Re: Stoichiometry and occupancy fractions of solid solutions

2016-12-20 Thread Wilkinson, Angus P 
If you have multiple atoms/vacancies on the same site, it is also worth 
considering what resonant scattering can do for you.


Zhang, Y.; Wilkinson, A. P.; Lee, P. L.; Shastri, S. D.; Shu, D.; Chung, D. Y.; 
Kanatzidis, M. G., Determining metal ion distributions using resonant 
scattering at very high-energy K-edges: Bi/Pb in Pb5Bi6Se14 J. Appl. 
Crystallogr. 2005, 38, 433-441.

http://dx.doi.org/10.1107/S0021889805005686


Wilkinson, A. P.; Lind, C.; Young, R. A.; Shastri, S. D.; Lee, P. L.; Nolas, G. 
S., Preparation, Transport Properties and Structure Analysis by Resonant X-ray 
Scattering of the Type-I Clathrate Cs8Cd4Sn42. Chem. Mater. 2002, 14, 1300-1305.

http://dx.doi.org/10.1021/cm0107880

Zhang, Y.; Lee, P. L.; Nolas, G. S.; Wilkinson, A. P., Gallium distribution in 
the clathrates Sr8Ga16Ge30 and Sr4Eu4Ga16Ge30 by resonant diffraction. Appl. 
Phys. Lett. 2002, 80, 2931-2933.

http://dx.doi.org/10.1063/1.1473236


Zhang, Y.; Wilkinson, A. P.; Nolas, G. S.; Lee, P. L.; Hodges, J. P., 
Strategies for solving neighboring-element problems: a case study using 
resonant X-ray diffraction and pulsed neutron diffraction to examine 
Sr8Ga16Ge30. J. Appl. Cryst. 2003, 36, 1182-1189.

http://dx.doi.org/10.1107/S0021889803013955


Best wishes, Angus


Angus P. Wilkinson

Assoc. Chair and Prof.

School of Chemistry and Biochemistry

and School of Materials Science and Engineering

901 Atlantic Dr.

Georgia Institute of Technology

Atlanta, GA 30332-0400


Office: MoSE building 1100J

Tel: 404 894 4036

Fax: 404 894 7452


Packages to:

A. P. Wilkinson

311 Ferst Dr.

Atlanta, GA 30332-0400




From: rietveld_l-requ...@ill.fr <rietveld_l-requ...@ill.fr> on behalf of Luca 
Lutterotti <luca.luttero...@unitn.it>
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

Combined X-Ray diffraction and fluorescence analysis in 
...<https://www.researchgate.net/publication/288701415_Combined_X-Ray_diffraction_and_fluorescence_analysis_in_the_cultural_heritage_field>
www.researchgate.net
X-Ray diffraction (XRD) and X-Ray fluorescence (XRF) techniques are broadly 
used for materials characterisation. In some respects, they can be regarded as 
...



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

Re: Stoichiometry and occupancy fractions of solid solutions

2016-12-20 Thread Luca Lutterotti 
Sorry, here is the direct link to the journal article if you have the 
subscription:

http://www.sciencedirect.com/science/article/pii/S0026265X15003756 
<http://www.sciencedirect.com/science/article/pii/S0026265X15003756>

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 20 Dec 2016, at 16:04, Cline, James Dr. (Fed) <james.cl...@nist.gov> wrote:
> 
> 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 20899-8523USA
> 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
>  
> <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)

RE: Stoichiometry and occupancy fractions of solid solutions

2016-12-20 Thread Cline, James Dr. (Fed) 
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 20899-8523USA
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
++
Please do NOT attach files to the whole list 
<alan.he...@neutronoptics.com<mailto:alan.he...@neutronoptics.com>>
Send commands to <lists...@ill.fr<mailto:lists...@ill.fr>> eg: HELP as th

Re: Stoichiometry and occupancy fractions of solid solutions

2016-12-20 Thread Luca Lutterotti 
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 
Maud page : 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  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 [rietveld_l-requ...@ill.fr] on behalf of 
> Othman Al Bahri [z3435...@zmail.unsw.edu.au]
> Sent: Monday, December 19, 2016 1:54 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 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
> ++
> Please do NOT attach files to the whole list 
> Send commands to  eg: HELP as the subject with no body text
> The Rietveld_L list archive is on 
> http://www.mail-archive.com/rietveld_l@ill.fr/
> ++

++
Please do NOT attach files to the whole list 
Send commands to  eg: HELP as the subject with no body text
The Rietveld_L list archive is on http://www.mail-archive.com/rietveld_l@ill.fr/
++

Re: Stoichiometry and occupancy fractions of solid solutions

2016-12-20 Thread babaryk 
Good luck with your project!

