[Ifeffit] XAFS for electride

[Matthew Marcus]

The obvious difficulty here is that the predicted phases requre TPa of pressure, which is way above what a DAC can provide.  Also, for a DAC, you need an edge at a high-enough energy to get
through the diamonds or gasket.  There exist ambient-pressure-stable electrides in which an alkali atom is inside a cryptand cage and its valence electron hangs around in blobs between the
cages.  The XAS could certainly be done there at the Cs edge, but there's nothing to compare to and see if there's any effect of the electron blob, because there's no phase just like it
without the electron blob.  I think you'd have a better shot at detecting the electron blob with crystallography.  If these phases are accessible through shock techniques, then maybe, just
maybe, a source like LCLS or one of the European hard X-ray FELs could take single-shot powder patterns.  It would be a hero experiment.
    mam
  ----- Original Message ----- 
  From: vikrai at comcast.net 
  To: XAFS Analysis using Ifeffit 
  Sent: Monday, February 06, 2012 1:57 PM
  Subject: Re: [Ifeffit] XAFS for electride


  Dear Yuan,

  These electron blobs will act as repulsive potentials for photoelectrons.

  To estimate (at least roughly) the value of the influence of these potentials on EXAFS signal you can approximate the electron density for the blob with some simple spherically symmetrical function, then find the potential created by this charge distribution and then calculate backscattering and forward scattering amplitudes according to any textbook on quantum mechanics or scattering theory. Then compare your results with scattering amplitudes calculated for some light atoms by e.g. FEFF

  The backscattering amplitude tells you about single scattering contribution. Forward scattering amplitude tells you about possible contribution to chain scattering if the last presents in that atomic structure.

  Victor Krayzman




------------------------------------------------------------------------------
  From: "Yuan Ping" <ping2 at llnl.gov>
  To: "XAFS Analysis using Ifeffit" <ifeffit at millenia.cars.aps.anl.gov>
  Sent: Monday, February 6, 2012 2:57:16 PM
  Subject: [Ifeffit] XAFS for electride


  Dear XAFSers,


  Recently there has been a growing interest in the electride phase of sp-bonded materials at extremely high pressures >Mbar (see attached paper for example). This phase is characterized by a matrix of positive ions and interstitial electron blobs. I am wondering how XAFS signal would change when these electron blobs are present. The x-ray diffraction probably won't detect them directly. But XAFS might be able to show their existence if they behave like additional scattering centers.


  Any thought or information would be appreciated.


  Yuan





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