Quantification - Multiphase Problem

katarinac

Hi,

I'm having problems determining the precise percentage of each phase in a multiphase problem.

My sample consists of two phases (with existing CIF files) - specifically, it's a hybrid material with one phase being the core and the other the shell. I can refine the pattern down to Rwp 5% (Rietveld refinement), but I'm not convinced the fractions/percentages that TOPAS gives me are correct.

I do have PXRD patterns of the physical mixtures with known weight fractions (50:50, 33:66, 75:25 etc.). I have refined the 50:50 PXRD and adjusted the scale factors to get the correct fractions in TOPAS - but now I don't know how to proceed.

How do I use the physical mixture PXRD patterns to obtain the correct fractions for the hybrid material?

Thank you,
Katarina

johnsoevans

Dear Katarina,

I've only got general advice as it's a potentially complex problem. Have you read e.g. the chapter by Madsen/Scarlett in the Dinnebier/Billinge book? It highlights some of the potential pitfalls. One issue they discuss is microabsorption and how it can effect samples with a large difference in absorption. Depending on the compositions of your core/shell and particle sizes this could be a major complicating factor.

A normal approach with physical mixtures would be to produce some kind of a calibration curve of Rietveld-determined composition against true composition. if the values agree well over a wide composition range then things are straightforward. If not, you have to think about what the origin of the discrepancy might be.

You shouldn't fix scale factors in topas to get the correct weight %. You should refine scale factors and see what weight % is reported , then think what it means.

John

rowlesmr

Hi Katerina

You may also want to have a read of this paper by Nathan Webster etal (http://scripts.iucr.org/cgi-bin/paper?S0021889810008484)

He looked at gibbsite precipitation seeded by goethite. THe goethite quantification went down due to the seed material being covered by gibbsite; no goethite was actually consumed in the reaction.

Matthew