Dear Masami Tsubota,
thanks for directing us to your interesting, free access article.
However, it is not stated in the article whether and how the new
criterion could be applied in usual Rietveld refinements of
non-certified samples.
Am I right that your new criterion (Σ|Δ2θR|(sum or all) = Min) relies on
the previous knowledge of the true, correct lattice parameter (which was
certified for LaB6 in case of the NIST material SRM 660a treated in the
article)?
I conclude that from combining the relations given in the paper: Δ2θR =
Δ2θexp + Δ2θana = Δ2θexp + 2·(arcsin(1/A · sinθ)-θ) = Δ2θexp +
2·(arcsin(a_SRM/a_refined · sinθ)-θ), i.e., the correction seems to rely
on the known certified value of lattice parameter a_SRM.
If this is correct, how could this criterion be applied to any other
sample? Or is it only meant for standard reference materials?
With best regards,
Johannes
Am 14.11.2017 um 06:15 schrieb TSUBOTA Masami:
Dear Larry,
Thank you for your comments.
The reasons why we showed the results applying the Howard's method
for the asymmetry function in our manuscript are:
1) the peaks were fairly symmetric for the in-house data,
2) no detailed information about the apparatus such as the goniometer
radius,
3) one cannot reproduce the diffraction peak asymmetry completely even
for
the Finger's method,
4) The effect of the asymmmetry on 2th angle affects in the low 2th
region,
however, the peak-shift in the high 2th region is much more important
to obtain the true lattice parameters,
5) the number of the parameters is smaller than the Finger's method,
6) we have confirmed that there is no difference of our findings between
the Howard's and Finger's methods.
You can also try the Finger's method as well. Prof. Le Bail shares
his high quality data with us on the Internet.
http://www.cristal.org/powdif/low_fwhm_and_rp.html
Best regards,
Masami
On 2017/11/14 12:22, Larry Finger wrote:
On 11/13/2017 08:54 PM, TSUBOTA Masami wrote:
Dear Rietvelters,
I'd like to introduce our following article.
https://www.nature.com/articles/s41598-017-15766-y
Accuracy of refinement parameters in the Rietveld method is not so
good.
We have carefully investigated the reason, focusing on the peak-shift.
Our results show that a proportional unit-cell compared to the true one
is obtained in the conventional Rietveld method. We propose an
additional
criterion to obtain the true lattice parameters accurately.
Hope this might be useful or interesting to you.
Masami,
The manuscript is very interesting, and appears to provide useful
information regarding the accuracy of lattice constants.
That said, why did you choose the ad-hoc method of Howard for
compensating for peak asymmetry, which is largely due to axial
divergence? The method of Finger, Cox and Jephcoat ("A correction for
powder diffraction peak asymmetry due to axial divergence", J. Appl.
Cryst. (1994). 27, 892-900) applies the optics of the diffractometer to
describe the asymmetry, AND the associated peak shift. Yes, the method
is computationally intensive, but it is physically superior to merely
adding sums of Gaussians. For most studies using small aperture slits
with minimal asymmetry, it probably does not matter; however, your study
in which a great deal is made of peak shifts, should use the best
methodology.
Sincerely,
Larry
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