Veniamin Sh. Shekhtman

Mössbauer study of single crystal Bi2Sr3Fe2Ox isostructural with Bi2Sr2CaCu2O8+x

Single and polycrystalline samples of Bi2Sr2SrFe2Ox (232Fe) are synthesized and studied by Mossbauer spectroscopy. The structure of 232Fe is similar to that of 2212Cu except for a commensurate structure modulation arising from the replacements of the latters Cu and Ca atoms by Fe and Sr. The Mossbauer spectra of the single crystals show two doublets with different parameters. Calculation of electric field gradient based upon the structure data confirms that the two doublets arise from two different Fe environments induced by the modulation wave: at the antinodes of the wave the oxygen coordination octahedrons are largely distorted and at the nodes far less. The experimental data are well explained accordingly. The Fe valence, isomer shift values, line widths and other Mossbauer data in comparison with the 2212Cu compound are also discussed.

In situ X-ray investigation of novel superprotonic conductors

Systematic X-ray diffraction studies of thermal expansion in CsHSO4, CsDSO4, RbHSO4 and Rb3H(SeO4)2 crystals have been carried out. An analysis of the results showed that the fast protonic conductivity is closely connected with the characteristic features of the structure, symmetry and orientation of hydrogens bonds in the crystals of MeXAO4 (Me = NH4, Rb, Cs; X = D, H; A = S, Se) family.

Graded X-ray Optics for Synchrotron Radiation Applications

Using X-ray diffractometry and spectral measurements, the structure and properties of graded X-ray optical elements have been examined. Experimental and theoretical data on X-ray supermirrors, which were prepared by the magnetron sputtering technique using precise thickness control, are reported. Measurements on graded aperiodic Si(1-x)Ge(x) single crystals, which were grown by the Czochralski technique, are also presented. The lattice parameter of such a crystal changes almost linearly with increasing Ge concentration. The measurements indicate that Si(1-x)Ge(x) crystals with concentrations up to 7 at.% Ge can be grown with a quality comparable to that of pure Si crystals.

Si1-xGex Laterally Graded Crystals as Monochromators for X-Ray Absorption Spectroscopy Studies

Using the BESSY I wavelength shifter (WLS) beamline we have examined the energy resolution of a laterally graded Si1-xGex crystal in the full divergent, white synchrotron beam. At the Fe-K absorption edge (E = 7112 eV) we have measured an energy resolution of E/ΔE=7.1104 for the (440) reflection and a vertical divergence of 0.63 mrad. For a pure Si(440) reference crystal and the same divergence we found E/ΔE=7.1103 which means a factor of 10 improvement in the case of the laterally graded crystal. Similar results have been obtained at the Co-K absorption edge (E = 7710 eV) with an increase of 2.6 for E/ΔE. We present the experimental set-up and a method for the characterization of the absolute lattice parameter with a precision of Δd/d=210-5. In addition, we discuss the application of the laterally graded Si1-xGex crystals in the crystal monochromator KMC-2 at the BESSY II storage ring.

Examination of Si1-xGex Laterally Graded Crystals for Use in High Brilliance Synchrotron Beams

We report about the feasibility of employing laterally graded Si 1-x Ge x crystals as x-ray monochromators for synchrotron radiation beams. This type of crystals can be used in a divergent beam with nearly the same energy resolution and spectral flux as a regular crystal in a parallel beam. The effect is achieved due to continous changes in the lattice parameter with varying Ge concentration in the Si crystal. Theoretical calculations for BESSY II dipole radiation show, that the optimum gradient of Ge along the surface should be about 1.0 at. % per cm. The properties of the crystals have been investigated by x-ray diffraction and fluorescence analysis with radiation from a conventional x-ray tube. We found nearly perfect crystal quality for Ge concentrations up to 7.0 at. % and gradients of Ge up to 1.1 at. % Ge per cm which would already be sufficient for applications in the x-ray monochromator.

In Situ X-Ray Investigation of Novel Protonic Conductors

Systematic X-ray diffraction studies of thermal expansion in CsHSO 4 , CsDSO 4 , RbHSO 4 and Rb 3 H(SeO 4 ) 2 crystals have been carried out. An analysis of the results showed that the fast protonic conductivity is closely connected with the characteristic features of the structure, symmetry and orientation of hydrogens bonds in the crystals of MeXAO 4 (Me=NI 4 , Rb, Cs; X= D,H; A= S,Se) family.