Stoichiometric phase compound

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. 

In the presence of excess fatty acid, different soap crystalline phase compounds can form, commonly referred to as acid—soaps. Acid—soap crystals are composed of stoichiometric amounts of soap and fatty acid and associate in similar bilayer stmctures as pure soap crystals. There are a number of different documented acid—soap crystals. The existence of crystals of the composition 2 acid—1 soap, 1 acid—1 soap, and 1 acid—2 soap has been reported (13). The presence of the acid—soaps can also have a dramatic impact on the physical and performance properties of the finished soap. The presence of acid—soaps increases the plasticity of the soap during processing and decreases product firmness, potentially to the point of stickiness during processing. Furthermore, the presence of the acid—soap changes the character of the lather, decreasing the bubble size and subsequently increasing lather stabiUty and... 

While Cordfuncke [997] believes that there are only four stable compounds in the U03—NH3—H20 system, the results of Stuart et al. [998, 999] indicate the existence of a continuous non-stoichiometric phase containing the NH4 ion and possessing zeolitic properties U02(0H)2 x (ONH4 )x yH20. 

Nonstoichiometric compounds such as FeO, Ni, xO, LaCo03+8 LaCo03+x/2 etc. are indexed under the notionally stoichiometric phase, i.e. FeO, NiO, LaCo03. 

The configurational entropy term, given by the degeneracy, gc, is included in AfG but not in AfGc. Let us assume the existence of two compounds with different formal oxidation states for the B atom, ABO3 and ABO2.5. The two compounds have the same (perovskite-type) structure and the non-stoichiometric phase... 

Figure 2.18. Examples of binary phase diagrams in each of which one stoichiometric phase is formed. In the Mg-Ge system we have the congruently melting Mg2Ge (33.3 at.% Ge) in Au-Sb, AuSb2 is formed through a peritectic reaction. In the Pt-Ag system one compound at 47 at.%...

Thermodynamically, the composition of any such phase is variable. In a number of cases, however, the possible variation in composition is very small. Invariant composition phases or stoichiometric phases, or compounds proper, also called point compounds in binary alloys, are represented by a point on the composition axis. 

In Fig. 2.19, on the contrary, we observe that intermediate solid phases with a variable composition are formed (non-stoichiometric phases). In the diagrams shown here we see therefore examples both of terminal and intermediate phases. (For instance, the Hf-Ru diagram shows the terminal solid solutions of Ru in a and (3Hf and of Hf in Ru and the intermediate compound containing about 50 at.% Ru). These phases are characterized by homogeneity ranges (solid solubility ranges), which, in the case of the terminal phases, include the pure components and which, generally, have a variable temperature-dependent extension. 

In the case of a ternary system, the formation of several, binary and ternary, stoichiometric phases, and different types of variable composition phases can be observed. One may differentiate between these phases by using terms such as point compounds (or point phases), that is, phases represented in the composition field by points, line phases , field phases , etc. 

The structure of H7I3O14 does not show any new type of catenation, because this compound exists in the solid state as a stoichiometric phase containing orthoperiodic and metaperiodic acids according to the formula (H0)sI0-2HI04. 

Whether a particular phase is a chemical compound or a solid solution can hardly be subject to any doubt in obvious cases such as in the Ni-Bi binary system with the intermetallics NiBi (homogeneity range HR < 0.3 at.%) and NiBi3 (stoichiometric phase) or in the Ti-Al binary system with the intermetallics Ti3Al (HR 12 at.% at 600°C), TiAl (HR 7 at.%), TiAl2 (HR < 1 at.%) and TiAl3 (stoichiometric phase).142 145 193... 

Nonideal solid-liquid TX diagram at 1 atm for Cu and Al (only about the left half of the diagram is shown). The two-phase regions are indicated. There is a very limited solubility of Cu in Al this is phase a. There is similarly a limited solubility of Cu in the stoichiometric phase or intermetallic compound CuAI2 (called the 6 phase). The liquid solution of Al in Cu freezes at the lowest possible temperature ( 540°C) for 32 mass % Cu this is the eutectic point (which is technologically useful in solders). 

Defect Complexes. The argument this far has reached the following stage. The state of minimum energy of any system at very low temperatures corresponds, without question, to the attainment of order. Third law considerations imply that a nonstoichiometric phase, with its inherent randomness, should be metastable at 0° K. with respect to a mixture of ideal, stoichiometric compounds. The relative status of complex intermediate phases and of stoichiometrically simple compounds,... 

By the direct fluorination with elemental fluorine, it is still difficult to prepare pure compounds C6oF.v and C70F of specific fluorine content because the reaction between fluorine gas and solid state C6o and C70 proceeds under heterogeneous conditions and, therefore, lacks control. Attempts to prepare stoichiometrically pure compounds have become successful using solid phase reactions between fullerenes and metal fluorides. These are described in the next section. It has been unsuccessful so far, but efforts are being made to produce fluorinated fullerenes C6oF v of lower fluorine content (x < 18) [24],... 

Consider the Al-S system shown in Figure 11.8. This phase diagram displays several features that are typical of many binary metal chalcogenides. There is only one stoichiometric line compound (bipartite phase) at room temperature, AI2S3. However, at one atm. 

SrjCrNbOj 53 is a semiconductor with a room-temperature resistivity, of ca. 250 Q-cm p increases rapidly with decreasing temperature and at low temperature p is larger than the limit of our instrument. Nevertheless, pRj is significantly lower than that in the stoichiometric parent compound, SrjCrNbO this behavior is attributed to the additional electrons of Nb " in the reduced phase. 

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