The mathematics of the two profile functions are different, and the details should be well documented in the HELP menu of Jade. Or you can find it in many textbooks on XRD. Note that if your peak fit is reasonably good. the %crystallinity result will be essentially the same for either function.

But to the second part of your question, what method are you proposing to use to determine %crystallinity? 

Pseudo-voigt shape uses a combination of Gauss and Lorentz functions. It is the most used shape profile for Rietveld refinement.

For mesure %crystallinity, I usually use the ab initio method, which means, insert a known phase in your material (alumina or quartz) as an intern pattern. After that, you can use Rietveld to measure the difference between the phases with and without the intern pattern. After that, you can use the following formula to calculate the %crystallinity:

%amorfous= (1-(Ws - Ws/Wp))/100- Ws . 10E4

as Ws the exact amout of intern pattern inserted (%) and Wp the quantified phase by Rietveld method.

100-%amorfous = %crystallinity

thank you, in attach file,this is XRD pattern that I get ,base line isn't same level . what shape function I should used?

Kn Smile, the base line change over the peak is a result of the normal background level combined with teh siganl from your material. Why don't you try both both shape functions and see which gives the lowest residual on fitting? As mentioned above, I  don't think it will make that much difference. A couple of other comments. Your background signal looks quite noisy, and your total counts are quite low -  for best fitting you need less noise, so count for longer; it may help if you increase the slit size a little. Also if you are doing background modelling you have not started the scan at a low enough angle in order to see the true background. Question- is the Cristobalite the added internal standard in this pattern? If it is then your material is already totally amorphous and it is not worth going through the effort of trying measure it.

It would help to know a bit more about this data set. Is it "spiked" with crystobalite as a standard as Ian has asked, or is it a mixture of amorphous silica and crystobalite for which you want to know the fraction of crystalline crystobalite in the mixture? If it is the former, then Ian is almost certainly correct that the answer is that your sample is 100% amorphous.

If the latter, then you need to take another data set to separate your instrument background from the amorphous scattering. Running a pure crystobalite (100% crystalline) sample under the same scanning conditions would do the trick for this. That will reveal what the "non-amorphous" background is. The remaining intensity is divided between the amorphous phase (the broad hump) and the crystalline phase (the sharp peaks) and the ratio of the integrated intensities is the approximate %crystallinity.

I have tried to sketch this on your figure (sorry for how bad it looks but I was in a bit of a rush) - assume the green area is the area under the background of the 100% crystalline phase, based on your yet-to-be-taken data. The remaining intensity above the green area is what you need to use for the crystallinity analysis.

thank you for all comment. It is a XRD pattern that mixed of amorphous silica and crystalline crystobalite. I prepared it and I 'd like to know the differrent of %crystallinity between I get from XRD and the real %crystallinity  that I prerared.

Kn Smile - thans for that information. You need to run it again, starting at a lower angle so that you can see where the rise in the background from the amporphous component starts.Then you can measure the true background. I would again advise that you obtain better signal to noise, count for longer, open the slit - you do not need good peak resolution for this excersize.

Measure the area under the curve for the total pattern, then use a background fitting tool to model the amorphous component and subtract this from the total to give only the crystalline component. Use the same area points on the crystalline only scan and obtain the ratio %crystllinity = (area of crystalline scan / area of total scan) *100

I have attached some slides to illustrate the process.

Kn Smile - I note you are using a very dated version (6.5) of MDI's Jade software. It must be well over a decade old, maybe more.