The Reflection Conditions

To decrease the processing time the laws of extinction (absent reflections) will be considered in PowderCell. These are contained in the symmetry file pcwspgr.dat as reflection condition. That means that a powder pattern can be calculated for that structures exclusively, described by a corresponding setting in pcwspgr.dat. However, the group-subgroup relations will be given for standard settings only. This includes the settings with different origins, but not the different settings or cell choices of the monoclinic and orthorhombic space-group types.

Systematic extinctions — the aim of this help screen — occurs then, if centered lattices (integral extinctions), glide planes (zonal extinctions) or screw axis (serial extinctions) have been defined as generators. Lattice centerings constitute only the result of a mathematical model. Therefore actual physical extinctions don't occur. They result from the comfortable definition of the translation lattice. If you describe the same structure using a primitive lattice you don't will find additional or absent reflections. Only the indexing of the calculated reflections will be changed. However, translation parts of symmetry elements always create systematic extinctions.

Accidental extinctions will be observed whenever atomic positions are located in an apparently higher symmetry. Excellently this can be shown with the first step of the automatical group-subgroups transition in PowderCell. There in practice the symmetry will be decreased and the same structure will be described in this lower symmetry oncemore. In the calculated powder patter some additional reflections occur but with no intensity, i.e. they are extinguished coincidentally. If you shift only one of the atoms outside a virtual (non-existing) symmetry element you will observe an intensity variation. The intensity of the extinguished reflections will increase.

Generally PowderCell is optimized in consideration of reflection conditions given in pcwspgr.dat. Nevertheless it is conceivable that systematically extinguished reflections will be calculated or usually observed are absent. If you find such case don't hesitate to contact us.

In the following tables all generator types characterized by additional translation parts are included. Furthermore the reflection conditions are given, respectively.