Chemical compound binary

Hydrides are compounds that contain hydrogen (qv) in a reduced or electron-rich state. Hydrides may be either simple binary compounds or complex ones. In the former, the negative hydrogen is bonded ionicaHy or covalendy to a metal, or is present as a soHd solution in the metal lattice. In the latter, which comprise a large group of chemical compounds, complex hydridic anions such as BH, A1H, and derivatives of these, exist. 

Solid solutions are very common among structurally related compounds. Just as metallic elements of similar structure and atomic properties form alloys, certain chemical compounds can be combined to produce derivative solid solutions, which may permit realization of properties not found in either of the precursors. The combinations of binary compounds with common anion or common cation element, such as the isovalent alloys of IV-VI, III-V, II-VI, or I-VII members, are of considerable scientific and technological interest as their solid-state properties (e.g., electric and optical such as type of conductivity, current carrier density, band gap) modulate regularly over a wide range through variations in composition. A general descriptive scheme for such alloys is as follows [41]. 

In a similar way, electrochemistry may provide an atomic level control over the deposit, using electric potential (rather than temperature) to restrict deposition of elements. A surface electrochemical reaction limited in this manner is merely underpotential deposition (UPD see Sect. 4.3 for a detailed discussion). In ECALE, thin films of chemical compounds are formed, an atomic layer at a time, by using UPD, in a cycle thus, the formation of a binary compound involves the oxidative UPD of one element and the reductive UPD of another. The potential for the former should be negative of that used for the latter in order for the deposit to remain stable while the other component elements are being deposited. Practically, this sequential deposition is implemented by using a dual bath system or a flow cell, so as to alternately expose an electrode surface to different electrolytes. When conditions are well defined, the electrolytic layers are prone to grow two dimensionally rather than three dimensionally. ECALE requires the definition of precise experimental conditions, such as potentials, reactants, concentration, pH, charge-time, which are strictly dependent on the particular compound one wants to form, and the substrate as well. The problems with this technique are that the electrode is required to be rinsed after each UPD deposition, which may result in loss of potential control, deposit reproducibility problems, and waste of time and solution. Automated deposition systems have been developed as an attempt to overcome these problems. 

Table V shows a classification of binary relations gene-gene relations, protein-protein relations, and other molecule-molecule relations. Different sets of binary relations can then be related to different types of graphs. For example, the generalized protein-protein interaction network is equivalent to a set of protein-protein binary relations. The KEGG metabolic pathway is a generalized protein-protein network, but it is also a network of chemical compounds that can be converted to a set of com-...

Timmermans, J. "Systems with Inorganic and Organic or Inorganic Compounds (Excepting Metallic Derivatives)" in "The Physico-Chemical Constants Binary Systems in Concentrated Solutions" Vol. 4, Interscience Publishers, Inc., New York, 1960. 

An example of a binary eutectic system AB is shown in Figure 15.3a where the eutectic is the mixture of components that has the lowest crystallisation temperature in the system. When a melt at X is cooled along XZ, crystals, theoretically of pure B, will start to be deposited at point Y. On further cooling, more crystals of pure component B will be deposited until, at the eutectic point E, the system solidifies completely. At Z, the crystals C are of pure B and the liquid L is a mixture of A and B where the mass proportion of solid phase (crystal) to liquid phase (residual melt) is given by ratio of the lengths LZ to CZ a relationship known as the lever arm rule. Mixtures represented by points above AE perform in a similar way, although here the crystals are of pure A. A liquid of the eutectic composition, cooled to the eutectic temperature, crystallises with unchanged composition and continues to deposit crystals until the whole system solidifies. Whilst a eutectic has a fixed composition, it is not a chemical compound, but is simply a physical mixture of the individual components, as may often be visible under a low-power microscope. 

Flood s equation physchem A relation used to determine the liquidus temperature in a binary fused salt system. fladz i.kwa zhan flores CHEM A form of a chemical compound made by the process of sublimation. flor ez 1... 

