fundamental parameters for transmission

annakater

Dear Topas-users,
I am trying to refine some patterns taken in transmission mode with a microfocus beam. Is there anyone who can help with the instrumental parameters, as opposed to reflection geometry? I am sure there is more to it than writing in the slit sizes.
Regards,
Anna Kater

cc0u50fd

A fair amount of instrumental parameters should be available by looking up the make and type of diffractometer you have.
ie:
diffractometer radii, monochromator angle, anti scatter slits, receiving slits, secondary soller....

the quoted values I would have thought would have been good enough.

The only time I have ever strayed away from using the simple axial model in favor of full axial was when getting particle sizes.
In which case I refined the instrumental parameters using a NIST standard and fixed them.

It all depends on what you want to do.

annakater

thanks for the reply cc0u50fd,
I follow the same method as you, with refining a standard and fixing the instrument parameters to use on patterns measured with the same setup. Whith the difference that I prefer full axial and use single axial model only for neutron data.

My question is, what is different in transmission geometry? Anything to look out for?

Cheers,
Anna

rowlesmr

Are you working with a capillary or a flat plate?

The absorption corrections will be different. If you've got a flat plate, and working in an asymmetric geometry, the peak shapes will be affected.

What, exactly, is your experimental setup?

cc0u50fd

I agree with rowlesmr,

The physical diffractometer doesn't change, but it depends now on your sample.
What elements (absorption) mainly.

Another thing to watch out for is if the diffractometer can be changed to different geometries (ie reflection or transmission) and then I think the radius will change.

What you shouldn't have to worry about is preferred orientation, unless you have not ground the sample properly (which could also be intentional).

djurdjijadzodan

Dear Topas-Users,

I am having few basic questions regarding a couple of terms to be used in FP approach in case of XRDs collected on capillaries on transmission geometry instruments:

1) In case of Cu source with Ge incident monochromator (CuK1.lam) of the STOE diffractometer, how do we determine the filament and sample lengths?
I am imagining that the filament length is in a way "restricted" by the monochromator (as well as by the divergence slit and collimator) before hitting the sample, but perhaps it is not important and we can still use the "12mm" as filament length value? Is the sample length in this case the diameter of the capillary (2r)?

2) If between the monochromator and capillary there are both divergence slit and collimator, the divergence slit we address with the "Divergence (value)" and the collimator with "Primary_soller_angle value" commands?

3) Is the "axial_n_beta value" command also relevant in case of transmission geometry instruments?

4) When using the cylindrical corrections:

prm muR 0.01792` min 0.01 max 1.0 ' Refined mueffR for absorption calculations

[
prm !packing_density 0.50 min 0.1 max 1.0 'typically 0.2 to 0.5
prm !capdia 0.5 'capillary diameter in mm
prm !linab = Get(mixture_MAC) Get(mixture_density_g_on_cm3);:148.89110`_0.00000 'in cm-1
prm muR = (capdia/20)linabpacking_density;
Cylindrical_I_Correction(muR) ' Intensity correction
]
Cylindrical_2Th_Correction(muR) ' Positional correction

4a) Do we first calculate the muR value by writing the diameter of the capillary in the appropriate line of code (or the TOPAS do it for us upon Rietveld?) and then fix the retrieved muR value in the Cylindrical_I_Correction(muR) and Cylindrical_2Th_Correction(muR) corrections or we should let the muR values to be refined upon each refinement cycle?

4b) How do we estimate the packing density value to be used as input?

5) In which cases we apply the "Apply_Brindley_Spherical_R_PD(R, PD):" correction? In the TOPAS technical reference and on the topas wiki I have read the short text about it, but I am wondering if and when the correction in practice is really needed (always?) and if can/should be combined with the macros under 2?

I am really sorry for so basic questions, considering that I am a beginner it would mean a lot to me to know in advance what I should do regarding the issues above since my inital trials on a standard haven't let me almost anywhere so far.

Thank you so much in advance.

Kind regards,
Djurdjija

johnsoevans

1-3. You need to talk to somebody who knows your instrument. Do you really need a fundamental parameters approach? It has it advantages but if you're just after structural information, straightforward size/strain or cell parameters it's not really needed. I tend to do 95+% of work with empirical peak shapes.

You can use this type of macro to either apply a correction based on the sample or refine the parameter freely. What's appropriate is up to you to decide. I would typically calculate the expected value for my sample and see how it compares with the refined value. You can estimate packing density by weighing the capillary before/after loading. It will typically be 50% or lower.
Try reading (e.g.) the chapter by Ian Madsen and Nikki Scarlett in the Billinge/Dinnebier text. My view is that if you need to apply a Brindley type correction you're in dangerous waters.

rowlesmr

muR for the capillary corrections refers to the size of the capillary.

R in brindley refers to the size of the particles. Brindley also assumes your particles are monodisperse, polycrystalline spheres. Also that their size is such that the correction is valid. Hint: They aren't.