No abstract

Polarized Cu K x-ray absorption near-edge structure spectra of CuO are measured and analyzed with the aim of extracting quadrupole and dipole partial spectral components. Theoretical spectral components are calculated within the framework of real-space multiple-scattering technique, relying on a non-self-consistent muffintin potential. A local coordinate system xЈyЈzЈ suitable for polarization analysis is defined by the sides of the CuO 4 quasirectangle rather than by directions of the Cu-O bonds. An exclusively quadrupole nature of the prepeak is established both experimentally and theoretically. By analyzing the experimental data, we find that those states which give rise to the prepeak lie within the CuO plane, and have mainly a d x Ј y Ј character. The theory correctly reproduces gross features of the polarized structure generated by dipole transitions, apart from a spurious peak at 8983 eV for the p z Ј component. A partial agreement between theory and experiment in the shoulder region suggests that this structure may arise partly from one-electron states and partly from manybody processes.

No abstract

Two model compoundsvaline C 5 H 11 NO 2 (L-Val) and isoleucine C 6 H 13 NO 2 (L-Ile)were chosen to study isomorphism of the compounds formed by enantiomeric molecules having similar shapes, but differing in the number of chiral centers. The X-ray powder diffraction method was applied to investigate samples with various ratios of valine and isoleucine crystallized from aqueous solutions by cooling and isothermal methods. Limited solid state miscibility between valine and isoleucine was established. A non-equimolar discrete compound with the molecular ratio Val:Ile = 2:1 (V 2 I) has been discovered, and its crystal structure was determined. Accordingly, the corresponding regions of solid solutions and two-phase areas in-between were distinguished. Solubility data in water support the phase behavior in the system.

We studied the anisotropy of the unoccupied electron states of three titanium dichalcogenides by means of polarized x-ray-absorption spectroscopy. The experimental spectra of TiS 2 , TiSe 2 , and TiTe 2 at the Ti K edge and of TiTe 2 at the Te L 1 edge were compared with the results of real-space multiple-scattering calculations. The local angular momentum projections of the densities of states at the titanium sites are presented for energies up to 25 eV above the Fermi level. The Ti K-edge spectra of all three dichalcogenides are quite similar to each other provided the differences in their lattice constants are suppressed by comparing the spectra in equivalent energy scales. However, the correspondence between the Te L 1 edge of TiTe 2 and the S K edge of TiS 2 is much less pronounced. For most of the spectra good agreement between experiment and theory was found. ͓S0163-1829͑98͒01336-8͔