I prepared a maghemite sample. It is perfectly reacted to a magnet. However, when I tried to analyze with XRD, I did not get any meaningful peak. What is the reason for that?
There are a few reasons why you might not be seeing any meaningful peaks in your XRD analysis of a magnetically responsive maghemite sample.
- Very small crystallite size: Maghemite nanoparticles with an extremely small crystallite size (< 5 nm) might not diffract X-rays strongly enough to produce clear peaks in a standard XRD experiment. This is because XRD relies on constructive interference of X-rays diffracted by crystalline planes within the material. Smaller crystallites diffract X-rays with lower intensity, making peaks difficult to detect.
- Amorphous phase presence: If your maghemite synthesis wasn't complete, you might have an amorphous phase along with the crystalline maghemite. The amorphous phase wouldn't generate any sharp peaks in the XRD pattern.
- XRD instrument limitations: Some factors related to the XRD instrument itself could affect peak visibility:
- Low instrument sensitivity: If the instrument you're using has lower sensitivity, it might not be able to detect the weak diffracted signal from small crystallites. Improper sample preparation: Make sure your sample is well-ground and evenly distributed on the sample holder for optimal X-ray penetration.
Although the answer by Mehmet Ali Recai Onal is true as far as it goes, it is probably not very useful. The answer by Sheffin khan a S is much more useful as it has more specifics.
Another factor is: most diffractometers use Cu-anodes which give Cu characteristic x-rays. Using this with an iron-rich sample is a very bad idea for two reasons:
1. background: the copper characteristic x-rays are very efficient at inducing x-ray fluorescence in iron. Unless you have a diffracted beam monochromator (or detector with on the order of 200 eV energy resolution), this will cause very high background. The high background makes all of the factors mentioned by the others much more likely to hide your diffraction peaks. High background means peaks must be larger to be detectable.
2. microabsorption: the absorption coefficient (μ) of Cu radiation in Fe and Fe-bearing compounds is very high (because it is so effective at inducing fluorescence), therefore microabsorption will be severe. This means that the phase with high μ in the sample will have systematically reduced peak intensities, compared to other phases. I have seen such phases with high μ completely disappear from diffraction patterns, even when present at 30%-50% fractions in the sample.
The solution to both of the above issues is the same: use an anode with a characteristic energy that is not highly absorbed by Fe, such as Co, Mn, Cr, or Mo. Any decent XRD lab will have at least one of those anodes available in addition to the "normal" Cu anode.
Note, that using a monochromator to reduce the background (effect 1, above), will not help effect 2 at all.
Reducing particle size will help effect 2, but you may also amorphize your sample with treatments designed to reduce particle size, thus hiding your phase of interest even more completely.
It can be related to the small particle size, magnitude of the sample and synthesis method. The crystallization of the product. in chemical methods such as co precipitation, the pH value and synthesis parameters are effective on the crystallization of the precipitate. If it is react with magnet then check the XRD radiation source. It is better to use cobalt radiation instead of Cu type. You can see this reference: : https://doi.org/10.1080/01490451.2017.1401183
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