Amorphous materials do not possess long periodicity (no long range order) compare to the crystalline materials (poses very long range order) and atoms are randomly distributed in 3D space. Amorphous materials have order only few atomic or molecular dimensions (very short range).

While diffracting amorphous materials with X-rays will be scattered in many directions leading to a large bump distributed in a wide angle or range (2 Theta) instead of high intensity narrower peaks.

Dear Orhan,

Diffraction = diffusion+interference (constructive or destructive)

If you have a cristallin atomic network, you can find the X-Ray patterns on the diffractogram with angles which are in relation with the cristallin structure (calculated from Bragg' law). However if you have an amorphous structure, you will not have the constructive interference. This is why you have a big potatos (amorphous halo).     

Dear Orhan,

All the amorph structures show this pattern.

Dear Lago,

But the question is WHY they show that pattern?

Thank you.

Amorphous materials do not possess long periodicity (no long range order) compare to the crystalline materials (poses very long range order) and atoms are randomly distributed in 3D space. Amorphous materials have order only few atomic or molecular dimensions (very short range).

While diffracting amorphous materials with X-rays will be scattered in many directions leading to a large bump distributed in a wide angle or range (2 Theta) instead of high intensity narrower peaks.

Sevgili Orhan,

I assume that the halo you mention is the one close to the center of the plate. If this is the case halo is due to "white radiation" This is the unfiltered radiation that finds its way to he specimen and the hao is produced. Just ignore it.

Hi Orhan

As mentioned previously all amorphous materials will show an 'amorphous' halo but in my opinion what has not been clarified is that, although amorphous materials have no long-range order and are macroscopically isotropic, the X-ray amorphous pattern is a fingerprint of the short-range order. 

If you consider silicate and aluminate glasses then the peak position of the amorphous halo, approx. 26 and 31 degrees 2 theta respectively, is related to that of the crystalline material but due to the distribution of the bond lengths and bond angles this causes the broadening of the peaks to give the characteristic amorphous halo.  If you then substitute a silicate glass with some alumina the peak position of the amorphous halo will tend to shift to higher degrees 2 theta values for the equivalent network connectivity. 

This 'hint' of the correlation of the XRD amorphous profile to the crystalline phase is also apparent when considering the deposition of apatite on bioactive materials where initially the amorphous halo is centred around 31 degrees 2 theta prior to the appearance of the 100 % peak of HA at 31.9 degrees 2 theta

http://spiral.imperial.ac.uk/bitstream/10044/1/10088/4/Law,%20R%20-%20The%20effect%20of%20phosphate%20content.pdf 

For organic materials see the below ref for an excellent discussion:

http://www.icdd.com/ppxrd/11/presentations/tuesday/simon_bates-amorphous_materials-a_structural_perspective.pdf     

Hope this helps

Clare

C. M. Crowley's answer is best, so far.

To put is briefly:  The amorphous halo is due to diffraction.  But amrphous materials do not have long range order required to give sharp peaks.  Short range order of amorphous solids and liquids give very broad diffraction peaks because there is a range of interatomic distances.

In my opinion the answer is simple. Narrow peaks are specified for pure elements which are patterns. The elements of these are planes for which there is a phenomenon Bragg. This is due, of course, with the crystalline lattice. The crystal structure of amorphous materials does not occur so the reflection of X-rays are scattered. Therefore, we observe a broad halo-called amorphous and not narrow peaks associated with the crystalline materials. Simply, there is no pattern for the amorphous material. I think the simple answer is sufficient.

I wonder if the position of this "Halo" is a function of wavelength. If not, there might be another reason for it.

Dear Orhan,

Previously I mentioned the halo close to the center of WAXS pattern which is due to white radiation. However there is sometimes a halo in general if the crystallinity in general and orientation (in the case of polymeric materials) lacks extensively a halo will be visible. This halo is a result of the average molecular separation in the ampotphous phase and is very natural. Due to amorphous nature the distance between the atoms will not bedefined as in the case of ordered structures, thus eliminating concentration of reflections at defined positions hence the halo.

Note that Scherrer formula for mean crystallite size determination is related with width of reflection peak. The well ordered crystallites above 100 nm give very sharp peaks, while very fine crystallites of few nanometers give very broad peaks. Therefore nanocrystalline materials looks like amorphous. I suppose that true amorphous materials like glasses can be considered as a very small well ordered regions giving contribution to broad peaks. Moreover the distance between planes varies from region to region what additionally broadens peak (overlapping of shifted components). Hope that my explanation is clear enough to be useful to somebody.

Thanks for all your answers. These are all so helpful to understand me more.

Amorphous materials have diffraction patterns too! Just not Bragg diffraction from lattice planes. Here are some useful references:

Fischer, H. E., A. C. Barnes, and P. S. Salmon. "Neutron and x-ray diffraction studies of liquids and glasses." Reports on Progress in Physics 69.1 (2005): 233.

Egami, T., and S. J. L. Billinge. Underneath the Bragg peaks: structural analysis of complex materials. Pergamon, 2003.

Warren, B. E. X-ray Diffraction. Dover, 1990.

Single atoms – scatters in ALL directions

Bring atoms TOGETHER – arranged RANDOMLY IDEALLY (non-interacting INERT gas) – neither constructive nor destructive interference – featureless diffraction pattern

Bring atoms together – arranged randomly NON-IDEALLY (liquids/amorphous solids) – some interatomic distances are preferred – these distances are statistically distributed, one with high probability with decreasing probabilities at both lesser and higher distances (bell shape) – halo/diffuse diffraction pattern

Bring atoms together – arranged orderly IDEALLY (perfect crystals) – few interatomic distances (satisfying Bragg’s law) are preferred – constructive interference in these few directions – peaks in diffraction patterns

Ref: B.D. Cullity, Chap 3 ‘Diffraction I: Direction of Diffracted Beams’ in Elements of X-ray Diffraction

It must be noted that this is only a broad superficial picture of general diffraction limited by the technique used in this case, XRD or WIDE angle X-ray scattering (WAXS). Medium size clusters existing in the otherwise amorphous structure may be revealed by Small Angle X-ray/Neutron Scattering (SAXS) and larger clusters by Ultra-small Angle X-ray/Neutron Scattering (USAXS).