Theoretically calculated intensities are independent of the size of the particular system while experimental one depends upon the size of the particles, although the relative ratios of the peaks obtained from the two methods being same/ similar.
Is there any discrepency in doing so?
Thank you
For powder XRD analysis, you should use McCrone micronising mill to wet-grind your sample and bring the particle size down to < 10um; this can minimise the particle size effect on XRD analysis.
Dear Sir
My question is related to theoretical calculation of xrd and not experimental xrd. Anyways thank you for the information regarding the experimental procedures.
regards
Dear Elangbam,
please have a look at your own question:
you ask for comparison of theoretical and experimental XRD relative peak heights. Gujie suggests how to prepare the powder pattern.
You have to take the XRD pattern of an adequately prepared sample in order to be able to compare with the theoretical ones. Minimizing the particle size effect, as Qujie mentioned, is one of the prerequisites to do that comparison. It is meant here (by Gujie's answer) to have a sufficient number of particles in the ROI of your sample in order to achieve a sufficient homogenous directional distribution of the crystallites in space. However 'small' crystallite size will increase the peak widths and thus will decrease the experimental peak heights.
Therefore it is recommended to take the ratio of the peak integrals instead of those of the peak heights. Here in that ratio the peak width- (and thus the peaks height-) dependence on crystallite size is effectively cancelled.
Another prerequiste is to take a sample having minor internal stress (stress causes peak broadening, unfortunately increasing with 2theta). But also here the ratio of peak integrals will 'help' you.
Then you have a very high propability that 'theoretical' peak height (better: peak integral) ratios resemble those from (powder!) experiments.
Thank you Dr Gerhard Martens for the detail explanations regarding the experimental techniques and necessary precautions and procedures.
Sir
I would like to explain a little bit more about my question.
Actually I am asking about the theoretical peak intensity. Calculation of theoretical intensities depends on the crystal system, composition and atomic positions but is independent of size.
Here I am considering size as a important factor but not actually about size effect. Apart from the size effect, say, if a sample is prepared at 100oC and then subjected to heat treatment of 1200oC (assuming in both temperature, the crystal structure is same), they will have different sizes but the same structure that is same standard xrd intensity (although slight changes) even if all the possible factors are taken into consideration.
And my question simply means that:
In such case, is it possible to normalize the theoretically calculated intensity according to the experimental one?
My question is about varying (normalization) theoretical one and keeping experimental one fixed.
Thank you so much for the nice explanation
Best Regards
Chitra
So simply take your peak height (integral) ratio of two peaks of your 'theoretical' pattern and compare it with the experimental one. They should be nearly equal if a) the experimental precautions mentioned above have been considered and b) if your theoretical model is 'ok'.
But again: please take the peak integral ratio at least for the case of the experimental peaks.
But sorry, I may do not really catch the point which your are adressing.
Perhaps someone else here on RG may comment.
Dear Chitra,
Can you elaborate a little about how the xrd pattern is calculated theoreticaly?
The best way of comparison is, do a Rietveld structure refinement on your powder xrd data which is collected as per the suggestion of Prof Gujie Qian and Prof Gerhard Martens . Rietveld refinement will allow you to incorporate allmost all the experimental conditions to model your calculated pattern to get a reasonable fit to the observed one. Also you will be able to incorporate the size, strain and many other sample effects, like transparency, absorption etc... to your calculated xrd pattern. basically this will model your experimentally observed pattern to the theoretically calculated pattern by incorporating these experimental and other effects like size and strain, so that you will be able to compare your observed and calculated xrd pattern in an unambigous way. Unless you incorporate these experimental conditions and sample effects to your theoretical calculations, a precise direct comparison may not be appreciated.
Thank you Dr Irshad for the detail explanation.
Yes, size, strain and other experimental conditions affects the intensity. But, still intensity is a function of the constituent atoms, their respective position, structure etc.
In fact my calculation starts from atomic position of the constituent atoms of the particular structure, which is simulated using a suitable program.
By comparing experimental one with a large number of model structures created by the program, the best structure is to be chosen so as to study the changes in the atomic positions within.
With best regards
Chitra
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