I am preparing a perovskite structure by three different methods. I prepared, getting all the same peaks in XRD Pattern of all three but with slight shift. Can anyone tell me the reason for the shift in XRD Pattern.
XRD peaks can shift for lots of various reasons.
Sometimes, the entire pattern will shift to higher or lower angles, which can happen if you have a variation in the height of the powder surface. This is subtle, but you can detect this by shifting the entire patterns by some angle and seeing if they match up.
If there is an actual change in the lattice spacing, then it will be impossible to shift the patterns so that all of the peaks line up, and the shift will be more apparent at higher angles. Oxides (especially perovskites) are notoriously sensitive to things like non-stoichiometry -- you can have a sample with, for example, ABO2.97 or ABO3.03, which will likely give you different XRD patterns as the interatomic spacing changes with oxygen content. I suspect that this is the problem you're running into, since you're using different synthesis methods, and since controlling oxygen content precisely and repeatably is one of the most difficult challenges of oxide synthesis.
That happens because of generation of stress in the sample, either during processing (rolling, forging etc.) of the materials or during sample preparation. Because of the stress, the dimension of the unit cell get changed, which causes the change in inter-planner distance (d) . During XRD the wavelength (λ) of the X-Ray is fixed, so according to the Bragg's Equation ( λ=2d Sinθ), there will be a slight change in the "θ" and hence the pick is shifted in the XRD pattern.
It might also be due to run-to-run variation between XRD scans, depends on how much shift we talk about here. To confirm that, you can simply run XRD on the same sample multiple times to check if there is shift, and if it is comparable to the shift you see between samples prepared differently.
XRD peaks can shift for lots of various reasons.
Sometimes, the entire pattern will shift to higher or lower angles, which can happen if you have a variation in the height of the powder surface. This is subtle, but you can detect this by shifting the entire patterns by some angle and seeing if they match up.
If there is an actual change in the lattice spacing, then it will be impossible to shift the patterns so that all of the peaks line up, and the shift will be more apparent at higher angles. Oxides (especially perovskites) are notoriously sensitive to things like non-stoichiometry -- you can have a sample with, for example, ABO2.97 or ABO3.03, which will likely give you different XRD patterns as the interatomic spacing changes with oxygen content. I suspect that this is the problem you're running into, since you're using different synthesis methods, and since controlling oxygen content precisely and repeatably is one of the most difficult challenges of oxide synthesis.
As well-known, WO3 has oxygen- deficient perovskite structure. In addition to the convincing reasons given by Stephen Boona, the use of different preparation or crystallization methods may result in some oxygen- vacancy ordering/disordering of the perovskite structure, due to which a shift of XRD peaks can be observed to higher or lower angles.
Ya Suriyanarayanan Sir ,I had taken different preparative conditions, but in two i m getting at same place and in one same peaks are there but with shift.
If you can spare little bit amount of sample, you can add Si as an internal standard and then confirm whether the shift is really observed. Once you confirm it with internal std method then you can explain the shift observed based on the preparation methods...
Since you prepared the samples differently, it is possible that the peak shifts in XRD are due to strain. See if you can determine the type of strain as either compressive or tensile (ie: are the lattice spacings increasing or decreasing?) and see if this makes qualitative sense in terms of your preparation techniques.
From my opinion the best explanation is given by Stephen. If we exclude simply alignment problems (sample not in the focus point, e.g. sample hight, or perhaps also different "packing" of the powder so that the radiation is coming from another depth) for perovskites non-stochiometry is a typical feature of this phase. It is also connected to the formation of different symmetries so that peak-splitting is observed. The best way for testing is the use of an internal standard as already proposed above. The other interpretations related to lattice strain are unlikely since lattice strain is measured at compact material and caused by the measurement of XRD pattern under different tilt angles of the sample (e.g. psi- tilt or rarely omega-tilt). It uses the effect that not only those lattice planes are investigated which are parallely aligned to the sample surface which is the standard procedure for theta/2theta scans. This means that only peak shifts appear if one tilts the same sample. However, if simply all peaks are shifted - as described here - another lattice parameter is the reason and thus very likely another chemistry. Shift by strain in a powder sample is from my point impossible. It can only generate a peak-broadening (as sum of a distribution of a phase with coontinuously changing lattice parameters).
It can be due to stress. It may be due to processing or presence of other elements, compounds in the sample.
The shift in a crystalline material could occur mainly due to strain/stress present in the lattice. This may be due to presence of some defects, impurities or even due to diffusion of atmospheric oxygen.
The best explanation is given by Stephen. In perovskite oxides, oxygen stoichiometry is highly effected by preparation method, condition, temperature and environment. In some case change in oxygen content just leads to change in unit cell volume and consequently shift in diffraction peak and in some case it may lead to change in crystal structure and corresponding physical properties. The most important example is case of Lanthanum Manganite, where orthorhombic, rhombohedral, monoclinic and cubic phases are stabilized just by varying the method of synthesis. In all the above four phase the difference is oxygen content.
- Badges
- Science method
- Similar topics
- Methodology
- Crystallography
- X-ray Diffraction