I have seen some people using W-H plots in non-nano materials. My understanding is that W-H analysis and Scherrer equation are applicable to nano particles only. If we apply it to micro samples, will it just be less accurate or completely meaningless? Can someone please throw some light onto it?
Easy answer: you can always apply any technique to any dataset. What's the meaning of the answer though? Both techniques are already qualitative (and NOT quantitative) and if you want to stick to the hypotheses behind hem, they could not be applied to real data. It is easy to understand what the problem is:
- in an ideal case (coherent beam over a length bigger than the domain size, no microstrain boadening), the breadth of the peaks is inversely proportional to the size. You don't have any instrumental effect, so if you measure with a sufficiently narrow step you can analyze even micron-sized particles
- in the real case you have other broadening effects that are convoluted to the size. it is easy to show that In a convolution between a broad and a narrow peak , the broader peak usually wins in determining the peak shape and breadth. Now the instrument has a peak breadth that is usually of the same order of magnitude of that caused by domains of ca. hundred nanometer in size (a bit more if you work with a high resolution synchrotron). The peak breadth and shape is therefore dominated by the instrument. If you add then the always present noise you see what the problem is.
- for large particles a further problem is statistics. If the specimen is NOT anymore a powder (cause you don't have enough grains under the beam to assume complete coverage of the orientation space), then the methods of analysis don't work anymore. It is easier to get a 2D pattern and calculate the size simply by looking at the intensity of the single crystal reflections (and you can index, map and measure each grain individually)
Thank you Matteo Leoni for the detailed reply. So if the sample I'm dealing with contains grains in the micrometer range, would it be wrong to assume that the contribution of crystallite size towards the peak broadening is negligible? In other words, if I am taking the XRD of all the samples having identical crystallite size in the same machine under identical conditions, can a comparison of FWHM be equivalent to a qualitative analysis of lattice strain?
if each grain is a monocrystal with diluted or no defects you can definitely assume it gives no contribution and therefore the broadening you see is given by the instrument (monotonic, angle dependent), by the microstrain (anisotropic, angle dependent).
Just be warned that in several cases large grains contain more than one domain
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