On 12/19/2016 09:29 AM, Cline, James Dr. (Fed) wrote:
Your data is from where? I wouldn’t trust numbers from refinements as these
from lab data.
Jim
James P. Cline
Materials Measurement Science Division
National Institute of Standards and Technology
100 Bureau Dr. stop 8520 [ B113 / Bldg 217 ]
Gaithersburg, MD 20899-8523 USA
jcl...@nist.gov <mailto:jcl...@nist.gov>
(301) 975 5793
FAX (301) 975 5334
*From:*rietveld_l-requ...@ill.fr [mailto:rietveld_l-requ...@ill.fr] *On Behalf
Of *Othman Al Bahri
*Sent:* Monday, December 19, 2016 3:54 AM
*To:* rietveld_l@ill.fr
*Subject:* Stoichiometry and occupancy fractions of solid solutions
Dear all,
I've made a series of solid solution powders using a solid state reaction in the
form A_2 B_3-x C_x O_12 at_x= 0.5 steps. A_2 B_3 O_12 is orthorhombic while A_2
C_3 O_12 is monoclinic. I'm refining the XRD data to find the atomic
distribution of the solute.
I've constrained the sum of the occupancy fractions for each relevant site to
equal 1. At low concentrations of the solute, I initially set the solute's
occupancy fractions to 0 and keep the solvent's occupancy at 1 then refine the
fractions (after following the usual Rietveld refinement steps). This seems to
give reasonable occupancy fraction values (no big numbers or negative values)
but the stoichiometry is way off. This is probably because each site has
different Wykoff multiplicities so constraining the sum of each site's fractions
to 1 is insufficient.
Let's assume that I knew the stoichiometry from Mass Spectroscopy or XPS - is
there a way to constrain the stiochiometry in a Rietveld refinement? I'm using
GSAS-II and comfortable with FullProf but feel free to give advice for any other
open-source software.
I've seen a few papers where the authors mention, typically in the supplementary
info, that their refinements' stoichiometry was off and that it should be
ignored. However I'm not comfortable with this approach and would appreciate
your advice.
This is my first time working with solid solutions so please feel free to offer
any general advice on what I should be careful with. I've tested for phase
mixtures (insolubility) by visually comparing my XRD patterns with the sum of
simulated XRD patterns of molar mixtures and through Rietveld refinements with
two phases. The system I'm working with has been reported but the original
authors didn't do Rietveld refinements - they were interested in physical
property measurements.
Kind Regards,
Othman
I am showing my age, but in my PhD thesis, I demonstrated that even when using
single-crystal data to refine distributions of such favorable species as Fe vs
Mg, constraining full occupancy was not sufficient. A stoichiometry constraint
was also needed. The reference is Finger, Larry W (1969) The crystal structure
and cation distribution of a grunerite, Mineral. Soc. Am. Spec. Paper, 2, 95-100.
If you know the overall chemical composition from electron microprobe data as in
my case, or from the chemicals used in the synthesis, use that information to
constrain the refinement!
Larry
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