A review of the crystal structures of the elements shows some prevailing tendencies of atomic arrangement. These are discussed as space, symmetry, and connection principles. Counteracting temperature and bond factors can be recognized.
The same principles and factors are responsible for the formation of alloy structures, taking into account additional factors due to the component's similarities and dissimilarities in size and electronegativity.
Similarity favors solid solution and dissimilarity favors compound formation. A q compound is here defined as a phase in a q-component system not connected with any other phase of the system by continuous solid solution. Similarly a q structure is defined as a structure type which needs only q components to be formed (considering present-day knowledge). For example, the binary compound Mg17Al12 has the elementary (1 ™) structure of α-manganese. As a rule, q compounds tend to form p structures with q > p.
A discussion of q structures with q = 1, 2, and 3 is given in some detail on the basis of known representatives to show: (1) the competition of geometrical principles and physicochemical factors in determining atomic arrangements of alloys; and (2) the value of rules for making guesses on the probable occurrence of compounds and their chemical composition in polycomponent systems yet unknown.