The Aim of the Program

The basic idea of the program is the specific use of crystallographic know-how for an intuitive generation of structure models. Very often the success of the so-called Rietveld programs — e.g. DBW3.2S (Wiles et al.), LHPM8 (Hill), EDINP/ALLHKL (Pawley), GSAS (Larson et al.), PREP/PROF (Rietveld) or RIETAN (Izumi) — depends on the initial structure model. Exactly this characterizes the aim of the program. The user shall be able to manipulate in an easy way known crystal structures (translation and rotation of atoms or molecules; change, delete and insert atoms or molecules, symmetry reduction etc.) or to create a new in a relative short time only by the use of crystallographic and crystal chemical knowledge. On this way PowderCell tries to support the structure determination.
The criterion for the quality of the created or modified model is the calculated powder pattern. That means that one of the most important requirements is an excellent X-ray or neutron measurement on a powder of the interesting substance. On the one hand the deviation of the experimental and theoretical diffractogram has their reason in an inacceptable adaption of the model on the real strcuture. On the other hand it is very possible that the measurements includes texture components. However, the comparison with a theoretical diffractogram requires its exact calculation. Therefore the simulation must allow the variation of essential experimental parameters, e.g. used radiation, consideration of anomaleous dispersion, diffraction geometry, fixed or variable slits, different intensity corrections, consideration of doublet, background etc.

However, not only in research it is useful. Also for the solution of problems connecting with phase identification, phase mixtures, unknown preferred orientations, the simulation of a powder pattern of a phase described exclusively by single crystal data, one can apply the different features of PowderCell. On the other hand it is very possible to employ the program within the education on universities or colleges. It is very simple e.g. to demonstrate the different diffraction conditions onto the resulting powder pattern or the symmetry relations between groups and subgroups. The necessary data (cell transformation) for the subgroup generation have its origin in the derivation of U. Müller (Gh Kassel, Germany).