Previous The Asymmetric Unit Next
PowderCell uses the data of the asymmetric unit to generate all atomic positions. Using the coordinates given in the asymmetric unit and the space-group symmetry defined in pcwspgr.dat the positions of atoms will be created. The atomic numbers listed in the structure file (e.g. import of *.cel-files) or the defined ionizations determine the coefficients for the calculation of the atomic scattering factors. The SOF (substitution and occupation will be multiplied to the Site Occupation Factor) represents the weight of the scattering power of the corresponding atomic position. The values given in the asymmetric unit represent the basis of each structure processed in PowderCell. For one and the same structure the number of atoms that must be entered is dependent on the point-group symmetry.

For each atomic position the following data must be known to carry out calculations within PowderCell:

  1. atomic number or ionization state (ion in the table of the dialog box structure data)
  2. relative atomic coordinates or alternatively the Wyckoff position (i.e. often it is enough to input the letter)
  3. substitution and/or occupation
  4. isotropic temperature factor or equivalent temperature factor (optional, per default 0).
But these data can be used only if either the space-group number and the setting or the Hermann-Mauguin symbol is known to derive the space-group type.
If the symmetry is unknown but all atomic positions within the unit cell are available it is possible to calculate the powder pattern using the space-group type P 1. Unfortunately, a lot of extinctions can be occured corresponding to the unknown symmetry. The shown diagram is always correct.

It is possible to display beside the full structure the entered atomic positions of the asymmetric unit, too. To this end a special button can be used.

© Dr. Gert Nolze & Werner Kraus (1998)

Federal Institute for Materials Research and Testing
Unter den Eichen 87, D-12205 Berlin,
Germany