A New Application of NonResonant XRay Raman Scattering: local symmetry and sitesubstitutional disord

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A New Application of Non-Resonant X-Ray Raman
Scattering local symmetry and
site-substitutional disorder in icosahedral B4C
G. T. Seidler (a, b), Y. Feng (a), J. O. Cross
(a, b), A. T. Macrander (c), J. J. Rehr (a) (a)
Physics Department, Univ. of Washington, Seattle,
WA 98195 (b) PNC-CAT, Advanced Photon Source,
Argonne, IL 60439 (c) Advanced Photon Source,
Argonne, IL 60439
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General Motivation

• The experimental quantity measured in core-shell
  spectroscopy is S(q,w) for photoelectron
  excitations. Theory and simulation of S(q,w) is
  now well-advanced, generally allowing detailed
  interpretation of local structure.
• XAFS Strictly dipole limit due to
  wavelength/radius 10.
• EELS q limited to at most 2 inverse Angstrom
• XRS Inelastic scattering of hard x-rays from the
  core shell of low-Z elements. Independent
  control of qvector and w, with q as large as 9
  inv angstrom
• Today (XR)NES and local symmetry

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Icosahedral Boron Carbide B4C
unit cell of 15 atoms, showing site-substitutional
disorder 3 carbon atoms on average per unit cell.
four different crystalline sites 1 blue 2
green 3 red 4 purple. Note site 3 is the
only site with inversion symmetry
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XRS Spectra
Put in reference to XAFS or EELS of B4C
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Site-Specific FMS Simulation with FEFF8
(solid curve) stoichiometric addition of B1, B2,
and B3 contributions. (dash curve)
contributions of B1 and B2 only.
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Spatial Extent of the Final State
FEFF simulation convergence indicates the range
of the final state.
Also Lanczos algorithm shows that gt100 legs
required for convergence. No small number of
paths dominates.
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Comparisons with Theory
Calculating S1s-2p(q) using 2p hydrogen-like
orbital for boron atom
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Single Selection Rule and Local Structure
XRS fully transferred momentum range
B3 only site at the center of inversion symmetry
d-type quadrople
s-type monopole
single definite parity for the final state
p-type dipole
single selection rule!
total spectrum
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Conclusion

• XRS reveals richer l-DOS information compared
  with traditional EELS and XAS techniques
• q-dependent XRS and ab initio calculation reveal
  a p-type exciton in B4C system
• This p-exciton solves a site substitutional
  disorder problem
• We illustrate the relationship between
  single-typed exciton and special local symmetry
  at different atomic sites

Acknowledgements
E. A. Stern, A. L. Ankudinov, J. A. Soininen, E.
L. Shirley, B. D. Chapman, E. A. Miller, Mellam
Family Foundation, Bosack and Kruger Charitable
Foundation, DOE DE-FGE03-97ER45628,
W-31-109-Eng-38, CMSN.