Luca,

I'd be interested to read your article, but I'm not about to join researchgate 
to do so.

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 
Luca Lutterotti
Sent: Tuesday, December 20, 2016 9:55 AM
To: rietveld_l@ill.fr
Subject: Re: Stoichiometry and occupancy fractions of solid solutions

To increase sensitivity to partial/mixed occupation and also solve the case of 
3 or more atoms in the same site (you have an infinite number of solutions that 
give you the same scattering factor, both x-ray and neutrons) we are developing 
a combined XRD-XRF analysis. Some preliminary application examples were 
published and we are preparing a full article on the theory behind.
https://www.researchgate.net/publication/288701415_Combined_X-Ray_diffraction_and_fluorescence_analysis_in_the_cultural_heritage_field

Best regards,

Luca

-----------------------Luca 
Lutterotti--------------------------------------------
Dipartimento di Ingegneria Industriale, Universita' di Trento,
via Sommarive, 9, 38123 Trento, Italy

e-mail address : luca.luttero...@unitn.it<mailto:luca.luttero...@ing.unitn.it>
Maud page : http://maud.radiographema<http://maud.radiographema/>.com

Phone number :+39-0461-28-2414
XRD lab:: +39-0461-282434
Fax : +39-0461-28-1977
----------------------------------------------------------------------------------------




On 19 Dec 2016, at 18:23, Vogel, Sven C <s...@lanl.gov<mailto:s...@lanl.gov>> 
wrote:

Hi Othman,

Another insightful thing to do might be to simulate diffraction patterns for 
various scenarios. If you have B on two sites and simulate if the diffraction 
pattern will look different for different occupancies on those two sites, 
resulting in a given overall stoichiometry, you can convince yourself whether 
diffraction is a good tool to determine SOFs. You can also check which 
diffraction peaks show the biggest change, which may affect your count time or 
region of interest. If you also simulate for neutrons and see a big change for 
neutrons, but not X-rays, you have a good case to supplement a neutron beam 
time proposal.

Good luck,

Sven


________________________________
From: rietveld_l-requ...@ill.fr<mailto:rietveld_l-requ...@ill.fr> 
[rietveld_l-requ...@ill.fr<mailto:rietveld_l-requ...@ill.fr>] on behalf of 
Othman Al Bahri [z3435...@zmail.unsw.edu.au<mailto:z3435...@zmail.unsw.edu.au>]
Sent: Monday, December 19, 2016 1:54 AM
To: rietveld_l@ill.fr<mailto: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 A2B3-xCxO12 at x= 0.5 steps. A2B3O12 is orthorhombic while A2C3O12 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
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