EM - Diffraction Standard Evaporated Aluminium

Aluminium Diffraction Pattern

In order to use a standard in electron diffraction properly, several conditions are necessary:

1. After obtaining the pattern of an unknown, it is necessary to expose the known standard to the same
   electrical and magnetic conditions, specifically the same lens current or high voltage.
2. The specimen must be in the same position as the unknown had been. Small movements of the
   stage are permissible in order to obtain a clearer pattern of the standard.

After developing the diffraction plates, the standard is measured first. The indices are assigned per the attached ASTM "d" spacings. After assigning the spacings, calculate "K" using the formula K = Sd; where "K" is a constant that represents wave length of the beam, camera length and associated variable crystallographic data, "S" is the diameter of the ring in cm and "d" is the interplanar spacing in angstroms. The values for "K" for the first five lines should be within 1% of each other. Use the mean value of "K".

Next, the pattern of the unknown is measured, and using "K", determined from the standard, the "d" spacings of the unknown are calculated. It is then necessary to establish identity of the unknown from the ASTM published data.

Aluminium: ASTM "d" spacings

Average unit cell from last five lines..............................4.0494