Formation of intermediate phases

The binary systems we have discussed so far have mainly included phases that are solid or liquid solutions of the two components or end members constituting the binary system. Intermediate phases, which generally have a chemical composition corresponding to stoichiometric combinations of the end members of the system, are evidently formed in a large number of real systems. Intermediate phases are in most cases formed due to an enthalpic stabilization with respect to the end members. Here the chemical and physical properties of the components are different, and the new intermediate phases are formed due to the more optimal conditions for bonding found for some specific ratios of the components. The stability of a ternary compound like BaCC 3 from the binary ones (BaO and CC 2(g)) may for example be interpreted in terms of factors related to electron transfer between the two binary oxides see Chapter 7. Entropy-stabilized intermediate phases are also frequently reported, although they are far less common than enthalpy-stabilized phases. Entropy-stabilized phases are only stable above a certain temperature, 

3 The system Ca-Zr-O is principally a ternary system. However, as long as the oxidation state of Zr and Ca are the same in all phases, the system can be redefined as a two-component system consisting of CaO and Zr02- 

4 It should be mentioned that oxygen vacancies are often formed in the perovskite-type structure ABO3 in cases where the B atom is a transition metal that readily exists in more than one oxidation state. 

While until now we have considered relatively simple phase diagrams and the fundamentals of the connection between phase diagrams and thermodynamics, we are here going to consider a somewhat more complex example, but only briefly. 

Here Raoultian standard states are used for both the pure metal and the impurity. The slope dxB/dr of the phase boundaries can now be derived by differentiation with respect to temperature. Let/(xB) denote the left-hand side of eq. (4.35) or (4.36) then (see Lupis, Further reading) 

---

Data for these systems are limited. There is no evidence for the formation of intermediate phases in either the Be-Zn or the Be-Cd system in the Be-Hg system, however, the existence of BeHgj has been postulated on the basis of the properties of the amalgam formed by electrolyzing a NaCI-BeClj molten salt mixture into the Hg cathode " . 

Precursor Structure Effects. The precursor structure can impact a broad range of properties, including crystallization temperature, the formation of intermediate phases during thermal treatment and film density, among other properties. Table 2.4 reports some of the key precursor properties that may affect densification and crystallization behavior, as well as the final film microstructure. 

Trends in the formation capability of binary compounds. A few general comments about the formation of intermediate phases in binary alloy systems can be made by using maps similar to those previously employed in Fig. 2.8 in order to give a summary of the mutual solubility. 

Figure 2.22. Compound formation capability in binary systems. The different element combinations are mapped on Mendeleev number coordinates and those systems are indicated in which the formation of intermediate phases has been observed (either from the liquid or in the solid state). Blank boxes indicate systems for which no certain data are available. Notice that the compound-forming alloys are crowded in a region corresponding to a large difference in the Mendeleev numbers of the elements involved (for instance, basic metals with semi-metals).

Am The formation of intermediate phases has been suggested but not confirmed. 

Thermal desorption studies revealed for Li-, Na, and K-borohydrides the formation of alkali hydride as decomposition products above 430 °C. While Mg-, Sc, and Zr-borohydrides decompose through the formation of intermediate phases and/or borides, Zn(BH4)2 decomposes directly to elemental Zn [5]. 

Apart from a few general rules, the alloying behaviour of metals is rather empirical. The classical rules of Hume-Rothery [220] explain this behaviour reasonably well. Such factors as size, electronegativity, valency, electron concentration, free energy, formation of intermediate phases and isomorphism are found to influence the alloying tendency of metals. However, size and electronegativity are the two most important factors, and they profoundly influence the solubility of the solute atoms and greatly affect the crystal structures of the alloys. 

Because of the importance of microstructure on dielectric and ferroelectric properties, the transformation pathway associated with conversion of the amorphous film into the crystalline state has been studied extensively. The basic mechanism involved is one of nucleation and growth, although the formation of intermediate phases that can impact the thermodynamic driving forces associated with the transformation frequently occurs. " Another key aspect of CSD films is that crystallization occurs well below the melting point of the materials. Therefore, compared to standard mixed-oxide processing of bulk materials, the thermodynamic driving forces associated with the transformation are much greater and the kinetics of mass transport are much less. 

For both series, the samples containing no cerium (x= 1) display different DTA profiles by comparison with the single phased samples (Fig. 1) and this behavior agrees well with the final presence of the praseodymium oxide phase Pr60u and the successive formation of intermediate phases [11]. 

Partial solid solubility with the formation of intermediate phases... 

ChaptCT VI on transport effects on interfadal stability. New material on solubilization rates and formation of intermediate phases during diffusion in surfactant systems sections on spontaneous emulsification and dynamic surface tension revised and expanded. 

The process of formation of intermediate phases at interdiffusion is accompanied by a competition between stable and metastable phases. Solid-state amorphizing reactions (SSAR) present a classical example, when at the initial stage a metastable amorphous layer is formed and grows rather than the stable intermetaUides [1-3]. It is clear that the search for possible ways to estabhsh control over phase competition is highly essential for technological processes. Another problem related to phase competition refers to the order of phases formed in thin films and multi-layers [4, 5],... 

The determination, from Equation 4.93, of the critical thickness of the CoSi layer beyond which the nucleation of CoSi2 between a CoSi layer and Si substrate may take place, shows that this thickness strongly depends on the composition of the Si-Ge solid solution, as observed experimentally (Figure 4.23b) in Ref [28]. Even so, it was noted that these critical thickness values are very sensitive to the standard Gibbs free energy of formation of intermediate phases for which experimental errors are of the order of some kilojoules per mole of atoms. The presented here results were published first in Ref. [32]. 

In the chapter, the pecuharities of phase formation in binary metal systems with competitive formation of intermediate phases resulting from diffusion interaction are analyzed. The previous kinetic analysis of diffusion phase interaction [46, 47] has shown that only after a suppressing phase reaches a certain critical thickness, the growth rate of the suppressed phase nucleus becomes positive. Thus, the kinetic... 

Reactivity of carbonates in superheated steam under 1 atm was investigated by using a newly developed apparatus. The water vapor clearly affected the decomposition of CaC03 and MnC03, and the solid state reaction between MnCQs and CaCQ3. It was concluded that the reaction in superheated steam was promoted by the Hedvall effect, which was caused by the formation of intermediate phase such as hydroxide. 

The carbides and nitrides of the early transition metals, vanadium, niobium, and molybdenum, are known to possess good catalytic properties. The compounds are synthesized by a temperature programmed reaction (TPR) method where a reactive gas is reacted with a precursor oxide as the temperature is uniformly increased. Results under similar reaction conditions are presented to compare the progress of the reaction, the formation of intermediate phases, and the development of surface areas. The increase in surface area is influenced by the phenomena of pseudomorphism and topotaxy. It is believed that pseudomorphism, found in all of the above syntheses, is associated with the development of internal pores, while topotaxy, found in some of the nitrides, maximizes this process to yield hi surface area products. 

---