EXAFS-spectroscopy method in the condensed matter physics: First results on energy-dispersive EXAFS station in RSC “Kurchatov Institute” Vadim Efimov Joint Institute for Nuclear Research, Dubna,Russia ____________________ Joint Institute for Nuclear Research, Dubna, Russia

Collaboration:  Kurchatov Center for Synchrotron Radiation and Nanotechnology, Moscow A.N. Artemev, A.V. Zabelin  Institute of Solid State Physics University of Latvia, Riga A. Kuzmin, Ju. Purans  Hamburger Synchrotronstahlungslabor, DESY, Hamburg, Germany E. Welter, D. Zajac  Institute of Solid State and Semiconductor Physics, Minsk, Belarus I.O. Troyanchuk, D. Karpinsky

Talk Outline I Introduction to X-ray Absorption Spectroscopy ▪ X-ray Absorption Spectroscopy experiment ▪ the procedure of EXAFS data treatment II An experimental EXAFS results and their analysis ▪ local atomic structure in the LaCoO 3 ▪ electronic structure in the LaCoO 3 III Energy-dispersive EXAFS spectrometer in Russian Synchrotron Center “Kurchatov Institute” IV Conclusions

Basics of X-ray Absorption Spectroscopy Incident X-rays ( I 0 )Transmitted X-rays ( I ) Fluorescence X-rays e-TEY Visible light XEOL Absorption  = (1/x) ln(I 0 /I) sample x Introduction to X-ray Absorption Spectroscopy

■ local electronic structure ■ chemical bond ■ density of available occupied electronic states ■ the valence and density of states of the absorber X-ray Absorption Near-Edge Structure ( XANES ) ■ local atomic structure near of the absorbing atom ■ distances of the nearest neighbor shells ■ number and type of the nearest neighbors ■ information about the correlated motion between the absorbing atom and its neighbors Extended X-ray Absorption Fine Structure ( EXAFS ) Introduction to X-ray Absorption Spectroscopy

X-ray Absorption Spectroscopy (XAS) The basic advantages of XAS as a method of investigation on condensed matter are Introduction to X-ray Absorption Spectroscopy ● types of samples are single-crystal, polycrystalline powder, glasses, thin films, liquid, gas ● a small required sample volume (usually, an amount less than 40 mg/cm 2 is enough). ● selectivity in the chemical-element type, which enables one to get information on the pair and multiatomic distribution functions for the local environment of each element of the material; ● sensitivity to the densities of vacant states near the Fermi level; ● high density sensitivity (10÷100 particles per mole) and relatively short periods of time (from milliseconds to tens of minutes) to detect experimental spectrum;

Experimental set-ups of the two types of the EXAFS spectrometers: classical and the energy-dispersive Introduction to X-ray Absorption Spectroscopy

The classical and energy-dispersive EXAFS spectrometers Types of the EXAFS spectrometers electron beam energy average stored current energy resolution accuracy of atom distance determination dimension of the sample time of measuring GeVmAeVÅmm 2 sec Classical ·10 3 Energy- dispersive ~ ·10 –3

Scheme of XAS experiment by using the classical EXAFS spectrometer Introduction to X-ray Absorption Spectroscopy experiment E o  threshold energy  o   absorption coefficient without contribution from neighboring atoms Δ  o ( E o )  evaluated at the edge step

EXAFS:  (k) An existence of XAFS is a consequence of interference effect, and depends on the wave-nature of the photoelectron. It’s convenient to analise XAFS in terms of photo-electron wavenumber, k, rather than x-ray energy: EXAFS  (k) is usually weighted by k 2 or k 3 to amplify the oscillations at high k: Identify the threshold energy E o we convert from E to k space: E o - threshold energy

f (k) scattering amplitude  (k) phase-shift R distance to the neighboring atom N coordination number of the neighboring atom    mean-square disorder of neighbor distance EXAFS equation and Fourier Transform of  (k) Fourier Transformation EXAFS  (k). k 2 converts from k to R space and is similar to an atomic radial distribution function where the sum is over “shells” of atoms or “scattering paths” for the photo-electron To model and interpret the spectrum in EXAFS region, we use the EXAFS Equation:

Perovskite structure of the cobaltite LaCoO 3 Co 3+ La 3+ local atomic and electronic structure in the LaCoO 3

X-ray absorption process and possible spin states of LaCoO 3

The temperature dependences of SQUID magnetization for LaCoO 3 measured from 5 to 350 K in field of 10 kOe

EXAFS spectra at the Co K-edge for LaCoO 3 from 10 to 290 K Absorption coefficient and experimental EXAFS χ(k)k 2 signals at the Co K-edge in LaCoO 3

Experimental Fourier transforms of the Co K-edge EXAFS  (k)k 2 signals for the LaCoO 3 polycrystalline powder from 10 to 290 K 2 nd shell 2

X-ray absorption near edge structure (XANES) spectra at the Co K-edge for LaCoO 3 XANES Analysis: Normalized and Curve fitting

Energy-dispersive EXAFS spectrometer at National center of Synchrotron Radiation “Kurchatov institute”, Moscow

Monochromator with triangular crystal Si (111) Type crystalEnergy range, keV Resolution  E/E Si(111)  10 –4

Holder for a sample 2D X-ray detector CCD matrix

First preliminary experimental EXAFS results on energy-dispersive EXAFS spectrometer for Zn и ZnO at the Zn K-edge

The opportunities of EXAFS-spectroscopy have been demonstrated: the combined EXAFS and XANES results to study the local atomic and electronic structure behavior of LaCoO 3 EXAFS-spectroscopy method and analysis experimental EXAFS spectra has been developed and used to study the local-range order structure of the samples CONCLUSIONS The energy-dispersive EXAFS spectrometer has been constructed and launched at the National Center of Synchrotron Radiation “Kurchatov institute”, Moscow

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