For a series of single crystal thin films (thicknesses between T=20-300 nm) I observe a narrowing of the (444) Bragg peak with increasing T, as expected from the Sherrer formula considering the crystallite size to be limited by the film thickness.
I tilted the samples by chi=70.53 ° to access the (4-44) planes. The material is cubic so the (444) and (4-44) diffraction peaks overlap. The (4-44) Bragg peaks are broader than the (444) Bragg peaks. Roughly the FWHM of the 200 nm film in the (4-44) orientation equals the FWHM of the 70 nm film in the (444) orientation. A simple math, shows that 200[nm]*cos(70.53°)=66[nm]. So, the crystallite size in the (4-44) direction seems to be the projection of the film thickness on this direction. Geometrically I would expect the crystallite size in the (4-44) direction to be T/cos(tilt) which would lead to larger crystallite size and narrower FWHM with increasing the sample tilt.
Attached are the XRD patterns.
If you tilt from the Bragg-Brentano focusing geometry you will broaden your peaks! (Unless you have parallel beam incident optics, in which case you will have broad peaks due to the detector resolution.)
You must always subtract instrumental broadening from observed peak widths to caculate sample broadening. The scan of a standard used to measure instrumental broadening, should always use the same instrumental setup and scan conditions, as the sample scan. For 2theta range, scan at least two peaks of your standard that are on either side of you sample peak of interest.
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