I am trying to synthesize MnO2/Graphene composite by hydrothermal method. Thereafter, when I recorded XRD, I observed there is slight shift in XRD peak towards lower theta value. As per my knowledge, this may be due to the elongation in crystal structure but I am not able to think what is the driving force leading elongation in MnO2 crystal structure. Please help me out.
If there is a systematic shift of all 2-theta values then it points to sample displacement error or zero error. Add a small quantity of a standard material (with few peaks away from peaks in your material) to your sample and then check the peak positions again. Software should be able to correct the sample displacement error by pulling the pattern back to the position defined by the standard. If the peaks are systematically too low in 2-theta your sample is below the focus of the diffractometer. For example, a displacement of about 0.01 degrees 2-theta at around 20 2-theta can be the result of the sample being 0.1mm too low.
peak position → dimension of the simple cell
peak intensity → content of the simple cell
peak broadening → strain/crystallite size
scaling factor→ quantitative phase amount
this is the summary of XRD
If you doubt about the possibility that your material might undergo some variations in the unit cell, then the shift could be due to sample displacement, if you're working in Bragg-Brentano geometry.
If there is a systematic shift of all 2-theta values then it points to sample displacement error or zero error. Add a small quantity of a standard material (with few peaks away from peaks in your material) to your sample and then check the peak positions again. Software should be able to correct the sample displacement error by pulling the pattern back to the position defined by the standard. If the peaks are systematically too low in 2-theta your sample is below the focus of the diffractometer. For example, a displacement of about 0.01 degrees 2-theta at around 20 2-theta can be the result of the sample being 0.1mm too low.
I quote Ian's answer, and I'd like to add a couple of things:
Is the structure cubic? If yes, stretching of the cell might result in a shift of all the reflections towards lower 2theta angles, although the extent of the shift would not be constant throughout the pattern.
If the extent of your shift is common to all of the reflections, then my suggestion is to try to record the PXRD pattern, then prepare the sample from scratch and record it again. If you notice differences in the peak positions between the two patterns, then it is sample displacement. In this case you just need to carefully prepare your samples in order to avoid the undesired effect.
Zero error is something depending only upon the instrumental apparatus, so, if you recorded the patterns for the pristine material and the composite with the same instrument, I'd tend to exclude it.
Some reasons exist for that.
1. Does your compound free of impurities? If not, solid solutions of some impurities in crystal lattice can be testified from low angle shift.
2. If low angle shifts are different for different peaks, you may expect distortion of your crystal lattice.
Before thinking of strange effects I agree with the suggestion of adding in the sample a reference compound. If the same shift is noted for the reference compound, it means that an instrumental error is present, otherwise the shift is due to some chracteristics of your sample.
I do agree with Ian, Marco and Giovanni. First try simple explanations and when they fail, think of something arcane. If the teta shift is the same for all reflections it may be due to zero error. If it increases it is likely due to focusing position (sample lower than expected).
Dear Ziyauddin . It was my unwanted mistake. Yes d increased with decreasing 2 Theta value. They are changed in a reverse order. However, it can be due to doping, increasing crystal size and ...
Excuse me for the mistake .
As it is well known, the XRD is represents the exact unit cell dimensions and then the unit cell volume.
The shift pick point positions regard the expansion of the lattice. That means the crystals under investigation (composite state) are in the nanoscale size which obtains an expansion in their lattice volume or lattice parameters due the crystal minimization to a nanoscale range size. The process depends on the particle size of MnO2. For more information you may see HRTEM solid surface images for imbedded grown crystals and see reference “Models for Mean bonding length, Melting point and Lattice thermal expansion for Nanoparticle Materials, Materials Research Bulletin 47, 3518(2012”
As have many others suggested, it could be either sample specific or instrument specific. If there is a true expansion of the unit cell as you are thinking, then the shift for each peak would not be the same; i.e. peaks at higher angles would have a larger shift compared to those at lower theta values.
If the shift for each peak is the same, then it is more to do with the sample displacement, which can change between each sample you prepare and put in the instrument.
Running the sample with a standard material of known lattice parameter is a good idea.
I suggest to add Si powders to your sample and get the XRD pattern in slow scan rate. If only the peaks of MoO2 shift to low angles, it indicates increase of lattice parameters. Another reason is the presence of tensile strength, but if so, the peaks will also become widder.
Calibration of instrument may help you or little changes in the crystal structures (d, a, b, c values) can lead to such conditions. You can explore basics on this link
http://www.instanano.com/characterization/d-value-calculation/xrd-d-value-calculation/
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