It depends on what you mean by missing peaks. If you mean why are there not diffraction peaks for all indices that is because of symmetry of crystal structure. For example in the fcc structure the only peaks that appear are those whose indices are all odd or all even, so you will get (111), (200), (220) etc., but not (100), (110) (0 is considered even). If a structure orders from fcc to primitive cubic you can then have those missing peaks but their intensity depends on the degree of ordering.

Other reasons for missing or very low intensity peaks could be strong texture in the sample. Check out a book on XRD like the one authored by Cullity.

your question is not so clear ..... add some more detail.....

If you try to say ....the peak from some particular planes are missing for particular element.... then the reason will be .... In that particular element the grain orientation will be such that ... majority of x-ray diffracted from particular plane ...while rest will be from other plane with different grain orientations. It can be possible for highly texture case... induced due to deformation process.......

It depends on what you mean by missing peaks. If you mean why are there not diffraction peaks for all indices that is because of symmetry of crystal structure. For example in the fcc structure the only peaks that appear are those whose indices are all odd or all even, so you will get (111), (200), (220) etc., but not (100), (110) (0 is considered even). If a structure orders from fcc to primitive cubic you can then have those missing peaks but their intensity depends on the degree of ordering.

Other reasons for missing or very low intensity peaks could be strong texture in the sample. Check out a book on XRD like the one authored by Cullity.

As we know, all reflections must satisfy the Bragg Equations. Some peaks does not appear since they are in the envelope of the other. Some peaks are very weak since the running current-voltage is not suitable. The purity of the material has also to be check.

It depends on the symmetry of the crystal, e.g. if the lattice is not primitive or in the presence of glide planes or screw axes. Systematic absences are useful to narrow the possible space groups.

As often on RG, the question is not clear at all. And it is not even clear what is the final aim of this question. Specify AT LEAST the material (better if you know the space group), the diffraction geometry and the form in which you have your specimen (powder vs film). And better clarify what you mean by missing peaks. Otherwise I really doubt you will need/understand the given answers

Pardon my lack of clarity. I have two samples of ZnS ( say A and B). On the diffractogram of sample A, I see only 3 prominent peaks indexed to the (111), (220) and (311) planes. On the diffractogram of sample B, I identified 3 additional (weak) peaks from the (200), (400) and (311) in addition to the peaks from the (111), (220) and (311) planes. Why is it that I see only 3 peaks from sample A but 6 from sample B?

I assume you have powder samples.

Questions: Are the samples equal, i.e., do they come from the same batch? How were they prepared for the diffraction measurements? Did you use a capillary or a film?

I have powder samples. Precipitated from different S sources i.e not from the same batch and I used a capillary.

What you get is reasonable, then. Are the heights of the main peaks comparable? Assuming of course that you have normalized peak heights to data acquisition time.

Abdullahi, you are probably a young student from the answers you give. One of the things students should learn quickly is to be accurate, especially when placing questions in sites like this. Many people place questions, but a few realise that there are scientist reading them and that those are the same scientists that you might find at conferences, in panels, as referees.

I jsut asked you for 3 easy things that are improtant to understand what's going on (well, the pattern would be the fourth): material, geometry, form. After some hard effort you gave two and a half of them. And the most important one is still missing: material. Saying ZnS is not sufficient. ZnS has two maximum degree of order polymorphs and it is subjected to heavy faulting (so both lattices coexist within the same crystal). Which of them do you have? Are all peaks broadened in the same way? This can tell you if you have troubles with crystallography or not

PS Sabino already pointed it out, but it was implicit that you did things in exactly the same way or both specimens, right?

Dr. Matteo you are right. I am a young scientist and this is my first time of analyzing powder diffraction data. I am sure you understand that because I am young and new in this field, translating theoretical knowledge to the real world problems requires more effort from me. My intension for posting the question is learn not to waste people's precious time. So once again pardon my lack of accuracy and clarity.

The ZnS I have were all produced from the same operating conditions and Zn sources but different sulfur sources (TAA and Tu). All the samples belong to the sphalerite phase of ZnS and they match PDF file no. 65-1691. The peak broadening and heights of the samples are not comparable (see attachment). The same procedure was followed to analyze the samples.

This look likes you have not a brilliant count number and your compound is nanosized making the peak broadening important.

then try plotting the data in log scale and you'll magically see the peaks. Actually you can also see extra ones in the Tu pattern (for sure one around 40 and one around 50). The Tu powder is more nano than the Taa

The information given by Dr, Leoni and Dr, Maggi are very useful and probably what you need.

Crystallinity and size of your samples are bothering you. You can estimate the crystallite size based on the three major peaks you observed.

PS: Beside log scale, you can also try SQRT scale for intensity, you might see extra peaks.

Dealing with crystallography problems required some knowledge such as crystal orientation, polymorphism, x-ray operating parameters, geometry, sample texture etc. If we run the sample (same sample but from the other fraction) for the second time with exactly the same ways as the first one, then the most probable problem may arise from the texture of the sample. This would tell us that the sample is lack of homogeneity.

The peak width can be related to the grain size in polycrystalline powder, using the Debby-Scherrer formula with a suitable form factor. The interesting question here seems to be whether you could convolute all the narrow peaks in one profile with one and the same broadening function and get the wider peaks in the other profile. I would guess, probably yes.