Dear colleagues,
I have analyzed the crystallinity of photocatalyst material (without calcination treatment and calcined samples) using XRD in two place XRD analysis services in my campus. When I comparing the data, there a significant movement between them. Obviously, the peaks have significantly shifted to the lower thetha (°) for calcined samples than control (untreated sample). Based on this fact, is it an instrument error or structure destruction?
If it is not the instrument error, then most probably the slight shifts of peak positions toward lower angles might be occurred due to the substitution of small radius ion by a larger one (Vegard’s law) which leads to an increase in the cell parameters that may cause a slight distortion in the lattice and a modification of the crystal structure.
@ F. Fabiyanto: Instrumental error can be checked by running standard and comparing with its recommended values of interplanar spacings (d). It 'd' values are not matching then structural changes may be responsible for shift towards lower two theta side.
Alaways believe the Instrument after verification. You could have changed the structure either by oxidation or change in size through sintering.
Every X-ray diffractometer offers some unintentional peak-broadening and peak-shifts. But, generally these errors are quite small for good instruments and if the voltage, current, monochromaticity of the source etc are reliable. Good companies also provide the verified deviations associated with the instrument. If you don't have the data, the standard procedure is to use a defect-free Si single crystal for the measurement of the deviations. However, it's quite costly.
In general, if you neglect the instrumental errors, the reasons behind broadening are: (a) increasing crystallite size, (b) defects and non-uniformity (a series of close sharp peaks merge and make a broader resultant peak) and (c) high strain.
The reason behind peak-shift is change in interplanar distance i.e., the unit cell conformation. From Bragg's formula: 2 d sin θ = nλ, it can be seen that, higher d causes lower theta and a left-shift in the XRD pattern and vice-versa.
The calcination-treatment can cause both broadening and peak-shifts as it promotes higher agglomeration-tendency.
If it is not the instrument error, then most probably the slight shifts of peak positions toward lower angles might be occurred due to the substitution of small radius ion by a larger one (Vegard’s law) which leads to an increase in the cell parameters that may cause a slight distortion in the lattice and a modification of the crystal structure.
The calcination-treatment may also result in new phase with new structure.
Hi F. Febiyanto, at a glance, this looks like zero shift error (likely due to sample height error). This is because the shift looks linear for both peaks, and the samples are dilated (expanded) with heat treatment, which is unusual. Usually wet samples before calcination contains incorporated water that will be removed after calcination, thus shrinking the unit cell.
Zero shift error is usually because the sample holders are not properly filled. You need to make sure the sample is filled correctly (not too much and not too little, so the sample height is the same as the top of the sample container) and consistently. Do not place it on flat surface. Once the experimental error is eliminated, you can now research what is the possible structural change. I recommend using quantitative X-ray analysis software like Fulprof (free) or Topas (paid, Bruker) to fit to known standard and extract the unit cell parameters and atomic positions.
There are a few phenomenon that can cause significant lattice dilation, like cation exchange or formation of solid solution. This means your raw, uncalcined material may contain smaller cation (e.g. Li) with larger cation sitting on separate phase (e.g. K). The resulting structure is related but the cell is then dilated because of the exchange. Please see this paper for more information
Article Hydrothermal synthesis of sodium potassium niobate solid sol...
@Albertus D. Handoko Thank you very much for your valuable answers and paper recommendation. Recently, I also think that it was caused by some error in analysis preparation so that the diffractions have shifted unusual.
@Febiyanto:
I see some shoulder peak has emerged on the left of the peak at 33 degree upon the calcination of the pristine sample. When the intensity of this shoulder peak has reduced, @600 C, the pattern looks nearly identical to the pristine one. Given the fact that, there are no instrumental error, there might be some possible phase separation/ dissociation. I suggest you to look into the origin of the shoulder peak carefully!
Good luck
The shift may also caused by induced stresses in the materials, because at higher anneal temperatures the structural properties may change by surmounted conditions or stresses may induced.
I suggest you to correct the zero error. If all peaks shift to lower Theta with same angle, which may indicate the occurance of zero error.
Xiangyan Hou I agree with your point. Routinely such errors should be checked to ensure that whatever data is generated is free from any instrumental errors. Various instrumental parameters influence data output.
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