Biomedical subjects

V Tsirelson

Publications and source records attributed to V Tsirelson.

5 recordsLinked to original sources

PubMed2003

Features of the electron density in magnesium diboride: reconstruction from X-ray diffraction data and comparison with TB-LMTO and FPLO calculations.

Features of the electron density in MgB(2) reconstructed from room-temperature single-crystal X-ray diffraction intensities using a multipole model are considered. Topological analysis of the total electron density has been applied to characterize the atomic interactions in magnesium diboride. The shared-type B-B interaction in the B-atom layer reveals that both sigma and pi components of the bonding are strong. A closed-shell-type weak B-B pi interaction along the c axis of the unit cell has also been found. The Mg-B closed-shell interaction exhibits a bond path that is significantly curved towards the vertical Mg-atom chain ([110] direction). The latter two facts reflect two sorts of bonding interactions along the [001] direction. Integration of the electron density over the zero-flux atomic basins reveals a charge transfer of approximately 1.4 (1) electrons from the Mg atoms to the B-atom network. The calculated electric-field gradients at nuclear positions are in good agreement with experimental NMR values. The anharmonic displacement of the B atoms is also discussed. Calculations of the electron density by tight-binding linear muffin-tin orbital (TB-LMTO) and full-potential non-orthogonal local orbital (FPLO) methods confirm the results of the reconstruction from X-ray diffraction; for example, a charge transfer of 1.5 and 1.6 electrons, respectively, was found.

Journal Article↗

PubMed2000

Electron density and atomic displacements in KTaO3.

The atomic interactions and anharmonicity of atomic displacements in the virtual ferroelectric KTaO3, potassium tantalate, have been studied using accurate single-crystal X-ray diffraction at room temperature. The multipole analysis of electron density and Gram-Charlier series description of anharmonic atomic displacements allowed us to perform a quantitative analysis of the chemical bonding and atomic motion in this crystal. Closed-shell interactions between Ta-O and K-O pairs of atoms were found, while no interaction was observed for the Ta-K and O-O atomic pairs. The character of the anharmonic atomic displacements is discussed and compared with those in SrTiO3.

Crystallography, X-Ray↗

PubMed2000

Electron density of KNiF3: analysis of the atomic interactions

The topological analysis of the electron density in the perovskite KNiF3, potassium nickel trifluoride, based on the accurate X-ray diffraction data, has been performed. The topological picture of the atomic interactions differs from that resulting from the classic crystal chemistry consideration. The shapes of atoms in KNiF3 defined by zero-flux surfaces in the electron density are, in general, far from spherical. At the same time, their asphericity in the close-packed layer is very small. The topological coordination numbers of K and Ni are the same as the geometrical ones, whereas topological coordination for the F atom (6) differs from the geometrical value. The latter results from a specific shape of the Ni-atom basin preventing the bond-path formation between F atoms in the same atomic close-packed layer, in spite of the fact that the closest F-F distance is the same as K-F. Judging by the electron density value and curvature at the bond critical points, the K-F interaction in KNiF3 can be considered ionic, while the Ni-F bond belongs to the polar covalent type. No correlation of the topological ionic radii with crystal or ionic radii was found in KNiF3. Critical points in the electrostatic potential have also been studied.

Journal Article↗

PubMed1999

Electron density and electrostatic potential of KNiF3: multipole, orbital and topological analyses of vacuum-camera-imaging plate and four-circle diffractometer data.

The electron density and electrostatic potential of KNiF(3), nickel potassium trifluoride, were studied using multipole and orbital model treatment of the precision X-ray diffraction data measured by vacuum-camera-imaging plate and four-circle diffractometer methods. Different experimental methods lead to similar multipole and atomic displacement parameters and to qualitatively the same electron densities. Good agreement was also achieved for the Laplacians of the electron density and the electrostatic potentials. Some pitfalls of the vacuum-camera-imaging plate method that could be improved are discussed.

Journal Article↗

PubMed1999

Electron density study of urea using TDS-corrected X-ray diffraction data: quantitative comparison of experimental and theoretical results.

The electron-density distribution in urea, CO(NH(2))(2), was studied by high-precision single-crystal X-ray diffraction analysis at 148 (1) K. An experimental correction for TDS was applied to the X-ray intensities. R(merge)(F(2)) = 0.015. The displacement parameters agree quite well with results from neutron diffraction. The deformation density was obtained by refinement of 145 unique low-order reflections with the Hansen & Coppens [Acta Cryst. (1978), A34, 909-921] multipole model, resulting in R = 0.008, wR = 0.011 and S = 1.09. Orbital calculations were carried out applying different potentials to account for correlation and exchange: Hartree-Fock (HF), density-functional theory/local density approximation (DFT/LDA) and density-functional theory/generalized gradient approximation (DFT/GGA). Extensive comparisons of the deformation densities and structure factors were made between the results of the various calculations and the outcome of the refinement. The agreement between the experimental and theoretical results is excellent, judged by the deformation density and the structure factors [wR(HF) = 0.023, wR(DFT) = 0.019] and fair with respect to the results of a topological analysis. Density-functional calculations seem to yield slightly better results than Hartree-Fock calculations.

Journal Article↗