I have got a W thin film grown on a Mo substrate. I measured XRD profiles both on Bragg-Brentano and grazing incidence geometries. On both profiles I see a clear shift of 211 and 220 reflection with respect the position one should expect from the cubic Im-3m crystal structure. I don't think it could be due to residual strain-stress because in that case the effect should just be visible on grazing angle geometry. Anyone can help me with this? See the grazing angle XRD attached.
why should one not see a shift in all film peaks (not only at grazing incidence)? XRD measures distances (in a given direction) and if in this particular direction d differs from the bulk value, then the peak will shift.
Yes, of course cell parameters can be different from bulk, but in that case all the peaks should shift accordingly (it's a cubic structure).
In case of thin films the unit cell can distort to accommodate the substrate strain. In this case the initial symmetry is lowered. The other possibility could be a systematic error of the system. If you trust your XRD machine, then the only possibility is that the W film do change.
Dear Marton,
I kindly disagree, because cubic cell distortion should lead to peak splitting and/or appearance of additional reflections.
Dear Filippo,
Its mostly happen in Thin films and nanomaterials. Make sure that the JCPDS card which you are using for this is matching perfect. It might fit with other JCPDS card. If you make sure that your analysis is right, then you have to consider the FWHM of the peak. First look of your XRD indicates that it might be on nanostructured materials. In case of nanomaterials it happens. [Peoples call it as quantum size effect (which really not exist if your particle size is not less then 10 nm)] because of increased surface area and high aspect ratio. I suggest you to consider all possible JCPDS cards for the analysis and after that check if your sample is in nanoform.
Dear Filippo,
In powders lowering of the symmetry (distortion from cubic) leads to peak splitting. In the case of textured or epitaxial thin films you could "miss" this effect, as the in-plane and out of plane distances will be different, but not "mixed". It is not possible to decide what is the situation in your case without detailed information about the film structure (thickness, texture, mosaicity, etc.) and measurement geometry. For example the 200 peak (at 58.2 deg 2theta) is missing in your measurement, a hint of strong texture.
Looks like a case of dissymmetrization (lattice distortion, deviation from cubic symmetry). If the deviation is weak, no peak splitting may be visible, especially in your case of nano-sized material with broad reflections. But the peak broadening should become anizotropic, what seems to be the case in your pattern.
Have seen something similar in Ni film/Au/Si.
How much is the shift (approx. order in 2 theta value)? And what is the order of thickness of your films?
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