356 nm and a fluorine or deuterium sphere with a radius of 0.530 or 0.470 nm, respectively. The structure of the fluorinated and deuterated fullerenes is discussed with allowance for the existence of covalent bonds between the shells inside the molecule and van der Waals bonds between the peripheral atoms of one molecule and neighbor molecules. The description of the structure of the fullerene derivatives is based on the assumption that deuterium or fluorine atoms are absent on the external shell in the places of contact with the neighbor molecule. The low-temperature behavior of the C60F48 and C60D36 compounds is studied. A phase transformation in the C60F48 fluoride atT=270 K is described, which is similar to the transformation of an orientation type in isostructural fullerene C60 at a close temperature. The observed changes in the intensities of the diffraction peaks are described under the assumption that the phase transformation is accompanied by a decrease in the volume at invariable radii of the carbon and fluorine spheres. Similar changes in the intensity have been determined in the C60D36 compound.

Neutron-diffraction study of YBa2Cu2.7Zn0.3O6+y isotope enriched samples

Abstract A neutron-diffraction study of the structure of YBa 2 Cu 2.7 Zn 0.3 O 6+ y has been performed using the isotope contrast method. Proportional distribution of Zn atoms between the Cu(1) and Cu(2) sites was found to be the most probable for a 10% concentration of Zn in the sample. From the data analysis it was found that due to the substitution of two-valent cations (Zn, Ni) for copper a positive charge transfer to the CuO 2 plane takes place, while for three-valent cation (Co, Fe) substitution a negative charge is transferred to the CuO 2 plane.

Investigation of the Hg-1201 structure under external pressure up to 5 GPa by means of neutron powder diffraction

Neutron powder diffraction investigation of the changes in the structure of the mercury superconductor HgBa2CuO4+δ in the range of pressures 0–5.07 GPa at room temperature has been carried out. The inhomogenious character of the unit cell parameters compressibilities has been proved.

Double-crystal diffractometer study of thermal neutron specular reflection

Abstract A double-crystal diffractometer with perfect crystals is used to study the specular reflection of thermal neutrons. If the mirror surface is placed between the crystals then it is possible to separate the effects of surface reflectivity and surface geometry. The critical angle can be determined with an accuracy of 1″ even if the mirror curvature reaches 1′.