Appendix 2 was compiled and formatted by eomposition paralleling the presentation for the fibrous minerals. The list, in eontrast with that of the minerals, shows a predominanee of simple chemical compounds that combine two or three elements. One-third of the reeorded synthetie fibers are of elements or binary alloys. Another third are compounds such as sulfides, phosphides, and halides that do not eontain oxygen (Table 2.9). Only three synthetic fibrous silicate compounds etre listed, although undoubtedly other experimental silicate combinations have been made but not recorded by us. 

As the author points out, the sulfuric acid-water system is one of the few examples of a liquid binary system in which certain properties change discontinuously. In this case dsjdp, where s is the specific gravity and p the per cent concentration of H2S04, has discontinuities at certain values of p. The most pronounced correspond to the compositions H2S04—H20 and H2S04—2H20. Mendeleev (102) believes that these characteristic points are connected with the formation in the solutions of definite chemical compounds. The more stable the compound the sharper the discontinuity in the property versus composition. 

BINARY. Descriptive of a system containing two and only two components. Such a system may be a chemical compound composed of two elements, an element and a group (hydroxyl, methyl, etc.), or two groups, (e.g.. oxalic acid). It may also be a two-component solution or alloy. 

Let us begin an analysis of the process of formation of chemical compounds in heterogeneous systems with the simplest case of growth of a solid layer between elementary substances A and B which form, according to the equilibrium phase diagram of the A-B binary system, only one chemical compound ApBq, p and q being positive numbers (Fig. 1.1). The substances A and B are considered to be solid at reaction temperature 7, and mutually insoluble. 

F.M. d Heurle evaluated a specific thickness of the layers (an analogue of the critical radius of nuclei in a homogeneous system for more detail, see Ref. 31) for compounds of the Ni-Si binary system. For Ni2Si, its value was found to be 0.15 nm, i.e. the nucleus does not contain even one lattice unit. Although higher values were obtained for other nickel silicides, they never exceeded 1 nm. Therefore, the nucleation process can hardly play any significant role in the formation of most transition-metal silicides, except in some special cases. This conclusion is likely to be valid for any other chemical compound layer. It should be noted, however, that there is also a different viewpoint.38 132... 

At x < xg>, the reactivity of the A surface towards the B atoms is less than the flux of these atoms across the ApBq layer. Therefore, there are excessive B atoms which may be used in the formation of either other chemical compounds (enriched in component A in comparison with the ApBq compound) of a multiphase binary system or a solid solution of B in A. 

At x< x f2, there is an excess of diffusing A atoms since the reactivity of the B surface towards these atoms is less than their flux across the ApBq layer. The excessive A atoms can be used in the formation of the layers of other chemical compounds of a given binary system enriched in component B in comparison with ApBq, if present on the equilibrium phase diagram. 

It should be noted that the value of each of the chemical constants kom and k0A2 depends on the physical-chemical properties of two reacting phases. The value of kom depends on the nature of substance A and the compound ApBq, while the value of k0A2 depends on the nature of substance B and the compound ApBq. Both physical (diffusional) constants depend only on the nature of the chemical compound ApBq and are therefore characteristic of this compound layer wherever it grows. However, as will be demostrated in the next chapters, the stoichiometry of adjacent phases must also be taken into account when estimating the growth rate of the ApBq layer in various reaction couples of the A-B binary system. 

Growth kinetics of two chemical compound layers in a binary heterogeneous system have been theoretically treated, from a diffusional viewpoint, by V.I. Arkharov,1 46 K.P. Gurov el al.,22 B. Schroder and V. Leute,52 A.T. Fromhold and N. Sato,53 G.-X. Li and G.W. Powell,55 and other investigators. Diffusional considerations predict that (z) both layers must occur simultaneously and (ii) the thickness of each of them as well as their total thickness should increase parabolically with passing time. 

