Title: A New Application of NonResonant XRay Raman Scattering: local symmetry and sitesubstitutional disord
1A 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
2General 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
3Icosahedral 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
4XRS Spectra
Put in reference to XAFS or EELS of B4C
5Site-Specific FMS Simulation with FEFF8
(solid curve) stoichiometric addition of B1, B2,
and B3 contributions. (dash curve)
contributions of B1 and B2 only.
6Spatial 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.
7Comparisons with Theory
Calculating S1s-2p(q) using 2p hydrogen-like
orbital for boron atom
8Single 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
9Conclusion
- 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.