discrepancy of lattice parameter between XRD refinement and PDF refinement

[gyuchul; park]

I refined the lattice pramater of the alloy at two different temperatures (A > B)

, and I got a different trend from XRD (Rietveld refinement) and PDF. For example, when compared to temperature B, I can qualitatively see a decrease in the atomic distances up to 30 angs. at temperature A (I see a shift of the PDF to the left), resulting in a reduction in lattice parameter. On the other hand, the lattice parameter at temperature A was greater than the lattice parameter at temperature B according to Rietveld refinement, which is contradicting to the PDF results. I think a trend should follow each other even though the absolute lattice parameter could vary. I am wondering about the possible reasons for the discrepancy.

Could it be because PDF is looking at local structure (within 30 A for my case), while XRD is looking at more larger area, which is corresponding to the size of the x-ray beam (~10 mm x 10 mm). 

I would appreciate for your help. 

Thanks.

Park 

[Simon Billinge]

Thanks Park,

It is a bit hard to say without more information, but it sounds like there could be an issue with data processing.  That would be what i would check first.

S

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Simon Billinge

Professor, Columbia University

Physicist, Brookhaven National Laboratory

[gyuchul; park]

Dr. Simon Bilinge,

Doesn't it make sense if the all the data process is correct? I can write about the data process procedure I went through. 

Park

[Alberto Martinelli]

Dear Park,

in these cases the presence of secondary phases may help. If you can detect one or more peak of a secondary phase in the diffraction pattern, look if they are shifted on the right/left as those of the main phase (i.e. with similar shift amplitude). In this case there could be some kind of artifact in your data. Conversely, if you see that the peak-shift for the secondary phase (that should be present on account of the thermal effect) behave differently than that of your alloy, than you can conclude that the change of the lattice parameters is reliable. The more the peaks of the secondary phase, the more reliable is your conclusion.

Alberto

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