Congruently, melting compound

Addition of the appropriate amount of water to anhydrous H3PO4, or crystallization from a concentrated aqueous solution of syrupy phosphoric acid, yields the hemihydrale 2H3PO4.H2O as a congruently melting compound (mp 29.3 "). The crystal structure shows the presence of 2 similar H3P()4 molecules which, together with the H2O molecule, are linked into... 

In the Au-Bi system the compound Au2Bi is stable in a restricted range of temperature only it is formed by a peritectic reaction (371°C) and, at a lower temperature (116°C), it is decomposed according to the eutectoidal reaction Au2Bi — (Au) + (Bi). In the Zn-Te system, finally, we have the congruently melting compound ZnTe. In this system a miscibility gap in the liquid state may also be noticed. 

Figure 3.50. The Yb-Cd binary phase diagram. The congruently melting compound YbCd57 is the first stable binary quasicrystal identified.

From the figure congruently melting compound is formed at = 0.667 which means that there are two moles of B for every mole ofH so that the compound has the empirical formula 2. Horizontal tie lines indicate that in area 1, Solid A and liquid are in equilibrium, in areas 2 and 4 liquid and solid AB2, in area 3 solid A and solid AB2, in area 5 solid B and liquid and in area 6 solid B and solid AB2... 

Fig. 6.9 T,x diagram of a binary system with a congruently melting compound.

Fig. 17. Pictorial representation of the Cl, CI5 and C16 structure type which are assumed by congruent-melting compounds. The coordination number of A and B are n and n respectively.

AB3 congruent-melting compounds according to their structure-type. 

Fig. 20. Pictorial representation of the DOjg, Ll2, and DOi type structures which are assumed by congruent-melting AB3 compounds. The special feature of A surrounded by B s and B surrounded by A s associated with congruent- melting compounds is to be noted.

A series of AB5 congruent-melting compounds (e.g., SnMgs, CaZn5, CeC 05, LaCu, SrAus ) are found at this stoichiometry. These compounds assume exclusively the D2d -type structure. This structure has two distinct B atomic sites with coordination numbers of 3 and 4 as shown in Fig. 22. Thus, the average coordination number for B is n = [(2x3) + (3x4)] = 18/5. Thus, the maximum of the liquidus line occurs at//m =n/(n + n )= 5/6... 

My own criticism on the theories proposed for superplasticity can be summarized in one word, electrons , or more accurately the lack of electronic consideration. This is similar to the theoretical consideration brought forward in the study of LME (liquid metal embrittlement) described in the previous chapter - no electronic consideration. As shown in Table 2, all superplastic alloys of binary system are found either at eutectic or eutectoid compositions. This is illustrated in Fig. 14 in which a few binary phase diagram involving superplastic alloys are shown. However, the people who made efforts in the formulation of theories did not consider this well-known fact important enough to incorporate into their theory formulation [24], In fact, this observation is so consistent one should ask the question of the special attributes associated with eutectic or eutectoid composition. Or the fact that the intermetallic compounds with superelastic property are all of the peritectic type. It must be emphasized that to this date there is no report of finding superplasticity in congruently-melting compounds. 

The phase diagram of a binary system as shown below is effectively two simple eutectic diagrams linked together. Therefore there are two eutectic points in this kind of the system. A maximum point appears on the liquidus, which is the melting point of the congruently-melting compound. 

In the previous diagrams, points e and / for the congruently-melting compound are both ternary eutectic. On the other hand, for the incongruently-melting compound die point e is the ternary eutectic, whereas the point/is the ternary peritectic. 

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.

Figure 11.14. Metastable extensions of equilibrium-phase boundaries. Solvus line extensions usually form a liquid miscibility dome. Extensions of incongruently melting compounds form a congruent melting point and extensions of congruently melting compounds often form eutectics with non-neighboring phases.

In the systems LiCl-TiCU, NaCl-TiCb, and RbCl-TiCb, no compounds are formed, while in the systems KCl-TiCb and CsCl-TiCb, the congruently melting compounds K2TiCl6 and Cs2TiCl6 originate respectively. 

From the complex physico-chemical analysis of the system LiF-KF-K2NbF7, it follows that the behavior and properties of the melts are affected by the formation of the congruently melting compound KsNbFg. Regarding the presence of the anion [NbFg] in the molten state the system behavior is not far from the ideal. 

The congruently melting compound KsNbFg was found to be not quite stable. At melting, it undergoes a partial thermal dissociation according to the scheme... 

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