V. A. Trounov

Temperature dependences of the parameters of atoms in the crystal structure of the intermediate-valence semiconductor SmB6: investigation by high-resolution powder neutron diffraction

The crystal structure of intermediate-valence (IV) samarium hexaboride has been studied on single-crystal double-isotope samples 154Sm11B6 by X-ray diffractometry at room temperature and by high-resolution powder neutron diffraction in the temperature range 23 K<or=T<or=300 K. The X-ray experiment revealed the occurrence of vacancies at the boron site, larger thermal vibrations of the Sm atom than of La in the isostructural non-IV material LaB6 and an aspherical charge distribution around the Sm nucleus. The neutron diffraction experiment confirmed the anomalous temperature dependence of the lattice parameter and revealed both a peculiar temperature dependence of the anisotropic thermal vibrations of the boron atom and a temperature-dependent change in the ratio of the isotropic thermal parameters of the Sm and B atoms. Thermal vibrations of the Sm ion can be satisfactorily described by the Einstein model with characteristic temperature Theta E approximately=120 K within the whole temperature range. The data obtained are discussed in terms of the influence of the fluctuating valence of the Sm ion on the structural parameters of atoms.

Mean-square displacements of atoms in hexaborides

Abstract High-resolution neutron powder and single-crystal X-ray diffraction studies has been used to investigate the temperature dependencies of atomic mean-square displacements in rare-earth hexaborides. The experimental data on elastic constants and phonon frequencies have been used to improve the valence force field parameters for SmB 6 and the temperature dependencies of atomic mean-square displacements in SmB 6 is compared with theoretical values. The correlation between ionic radii and mean-square displacements of rare-earth atoms are shown.

Investigation of the hydrogen capacity of composites based on ZnOCu

The composites ZnOCuH(D) saturated with hydrogen (deuterium) to a content of ∼1 wt % are investigated by the neutron scattering methods. Upon cooling of the samples (the ZnO matrix containing Cu crystals ∼10 nm in size) from 300 to 4 K, hydrogen (deuterium) is condensed on the cluster surface and penetrates inside the clusters in which the atomic hydrogen content with respect to copper can be as high as 30% at 20 K. Simultaneously, hydrogen fills nanopores of the ZnO matrix. It is revealed that, at temperatures of 90–300 K, approximately one-third of the hydrogen amount participates in the fast diffusion (the diffusion constant is approximately equal to 8 × 10−5 cm2/s) and the other two-thirds are immobilized. At 20 K, the fraction of mobile hydrogen decreases to ∼10%. An analysis of the results obtained demonstrates that the energy barriers retaining hydrogen in defect regions are relatively low.

Some capabilities of neutron methods for investigating materials and components of devices used in hydrogen power engineering

Structural units of a hydrogen fuel cell whose characteristics are advisable to investigate by different neutron scattering methods are discussed. The results obtained with the use of Bragg diffraction and small-angle neutron scattering from fuel membranes, nanocarbon-platinum catalysts, and Zn1 − xCuxO hydrogen storage systems are presented.

Synthesis and crystal structure determination of 6-phenyl-5-phenylsulfonyl-1,2,3,4-tetrahydropyrimidine-2-thione from neutron powder diffraction data

Abstract The title compound was synthesized by reaction of N-(azidomethyl)thiourea or N-(tosylmethyl)thiourea with α-phenylsulfonylacetophenone in the presence of sodium hydride followed by p-toluenesulfonic acid treatment of the obtained N-[3-oxo-3-phenyl-2-(phenylsulfonyl)propyl]thiourea. The molecular and crystal structure of 6-phenyl-5-phenylsulfonyl-1,2,3,4-tetrahydropyrimidine-2-thione (C14H13N2S2O2; space group P21/a; Z=4; a=17.396(1) Å, b=5.887(1) Å, c=15.665(2) Å, β=104.89(1)o) have been determined and refined from neutron (the neutron diffraction was very effective for describing of this system due to using of partial hydrogen-deuterium substitution) powder diffraction data using grid search and a Rietveld procedures. The crystal structure is formed by centrosymmetric hydrogen-bonded dimers along a axis.