Remarks on crystallochemical aspects in thermodynamic modeling

The problem of the thermodynamic modeling has been widely discussed and approached along different lines. From a crystallochemical point of view two groups of questions are worthwhile to be mentioned: 1. a) The crystallographically sound thermodynamic description of a complex phase, having a structure corresponding to several sets of equipoints (or Wyckoff positions). 2. b) The subdivision of the atoms pertaining, in a certain space group, to a specific Wyckoff position into two or more groups of equipoints of lower multiplicity, when some ordering takes place, i.e. on passing from a structure to its derivative ordered variants (cF4-Cu → cP4-AuCu3, for inst.). It may be remarked that a coherent crystallographic approach is very useful in modeling thermodynamic properties of the involved phases. In [97Ans] various important structure types (σ, μ, χ, Laves) have been analysed and suggestions given for a convenient thermodynamic modeling, coherent with a correct crystallographic description. It is well known, on the other hand, that alternative, complementary descriptions of the different structure types are available. In selected systematic collections, critically revised descriptions of the structure types are given which include, for each prototype, not only Pearson symbol, space group, lattice parameters and occupied Wyckoff positions, but also a description of the coordination (coordination polyhedra and next neighbour hystograms). On the other hand the role of the coordination, described in terms of “atomic environment types” (AET) has been discussed and used for a systematic presentation of the different prototypes, their possible classification and grouping in inter-related families. These aspects are certainly relevant while discussing phase (and alloy system) properties and possibly while looking for general criteria for their thermodynamic modeling in the framework of the Compound Energy Formalism. Examples of their use in the thermodynamic modeling of a few selected phases are presented and discussed.