Binary systems incongruent melting

In the binary Fe-S system pyrite melts incongruently to liquid sulfur and pyrrhotite ... 

Fig. 11.7. Isobaric phase diagram of a binary system where the following reaction takes place A + B — C resulting in (a) a congruent melting compound and (b) an incongruent melting compound.

Systems with formation of an incongruently melting binary compound... 

Similarly as in the previous case, incongruently melting binary compounds can originate in the ternary system as well. The vertical projection of the phase diagram of such a system is shown in Figure 3.32. 

The phase diagram of the ternary system with the formation of an incongruently melting compound has four planes. Planes pc(A), pc(B), pc(C), and pc(A4B) are the projections of the planes of primary crystallization of the compounds A, B, C, and A4B, respectively. The figurative point of the binary compound A4B lies in the boundary of the binary... 

Figure 4.22 Phase diagram of the quasi-binary system Ca0-P205-H20 at a water partial pressure of 65.5 kPa. (After Riboud (1973).) Note that incongruent melting of hydroxyapatite (HAp) occurs at 1570°C under formation of a -C3P (a -TCP) and C4P (TTCP).

The solute and solvent of a binary system may, and frequently do, combine to form one or more different compounds. In aqueous solutions these compounds are called hydrates for non-aqueous systems the term solvate is sometimes used. Two types of compound can be considered one can coexist in stable equilibrium with a liquid of the same composition, and the other cannot behave in this manner. In the former case the compound is said to have a congruent melting point in the latter, to have an incongruent melting point. 

Figure 3.21 Solvates (a) and polymorphs (b) characterized by their solubility curves in water. The phase diagrams of Mg(N03)2/H20 and NH4NO3/H2O are depicted in (a) and (b), respectively. Solvates are indicated by vertical lines in the binary system, thus characterizing intermediate compounds that can melt congruently or incongruently. In the case shown, different hydrates of magnesium nitrate containing 2, 6, and 9 mol water in the crystal...

Many binary systems consisting of halides of monovalent metals and halides of trivalent metals form intermediate compounds of MesMe Xe (X=F, Cl, Br, I) type. Some of them are congruently melting compounds and the others are incongruently melting compounds. Table 6.6 lists the melting types and relevant atomic parameters of 89 intermediate compounds of this valence type. 

The following phase diagrams are for ABC ternary system forming a binary compound AB which melts incongruently, as it is unstable at its melting point ... 

C Covellite (CuS) melts incongruently in the binary Cu-S system CuS-c S(liquid) CUjgS + V... 

It is difficult to prepare many of the compounds of the A° —systems by synthesis from the elemental components and alloys, because of the comparatively hi melting points of many binary materials (CdS, CdSe, ZnS, and others) or the incongruent ternary-phase melting. Therefore, to prepare new semiconducting compounds, it is sometimes advisable to use doubledecomposition reactions [16,17]. The thermal effect of these reactions, which is necessarily dependent on the direction of the exchange reaction, is calculated from the heats of formation or the energies of the crystal lattices. Various experimental results demonstrate the success of this approach [16]. An example is the preparation of lead metaselenoarsenite ... 

The sodium chlorides Na2RCls (R=Sm-Gd) melt incongruently (but crystals suitable for X-ray structure determination could be grown from off-stoichiometric mixtures of the binary educts (Thiel and Seifert 1988, Seifert and Sandrock 1990, Seifert et al. 1991). No compound formation could be detected in the NaCl/RCb systems with the R + ion larger than Sm " these systems are simply eutectic (Seifert et al. 1985,1986,1987,1988). With R " ions smaller than Gd ", chlorides with other compositions such as NasRCle and NaRCU become more stable and no Na2RCl5 formation is observed. No Na2RXs halides other than chlorides are known and the same is true for A=Cs. 

As the data in table 32.7 indicate, several of the R-X equilibrium diagrams are very complex. Thermal analysis data, cf. table 32.3, show that in general the intermediate phases melt incongruently. The Tm-Cl system (Caro and Corbett, 1969), is obviously the most complex of the known systems. In addition to the binary phases listed in table 32.7, several mixed-metal phases have also been described. These include the face centered cubic (Nd, Ce)Cl2.2o and the (Nd,Ce)Cl2, 37 compositions in which divalent cerium is observed (Druding and Corbett, 1961). The PrCl2.37 phase, which decomposes below 594°C, is stabilized at low temperatures by substitution of Nd (Druding et al., 1963). 

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