Cheers,
Artem

> Dear Artem,
>
>
> Thank you very much for pointing me to these resources. They have also
> been extremely useful.
>
> Have a wonderful holiday and a happy new year.
>
>
> Kind regards,
>
>
> Othman
>
> 
> From: baba...@univ.kiev.ua <baba...@univ.kiev.ua>
> Sent: Monday, 19 December 2016 9:31:29 PM
> To: Othman Al Bahri
> Subject: Re: Stoichiometry and occupancy fractions of solid solutions
>
> Dear Othman,
>
> General notes and precaution are concisely described in the
> Rietveld refnement guidelines "J. Appl. Cryst. (1999). 32, 36-50
> (paragraph 7).
> Then, you may refer to Madsen & Hill's Variable Counting Time concept,
> J. Appl. Cryst. (1994). 27, 385-392. Definitely, that's the solution
> for reliable refinement of site occupancies, when lab PXRD is utilized.
> Those considerations are simplified by condition of chemically averaged
> grain compositions, as Uwe has noted.
>
> Sincerely,
> Artem
>
>
>> 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
>> ++
>> Please do NOT attach files to the whole list
>> <alan.he...@neutronoptics.com>
>> Send commands to <lists...@ill.fr> eg: HELP as the subject with no body
>> text
>> The Rietveld_L list archive is on
>> http://www.mail-archive.com/rietveld_l@ill.fr/
>> ++
>>
>>
>
>
>


++
Please do NOT attach files to the whole list <alan.he...@neutronoptics.com>
Send commands to <lists...@ill.fr> eg: HELP as the subject with no body text
The Rietveld_L list archive is on http://www.mail-archive.com/rietveld_l@ill.fr/
++

Re: Stoichiometry and occupancy fractions of solid solutions

2016-12-19 Thread Leopoldo Suescun 
Dear Othman,

I agree with previous comments and warnings from colleagues.

But regarding your specific question, you need to check the manuals for
GSAS II or FULLPROF on how to setup the constraints on the site occupancies
of all the atoms and sites involved.

It is not sufficient to constrain the occupancies of each site add up to 1.

You also need to cross-constrain occupancies in such a way that the sum of
occupancy*multiplicity for each element equals your known chemical
composition.

For example, if you have 2 sites a and b for B/C, the two sites have
multiplicities MULTa and MULTb (as defined in the International Tables Vol.
A), your composition is B0.5C2.5 and the number of formula units A2B/C3O12
is Z, your constrains should look something like this: (I will call SOF-Ba
and SOF-Bb the fractional occupancies of B in a and b sites respectively
and SOF-Ca and SOF-Cb the fractional occupancies of C in a and b sites
respectively):

SOF-Ba+SOF-Ca=1
SOF-Bb+SOF-Cb=1
(you said you already set up these two and those make sure you don´t have
more or less than 3*Z atoms in the sites)

SOF-Ba*MULTa+SOF-Bb*MULTb=0.5*Z
SOF-Ca*MULTa+SOF-Cb*MULTb=2.5*Z
these constraints keep your B and C composition in the expected values.

Be careful with the number of formula units per unit cell or your constrain
will be wrong. If you have more than two sites you need to proceed
accordingly adding a third constraint for the total occupancy of the third
site and adding the corresponding term in the two sum constraints.

Depending on the specific software this will be written differently but the
idea should be to keep these relations.

Best luck with your refinement,
Leo


2016-12-19 5:54 GMT-03:00 Othman Al Bahri :

> 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 A2C
> 3O12 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
>
> ++
> Please do NOT attach files to the whole list  >
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Re: Stoichiometry and occupancy fractions of solid solutions

2016-12-19 Thread Larry Finger 

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 20899-8523USA
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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RE: Stoichiometry and occupancy fractions of solid solutions

2016-12-19 Thread Cline, James Dr. (Fed) 
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 20899-8523USA
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 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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