I would like to refer you to my article "Microstructural and Defect Analysis of Metal Nanoparticles in Functional Catalysts by Diffraction and Electron Microscopy: The Cu/ZnO Catalyst for Methanol Synthesis" which was written to the non-XRD-Expert to aswer all these kind of questions. The full text is available on my profile.

I would like to refer you to my article "Microstructural and Defect Analysis of Metal Nanoparticles in Functional Catalysts by Diffraction and Electron Microscopy: The Cu/ZnO Catalyst for Methanol Synthesis" which was written to the non-XRD-Expert to aswer all these kind of questions. The full text is available on my profile.

There have been several posts re peak broadening see:https://www.researchgate.net/post/What_are_the_factors_which_influence_the_XRD_patterns_of_nanoparticles

https://www.researchgate.net/post/What_are_the_reasons_for_peak_broadening_in_the_case_of_nanoparticles_compared_to_the_bulk2

But this ppt presentation: Estimating Crystallite Size Using XRD by Prof Speakman is good:

https://www.google.ie/url?sa=t&rct=j&q=&esrc=s&source=web&cd=6&cad=rja&uact=8&ved=0CEIQFjAF&url=http%3A%2F%2Fprism.mit.edu%2Fxray%2Foldsite%2FCrystalSizeAnalysis.ppt&ei=YLVgVO-1Bcac7gbk24HoBA&usg=AFQjCNGtJ9aO3RdkzWp6B0dnhllYHCNlGg&bvm=bv.79189006,d.ZGU

Size effect  of diffraction, the diffraction line broaden with the decrease in the number of lattice planes reflecting the x rays. when the crystal become smaller, the number of lattice planes becomes less, so the diffraction line will be broadened. You can find full explanation in textbooks about XRD.

The explanation given by ZhiQing Yang is right. As the  size of crystal become nanosize, the number of lattice planes becomes less, so the diffraction line will be broadened.

This peak broadening is also used in the Scherrer equation from 1918 to calculate the average grain size in polycristalline materials from XRD measurements. A short explanation is given here: http://en.wikipedia.org/wiki/Scherrer_equation

I agree with Rene. It depends from grain size of constituent phases. This effect is described by Scherrer equation.

i agree with all those previous answers that mentioned Scherrer equation. But the results not always mean that we reached the Nano Scale.other factors can broaden the XRD peaks too such as mis-orientation of grains or Mosiac structure. So to get sure about our result we usually mix our samples with the Tungestan Carbid that has a large grains and almost all the width of the pic is because of the the other factors that broaden the peak.

you correct the width of the peak with this equation

B(true)= (B^2(sample)- B^2(callibration) )^0.5 

The Scherrer approach must be used with an abundance of caution. It is essential in this approach to assume a correct shape for the diffracting domain size for precise analyses.

The Williamson-Hall method may help deconvolute the "strain effect".

"B(true)= (B^2(sample)- B^2(callibration) )^0.5" for a Gaussian Bragg profile shape.

I'll post some references as I find them.