0
$\begingroup$

As far as I am concerned, when we perform an X-ray diffraction experiment using a Bragg-Brentano diffractometer using the "powder method" we have that the scattering pattern depends on only one angle because the oter one is "integrated out" (considering spherical coordinates ($r,\theta,\phi$) we have symmetry with respect to $\phi$ due to the random orientation of the various crystals reduced to powder).

enter image description here

Now, I have some data regarding an X-ray diffraction experiment performed on a thin film (NOT on a powder sample as shown above) using a Bragg-Brentano diffractometer and I need some help to interpret the data I have.

1) In this case (thin film) I think that the diffraction pattern does not provide a complete description of the physics because I cannot say to have simmetry with respect to $\phi$ angle as in the powder case. (I have a thin film, so I guess I cannot say that crystals are randomly oriented, so I should consider also the dependence with respect to the $\phi$ angle). How should I interpret in this case the diffraction pattern below? enter image description here

2) I asked the tecnician who performed the experiment what does represent the quantity on the Y axis in the plot I posted and he said that it is a count and it is in arbitrary units, it represents the number of signals collected in a certain position by the detector. Moreover he said that the only thing which matter in the plot is the position of the peaks, not their amplitude. He giustified this by saying that the superimposition principle does not hold in the diffraction pattern due to the "decimation of some d.o.f." performed by the detector. His explanation was not very satisfactory, is someone able to explain me:

a) what does the counts [arbitrary units] represents in the y axis?

b) why amplitudes are not important in this plot?

c) does it make any sense to use a Bragg-Brentano diffractometer to perform diffraction on a thin film intead of on a powder pattern?

d) reading in internet I encountered that in order to investigate surface properties we use "grazing incidence X-ray"...is it possible that he forgot to say us that he was doing the diffraction experiment using only grazing angle X-ray?

$\endgroup$
3
  • 1
    $\begingroup$ You are much much more likely to get answers if you ask one specific question instead of four. Asking four questions prevents anyone who might answer one, two, or three of them from posting an answer. Please ask your four questions in four separate posts. $\endgroup$
    – DanielSank
    Commented Jul 16, 2015 at 17:31
  • $\begingroup$ Well is not completely clear what your aim is in this case. I have seen similar X-ray diffraction set ups where the aim is to excite the electron shells of the atoms and measure the resulting X-rays emitted subsequently. Here is clear that the counts will be proportional to the abundances of each atom, hence the relative heights of the peaks hold the ratios of the relative concentrations. However is unimportant the absolute values of the counts in the peaks to reflect concentrations one needs to correct for detector efficiency, angle coverage, initial X-ray source intensity, etc. $\endgroup$
    – rmhleo
    Commented Jul 17, 2015 at 16:05
  • $\begingroup$ Tag edits because I think that you actual question are more related to the experimental realities of this work than the underlying theory. Feel free to roll them back if you think that I am wrong. $\endgroup$ Commented Jul 21, 2015 at 16:15

1 Answer 1

2
+25
$\begingroup$

Some brief answers, and references for further reading below.

1) You are right to be concerned about your sample having crystals with a preferred orientation. A single crystal would produce points instead of circles. The points would fall somewhere on the circles. But the image you provided shows that it is possible for the point to be off the film.

However, modern instruments do not use film. You would want to understand the geometry and operation of your instrument.

The fact that you do find several peaks is a good sign. Perhaps you have a polycrystalline film with many orientations. Perhaps the detector is big enough to sample a large part of the circles.

You can check what materials are matched by the peaks you have. Keep in mind that some peaks could be missing.

2a) X-ray intensity

2b) Amplitude does matter for some applications. But if you want to identify what material is present, just knowing what peaks are present is enough. You compare the peaks you found to a list of peaks expected for various materials. For this, knowing which peaks are big or little is just a detail.

2c) Yes

2d) It is not necessarily true that he forgot. Grazing angle is one possible geometry. There are others.

dof may mean degree of freedom?


MIT has useful new and old websites on x-ray diffraction.

Here is a presentation on Basics of X-Ray Powder Diffraction oriented toward technicians of a Bragg-Brentano diffractometer. Scattered throughout it are discussions of many of the issues you brought up.

Here and here are some resources for further reading.

$\endgroup$
1
  • $\begingroup$ thank you for your answer, and yes, dof means degree of freedom. $\endgroup$
    – Chaos
    Commented Jul 21, 2015 at 13:27

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.