I doped a fluoride perovskite LiMgF3 with different concentrations of Cr (0.25%mol and 0.5%mol). The XRD pattern of LiMgF3 shows the highest peak at 38.9847, but LiMgF3:Cr 0.25%mol shows this peak at 39.1878 and now the intensity is lower. Can I say there is a shift due to the Cr? Because the shift is very small (~0.2 in each peak). And also, Can I say the changes in intensity are due to Cr as well? (The intensity of all XRD patterns was normalized)
I would advise you to enter the crystal structure in VESTA (http://www.jp-minerals.org/vesta/en/) and vary the chemical composition there. You can then create an XRD pattern in VESTA and after some variation you can get a feeling for the influence of the chemical composition.
The shift is significant and definitely the dopant is playing a role in modifying the interplanar spacing of the LiMgF3:Cr system. But, you need to venture a number of more sophisticated experiments to understand the situation. Let me explain.
The Bragg's law is simple: 2d{hkl}sinθ = nλ, so if d increases θ decreases for a particular peak and the XRD pattern shifts left and vice-versa. As if you have doped Cr, d got definitely changed. But, as Cr has higher ionic radius, for substitutional doping the probability of higher d is likely.
So, you need to do Rietveld refinement and XPS to understand where exactly the doped Cr atoms are located. Do they substitute any or both of Li & Mg, or they make some sort of point-defect by occupying an intermediate position. Rietveld refinement will give you the complete crystallographic info e.g., lattice parameters, stoichiometry and unit cell volume. So, you may compare the pure and doped material.
You may also go for SAED along with HRTEM or DFT calculations to understand which site is most stable.
Shift in peaks is well observed in XRD analysis if crystallite structures (metal) are doped with another metal. Actually doping of any metal on semiconductor leads to small red shift toward more 2 theta. This nature will produce a less crystal size after doping, this process depends on the interaction between doped metal and the main metal in crystal. If the doped metal has a less ionic radius compared with the ionic radii of the main metal in semiconductor, that will cause a good interaction and less crystal size. But in your case the ionic radius is higher than the compositional metal of the existing semiconductor. So the chance of substitution doping is higher which likely to portrait the XRD nature exactly opposite of yours. I suggest please repeat the experiment, if same result persist do a crystallographic scan. But yes the XRD peak can be shifted in both side depending on the crystallographic structure after doping.
In principle, you have two effects: Bragg angle shifts (that are qualitatively explained above by Souvik and Gourab) and Bragg peak intensity changes. You may qualitatively (and quantitively) assess the latter by considering the structure factor F of your sample: F is determined by the positions of all the atoms with their atomic scattering factors (S) in the unit cell. If you add an extra atom or substitute an atom of the original unit cell by a dopant atom, the structure factor of this new unit cell will in general be different from the original one, even if you substitute a dopant on a regular lattice position, each atom will have different atomic scattering factors and thereby provide a change of the entire structure factor. If you then take into account that only every nth unit cell will be modified (as determined by the amount of the dopant you introduce) you may get an idea of the changes in Bragg peak intensity ... Hope this helps, enjoy the week-end, Dirk
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