General remarks, unit cell description

The characterization, the full identification, of a phase requires a complete and detailed description of its structure. As examples, consider the data, obtained from X-ray diffraction experiments, reported in Table 3.2 for stoichiometric and variable composition phases. An explanation of the various symbols used will be given in the following paragraphs. For a general reference to symbols, definitions and 

The two special (a) and (b) Wyckoff positions have no free coordinate parameter. The two occupancy parameters are 100%. The Pearson symbol is cP2. 

No free parameters in the atomic positions of Mg and Ge. In this case the two occupancy parameters have been found to be 100%. The Pearson symbol is cF12 (CaF2 is considered the prototype of this structure). 

From this description, however, alternative (and/or complementary) presentations of the structure and different symbolic representations can be deduced. These are often differently defined for specific groups of compounds and may be useful to obtain a clearer view of the atomic assembly and/or to make an easier comparison between different compounds. In other words it must be underlined that there is no ideal way of describing all structure types. The most appropriate way of description depends on the structure itself but also on a number of points we are interested in emphasizing (comparison with other structural types, comparison with other compounds of the same element, etc.). These points will be discussed in a few subsequent sections after the presentation of the conventional description. 

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