My sample which is in powder form shows a huge preferred orientation effect in its PXRD pattern along 006 hkl plane. I would want to refine it using GSAS software. I learnt that GSAS offers two ways of handling the preferred orientation. One is march Dollas way and another is spherical harmonics. I want to know the prime difference between these two approaches. Which one gives the best fit for the powder sample? I blindly followed March Dollas way. i obtained the best fit for 006 peak with march Dollas co-efficient nearly 4. but this is reducing the Rw of the fit. What is the reason for it? How do I proceed?
Hello Rakshitha H a.
With March Dollase, you choose 1 reflection and then adjust the factor, so the intensity of this reflection will be correspondent to your patter. With Spherical harmonics, it's generalized, as long as you increase the factors, there will be more "liberty" to fit your pattern.
But first, did you find a March Dollase factor of 4? Not 0.4, did you mean 4? This is strange, because March Dollase factor starts with 1, and as long as you need to increase the intensity of the reflection, you reduce the factor. A factor of 4 would mean that your peak is lower than it should be? If this is the point, I think we should do a deeper analysis, because as far as I know, there is no preferred orientation when a peak is lower than it should be.
Feel free to send a reply, so we can undertand what is going on with your sample.
Best regards,
Ricardo Tadeu
Ricardo Tadeu Maia , Thanks for the reply.
Sorry, March Dollas factor is 0.4 not 4. Why is there a loss in Rw as the fit is getting better for the oriented hkl plane? Is there any factor that I take into account while refining to avoid loss of Rw while applying March Dollas factor? One more query is, Can I use the March dollas way for the preferred orientation along another hkl plane in the same PXRD pattern? for example, My sample shows preferred orientation along 006, 004 and 0010 planes.
So, you got a problem with intensitiy of peaks. We really need to make a deeper analysis, because a intensity difference doesn't mean that it's always preferred orientation. If you could share your result, I could give a better guess about what to do.
Ricardo Tadeu Maia , Please have a look at the PXRD data image that I have attached. If you are not convinced that the preferred orientation is not present in this case, please convince me too...
Indeed, this is preferred orientation. As you said, in more than one reflection. I do think that GSAS-II can handle it, but I don't think March-Dollase can model more than one peak, so Spheric harmonics may be the way to do. You are using GSAS-II, right?
Ricardo Tadeu Maia , Yes I am using GSAS-II. How to proceed with the spherical harmonics way? are there any tutorial or guide books to use spherical harmonics in refinement?
I never managed to use it successfully, but it's worth a try. I've never seen a tutorial of it, but you can try to use it in the same section GSAS-II presents March-Dollase. If you want to, you can send me the data and I will try too.
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