X-rays are electromagnetic radiation used mainly in medical imaging (radiology) and crystallography (study of crystalline substances). They were discovered in 1895 by the German physicist Wilhelm Röntgen to whom he was awarded the Nobel Prize in Physics. X-rays easily penetrate matter, including flesh, which is why radiologists use it. Prolonged exposure to X-rays can lead to burns, cancer and other diseases.
Apparatus used in crystallography: the diffractometer
The diffractometer has a goniometer (on the right in the photograph that looks like the barrel of a drill) to manipulate the single crystal in the X-ray beam from all angles.
he X-rays come out through the vertical tube at the top
the crystal in the center of the photo is too small to be seen. It is attached to the end of a thin glass needle manipulated by the goniometric head and rotates the crystal in all directions along three successive axes (a vertical, a 45 ° and a horizontal) while maintaining it in the X-ray beam
a video camera (in black on the top left) makes it possible to control that the crystal is well centered
a well in the middle is held by a blade. It is used to stop direct X-rays that do not interact with the crystal
a cooling system (right tube with letters in red) can cool the crystal
Data exploitation of data
from the diffracted intensities and the inverse relation (reciprocal lattice - real lattice), the software determines the axes and centers of symmetry of the crystal and proposes the most likely crystalline system among the 7 existing ones
the user then chooses the most appropriate space group (Bravais network): the system chosen is the one with the highest symmetry in order to have the best resolution.
mesh parameters are then proposed
the reliability factor R makes it possible to calculate the degree of reliability of the proposed mesh with respect to the actual crystalline structure. When it reaches a sufficiently low value it means that the mesh pattern is acceptable
We proceed to the refinement of the mesh parameters and the integration of the diffracted intensities:
the diffracted intensities are characteristic of the nature and the position of the atoms and the electronic density at any point of the mesh is a function of the intensities diffracted. The program used allows the correlation between the mesh parameters, the reference file obtained previously and the expected molecular compound formula
we enter the molecular formula of the expected compound in the program that will affect the atoms to electronic densities, the intensity maxima corresponding to the largest atoms
the crystalline structure is then refined by trial and error.
