Hello Vladimir Pankratov

The principle of Powder XRD is “governed” by Braggs' Law:

n * lambda = 2 * d * sin(theta)

Each one of the peaks you get in a XRD pattern corresponds to a crystallographic plane of the unit cell, like (100), (111), (200), etc. Each one of these peaks has a d, called interplanar distance. This "d" is one of the variables of Braggs equation. What the equation means is that a peak in the pattern will only be detected when the equation is satisfied. So, for each d, there will be a correspondent theta, and consequently 2theta.

This is it, as long as you keep increasing theta, you will measure more peaks. But, there is a limit. Theoretically, you can only go from a range of 0 to 180 degrees. In practice, much less than it.

Let do a math, and rearrange this equation

d = n * lambda / (2 * sin (theta))

You can see that theta and d are inversely proportional. As theta increases, d decreases. So, at high angles, you will detect crystallographic planes with smaller interplanar distances, which is better for your analysis, because you will get more information about the structure.

What about the synchrotron? Synchrotron uses X-rays with shorter wavelengths. A common XRD equipment uses copper Kalpha radiation, with lambda = 1.54 Angstrom. Synchrotron can use X-rays with lambda shorter than 0.5 angstrom. This means that using synchrotron instead of copper radiation, you can detect more peaks, using the same range of analysis.

Feel free to ask for more explanation about this

Best regards

Ricardo Tadeu

Dear Vladimir Pankratov,

in addition to the answer of Ricardo Tadeu Maia,

I would like to add the following:

a) you know, that radiation from a synchrotron has a quite high intensity compared to the x-rays lines arising from an x-ray tube, and

b) the divergence of the primary beam is close to zero, i.e. the primary beam is almost a parallel one.

The high intensity of the synchrotron radiation*) allows time resolved diffraction measurements, especially when 1D or 2D detectors are used. Furthermore, there is no necessity of having a broad primary beam cross-section, so one is able to analyze i) small samples or ii) map diffraction pattern across an extended sample via x-y scanning of the sample in a reasonable timescale.

Due to the small divergence of the primary beam*) its contribution to the XRD peak widths is negligible compared to that of an x-ray tube set-up. In consequence one will have smaller XRD peak widths.

Another aspect is related to the monochromator; one has a free choice**) of the wavelength (or photon energy) compared to the few characteristic lines offered by x-ray tubes. In addition, monochromators will give single lines and not doublets as X-ray tubes do via the K-alpha lines...

*) monochromatization already included

**) for example to exclude fluorescence of the sample

Best regards

G.M.

I think the demerit of SR-XRD is over merit. Laboratory XRD in your institute is usually better than SR beamline.

1. The experimental optics are not always the same for the same SR beamline.

2. Travel to SR facility is tedious.

3. You can discuss with the technician for special arrangement of in-house XRD.

4. You can measure automatic measurement through night for an in-house XRD.

5. The structural analysis of even biological or pharmaceutical samples becomes easy without SR, if you use "crystal sponge".

Of course, there are a kind of samples which need the SR experiment.