[Wien] [SPAM?] How to obtain ELNES simulation results consistent with experiments ?

[Laurence Marks]

There are two fundamental issues:
a) How to handle the dynamical diffraction of the swift electrons.
b) How to handle the core hole.

To my knowledge no DFT code handles a) correctly. (Some details are in Chpt
5 of "High-Resolution Transmission Electron Microscopy: and Associated
Techniques" Eds Buseck, Cowley  and Eyring.) Particularly if you are on a
zone axis dynamical diffraction effects (often called channeling, but this
is not fully rigorous in my opinion) can be very strong. One way to
minimize the effect is to tilt off the zone axis to a pseudo-kinematical
condition. Of course, by doing this you lose imaging. You might (very
might) be able to reduce the issue by using a larger collection angle
making it similar to Precession (e.g. DOI 10.1016/j.ultramic.2012.11.007).
I am not sure if people have really explored this enough.

For the core hole, from what I have seen (I am not a full EELS expert)
often how much of a core hole one wants is an "adjustable parameter". At
least with Slater Transition Theory it should only be 1/2 a hole.

Not an easy problem, and I am not convinced that it has been fully solved
although I would be willing to be proved wrong.

On Mon, Oct 15, 2018 at 6:54 AM yfding <yfding0375 at foxmail.com> wrote:

> This problem haunts me for nearly a year, and my questions are still
> unsolved after consulting a lot of literature and mailing list.
>
> I tried to get Oxygen K edge of the 3*3*3 SrCrO3 supercell.
>
> In my calculations, I first converted the SrCrO3 (space group P4/mmm) into
> a SrCrO3 3*3*3 supercell (space group P1) in CrystalMaker, then transformed
> the SrCrO3supercell.cif file into SrCrO3supercell.struct file under the
> Wien2k case directory, labelled one oxygen atom to make this atom unique,
> did initialization, and accepted all the queries from nn complaints. The
> initialization parameters were as follows:
>
>   Rmt: Sr:2.40  Cr:1.78  O:1.61
>   potential: GGA of PBE-96
>   separation energy: -6.0 Ry
>   RKmax: 7.0
>   Max L in WF: 10
>   number of k points in the whole BZ: 5*5*5
>   number of k points in the irreducible wedge of the BZ: 18
>
> Then I edited SrCrO3.inc and removed one core electron from the 7th atom,
> i.e. 1st Oxygen atom 1s state. Next I changed 0.0 to -1.0 in SrCrO3.inm.
> Then run SCF in parallel (energy convergence 0.0001 Ry).
>
> Then,
> -edited case.innes :
>
>   SrCrO3 O K edge
>   7            (atom)
>   1, 0        (n, l core)
>   532       (E-Loss of 1st edge in eV)
>   200   (energy of the incident electrons in keV)
>   -5.0 20.0 0.1        (minimum energy, energy step, maximum energy)
>   0.37  0.75   (collection semiangle, convergence semiangle, both in mrad)
>   50 1   (NR, NT, defining the integration mesh in the detector plane)
>   0.7   (spectrometer broadening in eV)
>   OUTPUT
>   0                   (full output)
>   DETECTOR POSITION
>   0.0 0.0             (thetax, thetay)
>   MODUS
>   energy              (dscs wrt what?)
>   ORIENTATION SENSITIVE
>   0.0 0.0 0.0
>   SELECTION RULE
>   n                   (selection rule)
>   LSELECTION RULE
>   d
>   INITIALIZATION
>   y y                (dos and xdos handling)
>   y y                (handling of rotation matrices)
>   QGRID
>   U
>   END
>
> - x lapw1
> - x lapw2 -qtl
> - x qtl -telnes
> - x telnes3
> - x broadening
>
> As it is supercell, I calculated every peak of unequivalent O K edge and
> summed them. I'm not sure all the steps I have done are right. The result
> is different from the TEM EELS experiment (The O K edge EELS spectra were
> studied on an JEOL ARM200F Cs-corrected TEM with a FEG and a Gatan parallel
> electron-energy-loss spectroscope). The relative strength and position of
> the peaks are completely inconsistent.
>
> It's not just this example, but I've had the same problem with all the
> other materials. I thought about all the possible reasons, and looked up
> lots of literature and mailing lists, but I couldn't get the calculation
> results closer to the experiment. I want to know how to obtain ELNES
> simulation results consistent with experiments ?
>
> Any comment(s) would be highly appreciated. Thanks in advance!
>
>
>

-- 
Professor Laurence Marks
"Research is to see what everybody else has seen, and to think what nobody
else has thought", Albert Szent-Gyorgi
www.numis.northwestern.edu ; Corrosion in 4D: MURI4D.numis.northwestern.edu
Partner of the CFW 100% program for gender equity, www.cfw.org/100-percent
Co-Editor, Acta Cryst A
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