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Lithium Fluoride (LiF) Crystal

Lithium fluoride is an inorganic compound with the chemical formula LiF. It is a colorless solid, that transitions to white with decreasing crystal size. Although odorless, lithium fluoride has a bitter-saline taste. Its structure is analogous to that of sodium chloride, but it is much less soluble in water.
The crystal structure of the Lithium Fluoride is therefore face centered cubic in which the fluorine and lithium ions alternate with the vertices of the cube. Details on the structure can be found in the post about sodium chloride: Sodium Chloride (NaCl) Crystal

Crystal XRD Patterns

The first test performed was the complete 10° to 120° scan of the crystal, using the non-filtered tube emission, configured at 30 KV and 50 μA. The result is shown in the graph below, in which the Bragg reflections for the Kα and Kβ lines of the copper are present. The reflections were highlighted for the order n = 1 and n = 2.

For the crystallographic analysis of lithium fluoride we used the Nickel filter to have monochromatic emission in correspondence only to the  line of the copper at 0.1542 nm. We examined the LiF crystal in the two orientations allowed by our protractor, corresponding to the crystal orientations (200) and (020). The images below show the setup of the measurements and the Bragg peaks obtained, both at about 45°. Having obtained the same results, within the experimental error, is the demonstration that we are dealing with a crystal of cubic structure.

By rotating by 45° the crystal, together with the crystal holder, with respect to the movable arm, we sought the presence of reflections corresponding to the planes (nn0), the graph below shows the result obtained with the Bragg peak corresponding to the plane (220).

Powder XRD Pattern

Further experimental investigations can be done using XRD from powders. The image below shows the sample obtained by mixing fine lithium fluoride powder with liquid paraffin in order to obtain a thick paste. Scanning in the diffractometer allows to find the three main Bragg peaks, even if of limited intensity, shown in the graph below.

Diffraction from Powder

Again with a sample of fine LiF powder we carried out the Laue – Debye test in order to obtain an image of the diffraction cones on a radiographic film. The image obtained, shown below, is quite clear and allows to see the traces left by diffraction cones corresponding to the crystallographic planes already highlighted with the other techniques.

“Solving” the Crystal Structure

From the checks made, described above, it has been established that the sodium chloride crystal has a cubic structure. The variants of the cubic lattice are the followings :

  • Primitive cubic
  • Body-centered cubic (bcc)
  • Face-centered cubic (fcc)
  • Diamond (or Zincblende)

each of these is characterized by a specific pattern of reflections. We verify that the experimentally obtained Bragg peaks are compatible with the fcc structure. Meanwhile for a cubic crystal with lattice constant a, the distance d between adjacent lattice planes (hkl) is computed as follows:

By combining the Bragg condition for the reflections with the above written equation we get :

From which we deduce that, for the diffraction peaks, the ratio remains constant and this allows to verify the type of cubic structure and to determine the lattice constant a, if the wavelength λ of the X-rays is known. For the measured Bragg reflections we can compile the following table for which we hypothesize an fcc structure, using the following formulas:

  • λ = 0.1542 nm
  • d = λ / 2senθ
  • a = d x √(h2+k2+l2)
hkl d(nm) h2+k2+l2 √(h2+k2+l2) a(nm)
 111  38.33  0.2349  3  1.732  0.407
 200  45  0.2015  4  2  0.403
 220  67.17  0.1394  8  2.828  0.394

The values ​​of a obtained are congruent with each other, a sign of the correctness of the initial hypotheses. They are also close to the correct value which turns out to be 0.403 nm.

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