Disordered phases, structural solution composition

The situation in the solid state is generally more complex. Several examples of binary systems were seen in which, in the solid state, a number of phases (intermediate and terminal) are formed. See for instance Figs 2.18-2.21. Both stoichiometric phases (compounds) and variable composition phases (solid solutions) may be considered and, as for their structures, both fully ordered or more or less completely disordered phases. This variety of types is characteristic for the solid alloys. After a few comments on liquid alloys, particular attention will therefore be dedicated in the following paragraphs to the description and classification of solid intermetallic phases. 

The Cu-Au system is the classic textbook example for discussing ordering reactions in solid solutions and the effects of atomic order on properties (see, e.g., Schulze, 1967 Honeycombe, 1968). At higher temperatures above 410 °C the Cu-Au alloys form the disordered Al structure with complete mutual solid-solubility of Cu and Au, whereas at lower temperatures ordering reactions occur which produce various intermetallic phases, depending on temperature and composition, with broad... 

Phases of the composition ARF4 are reported for A = Li, Na, Ag, K, Rb, and Cs. From a structural view-point, they can all be considered as metal difluorides with a disordered or partially/fully ordered cation sublattice. Disordered phases can be expected mainly if A and have similar radii and in fact, cubic fluorite-related (Na, R)F2 phases are observed at high temperatures as well as a few (K, R)F2 phases. In most cases, they are part of solid solutions (A, R)F2-j, which are structurally related to yttrofluorites as far as the anion-excess phases... 

In the recent past, wide-range nonstoichiometric phases were commonly considered to be random solid solutions. Later, more and more were found to consist of closely spaced series of ordered structures (Bursill and Hyde 1972). The disordered phases in the rare-earth oxides are among the few wide-composition, nonstoichiometric equilibrium phases remaining and there are still questions about the structural details. 

For the disordered (A2) phases, rjeq = 0, Eq. 17.19 is satisfied automatically, and equilibrium tie-lines are present if W < 0 and T < Tcrit = W/k, as illustrated in Fig. 17.5. In the nearest-neighbor model, ordered B2 solutions can appear at any uniform composition Xb Nonzero equilibrium structural order parameters appear only if W > 0 at temperatures T satisfying... 

Exactly similar arguments apply to the y phases. Here the extent of the solid solution has often led to the assignation of incorrect formulae, especially since the composition satisfying Hume-Rothery s rule is relatively complex so that a composition differing but little from it can often be expressed by a simpler formula. Disordered structures are again common, and even when an ordered arrangement obtains the distribution of the atoms in closely related structures is not necessarily... 

---