Basics of X-ray diffraction

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categorie: Engleza

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About 95% of all solid materials can be described as crystalline. When X-rays interact with a crystalline substance (Phase), one gets a diffraction pattern.In 1919 A.W.Hull gave a paper titled, "A New Method of Chemical Analysis". Here he pointed out that "... .every crystalline substance gives a pattern; the same substance always gives the same pattern; and in a mixture of substances each produce[...]
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About 95% of all solid materials can be described as crystalline. When X-rays interact with a crystalline substance (Phase), one gets a diffraction pattern.In 1919 A.W.Hull gave a paper titled, "A New Method of Chemical Analysis". Here he pointed out that "... .every crystalline substance gives a pattern; the same substance always gives the same pattern; and in a mixture of substances each produces its pattern independently of the others. "

The X-ray diffraction pattern of a pure substance is, therefore, like a fingerprint of the substance. The powder diffraction method is thus ideally suited for characterization and identification of polycrystalline phases.Today about 50,000 inorganic and 25,000 organic single component, crystalline phases, diffraction patterns have been collected and stored on magnetic or optical media as standards. The main use of powder diffraction is to identify components in a sample by a search/match procedure. Furthermore, the areas under the peak are related to the amount of each phase present in the sample.

In order to better convey an understanding of the fundamental principles and buzz words of X-ray diffraction instruments, let us quickly look at the theory behind these systems. (the theoretical considerations are rather primitive, hopefully they are not too insulting).

Solid matter can be described as : Amorphous : The atoms are arranged in a random way similar to the disorder we find in a liquid.
Glasses are amorphous materials.Crystalline : The atoms are arranged in a regular pattern, and there is as smallest volume element that by repetition in three dimensions describes the crystal. E.g. we can describe a brick wall by the shape and orientation of a single brick. This smallest volume element is called a unit cell. The dimensions of the unit cell is described by three axes :a, b, c and the angles between them alpha, beta, gamma.

About 95% of all solids can be described as crystalline. An electron in an alternating electromagnetic field will oscillate with the same frequency as the field. When an X-ray beam hits an atom, the electrons around the atom start to oscillate with the same frequency as the incoming beam. In almost all directions we will have destructive interference, that is, the combining waves are out of phase and there is
no resultant energy leaving the solid sample.

However the atoms in a crystal are arranged in a regular pattern, and in a very few directions we will have constructive interference. The waves will be in phase and there will be well defined X-ray beams leaving the sample at various directions. Hence, a diffracted beam may be described as a beam composed of a large number of scattered rays mutually reinforcing one another.
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