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... 

Also, formation of a solid solution is often considered to be a prerequisite for the occurrence of a chemical compound layer, with the latter being a result of supersaturation of the former. In fact, however, these are two concurrent, competing processes, if both solid solutions and chemical compounds are present on the phase diagram of a binary system. In any... 

Relations between different constants will be considered in greater detail, when comparing the growth rates of the same chemical compound layer in various reaction couples of a multiphase binary system (Chapter 4). 

It must be quite clear that in the great majority of binary systems with two chemical compounds on equilibrium phase diagrams both layers can hardly be expected to occur at the A-B inteface simultaneously. Rather, their formation must be sequential. This immediately follows even from a formal probability consideration. 

It is clear that in general the kinetic dependences considered in this chapter gradually transform into each other with passing time. In contrast to the diffusional theory, the physicochemical approach thus gives a more complicated, not simply parabolic, relationship between the thickness of two chemical compound layers and the time, in accordance with the available experimental data in binary systems. 

Evidently, in the course of layer formation the plane of inert markers cannot coincide with the initial interface between substances A and B. It would mean that compound layers could grow at the expense of one component. Chemically, this is impossible since any binary compound consists of two components. Position of the layers relative to the initial interface is mainly dependent upon the stoichiometry of chemical compounds, if both ends of a couple are equally free to move. Coincidence of initial and marker planes provides evidence for the lack of contact between reacting phases at that place. 

Many binary systems are multiphase, with the number of chemical compounds on the A-B equilibrium phase diagram reaching or even exceeding ten.120 123 142 145 146 215 225 Therefore, the primary question the experimentalist faced when starting to investigate a particular reaction couple is how many and precisely which compounds, of their variety shown on the phase diagram, can form separate layers at the interface between initial elementary substances under given conditions of temperature and pressure. 

To understand the reculiarities of multiple layer formation, it suffices to consider the A-B binary system with three chemical compounds ApBq, ArBs and AiBn on the equilibrium phase diagram (Fig. 3.1). The scheme of analysis of the process of their occurrence at the A-B interface is analogous to that of two compound layers (see Chapter 2). First of all, the equations of partial chemical reactions taking place at phase interfaces must be written. These are as follows. 

The equilibrium phase diagram is doubtless the main source from which the researcher obtains the required primary data, when starting to investigate the kinetics of formation of chemical compound layers in a particular binary system. It immeaditely indicates which compounds may form separate layers but by no means dictates that those must occur at A-B interface simultaneously. 

Thus, looking at the equilibrium phase diagram and knowing the physical-chemical properties of the elemets A and B and their compounds, it is possible to draw certain conclusions concerning the sequence of compound-layer formation in a multiphase binary system. It must be remembered, however, that any predictions based on the above-mentioned or other criteria hitherto proposed are only weak correlations, rather than the precise rules. As both the researcher and technologist are always interested in knowing the sequence of occurrence of chemical compounds in a particular reaction couple, they can hardly be satisfied even with a correlation valid in 99 out of 100 cases, because it remains unknown whether this couple falls in the range of those 99 or is the only exception. Further theoretical work in this direction is badly needed. 

Hence, none of the two above-mentioned criteria can be used as a reliable basis for predicting the sequence of occurrence of chemical compound layers in multiphase binary systems. The experimental data on the formation of nickel and cobalt silicides provides additional evidence for the validity of this conclusion. 

This does not mean, however, that the rules based on those assumptions must necessarily be incorrect. Though, for example, the original derivation of Evans equation is definitely incorrect, the final equation itself is quite correct (see Chapter 1). Further work is required to check the applicability of the proposed rules to other binary systems of different chemical nature. Also, much efforts are to be undertaken to find out other relationships between the thermodynamic properties of chemical compounds and the sequence of occurrence of their layers at the A-B interface. This sequence seems to be more dependent on the partial, rather than on the integral values of thermodynamic potentials. 

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