Limiting types of binary compound

Corresponding to the two limiting types of element, metal and nomnetal, there are three limiting types of binary compound  

These are formed principally by the combination of a metal with a metal, and have the characteristics of a metal. 

These have the same characteristic properties as metallic elements. 

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In this chapter, I shall discuss the limiting types of binary compound. I shall do this on the assumption that matter is composed of atoms, but without any reference to the structure of the atom as we now understand it. This something that I shall bring in later. 

The limiting types of higher-order compounds are the same as the limiting types of binary compound, namely ... 

The three types of binary compound that we have been considering are what we have called limiting types . In other words they represent extremes, and most binary compounds fall somewhere in between these extremes. 

Between these limiting types are bonds of intermediate character, corresponding to the intermediate types of binary compound ... 

There are several structurally different types or polymers that are suitable precursors for ternary Si-C-N ceramics. By far the most investigated precursors are polysilazanes of the general type [Si(R )(R°)N(R°)] (R, R°, R° = H, alkyl, aryl, alkenyl, etc.). In contrast to the limited number of starting compounds, H SiCl(4 ) (x = 0-3) as the silicon source and NH3 or H2N-NH2 as the nitrogen source for synthesis of polysilazanes as precursors for binary Si-N ceramics, the chemistry of polycarbosilazanes, that is, carbon-containing or modified polysilazanes, is very multifaceted. The attachment of various organic groups to the silicon atoms allows adjustment of their physicochemical properties, to control their thermolysis chemistry, and also to influence materials properties. The first... 

U.S. Bureau of Mines Bull. 672, 674, and 677. Bulletins 672 and 674 cover the elements, binary oxides and binary halides in a very complete fashion. Bulletin 677, summarizes the values from Bulletins 672 and 674, and adds a modest selection of tables for arsenides, antimonides, borides, carbides, carbonates, hydrides, nitrides, phosphides, selenides, silicates, silicides, sulfates, sulfides and tellurides. The coverage of these added compound types, however, is far from complete for example, there are no tables for PbS04, SnS04, GaS and Li2S. The only ternary compounds included are the carbonates, sulfates and silicates, and no quaternary compounds are listed except for a limited number of hydrated compounds. Only brief references are given to the data sources, without attempt to explain the choice between conflicting values. 

In this chapter, we described the characteristics of binary compounds containing various types additives. These additives include various types of stabilizers, surface property modifiers, antistatic agents, nucleating agents, and curatives. Many of these are polar organic compounds that have limited solubility in hydrocarbon polymer matrices. 

Several structural types, corresponding to about 5500 binary compounds and alloys, were considered 147 structure types were classified as 97 coordination types. The applications of these maps, which, in the most favourable cases make it possible to predict not only the CN and polyhedron but also the structure type or a limited number of possibilities, were discussed. The possible extension to ternary and quaternary phases was also considered. 

Notice, moreover, that for a family of binary and complex phases such as the Laves phases (Cu2Mg, MgZn2, Ni2Mg types) an overall number of about 1400 has been estimated. The restriction of the phase concentration to a limited number of stoichiometric ratios is also valid (and, perhaps, more evident) for the ternary phases. We may notice in Fig. 7.2, adapted from a paper by Rodgers and Villars (1993), that seven stoichiometric ratios (1 1 1, 2 1 1, 3 1 1, 4 1 1, 2 2 1, 3 2 1, 4 2 1) are the most prevalent. According to Rodgers and Villars they represent over 80% of all known ternary compounds. 

In pure titanium, the crystal structure is dose-packed hexagonal (a) up to 882°C and body-centered cubic (p) to the melting point. The addition of alloying dements alters the a—p transformation temperature. Elements that raise the transformation temperature are called a-stabilizers those that depress the transformation temperature, p-stabilizers the latter are divided into p-isomorphous and p-eutectoid types. The p-isomorphous elements have limited a-solubility and increasing additions of these dements progressively depresses the transformation temperature. The p-eutectoid elements have restricted p-solubility and form intermetallic compounds by eutectoid decomposition of the p-phase. The binary phase diagram illustrating these three types of alloy... 

At that date, palladium hydride was regarded as a special case. Lacher s approach was subsequently developed by the author (1946) (I) and by Rees (1954) (34) into attempts to frame a general theory of the nature and existence of solid compounds. The one model starts with the idea of the crystal of a binary compound, of perfect stoichiometric composition, but with intrinsic lattice disorder —e.g., of Frenkel type. As the stoichiometry adjusts itself to higher or lower partial pressures of one or other component, by incorporating cation vacancies or interstitial cations, the relevant feature is the interaction of point defects located on adjacent sites. These interactions contribute to the partition function of the crystal and set a maximum attainable concentration of each type of defect. Conjugate with the maximum concentration of, for example, cation vacancies, Nh 9 and fixed by the intrinsic lattice disorder, is a minimum concentration of interstitials, N. The difference, Nh — Ni, measures the nonstoichiometry at the nonmetal-rich phase limit. The metal-rich limit is similarly determined by the maximum attainable concentration of interstitials. With the maximum concentrations of defects, so defined, may be compared the intrinsic disorder in the stoichiometric crystals, and from the several energies concerned there can be specified the conditions under which the stoichiometric crystal lies outside the stability limits. 

Binary or ternary catalyst systems from nickel compounds with Group I—III metal—alkyls have many features in common with those from cobalt and it may be inferred that a similar type of catalytic entity is involved. The composition for optimum activity may be different, however, and in the soluble catalyst Ni(naphthenate)2/BF3. EtjO/AlEtj (Ni/B/Al = 1/7.3/6.5) [68] the ratio of transition metal to aluminium is much higher than in cobalt systems. Rates were proportional to [M] and [Ni], molecular weight was limited by transfer with monomer and catalyst efficiency was relatively low (Table 5, p. 178). With the system AlEtj/ Ni(Oct)2/BF3—Et2 0 (17/1/15) the molecular weight rose with increase in [M] /[Ni] ratio and 3—9 chains were produced per nickel atom. It was observed that as molecular weight increased so the cis content of the polymer increased — from ca. 50% up to ca. 90% [292]. 

The close relation between structure and composition and simple geometrical considerations is nicely illustrated by some binary and ternary nitrides. In many of these compounds there is tetrahedral coordination of the smaller metal ions in particular, there are many ternary nitrides containing tetrahedrally coordinated Li ions. From the simple theorem that not more than eight regular tetrahedra can meet at a point (p. 159) it follows, for example, that there cannot be tetrahedral coordination of Li by N in LisN, a point of some interest in view of the unexpected (and unique) structure of this compound and the apparent non-existence of other alkali nitrides of type M3N. It also follows that the limit of substitution of Be by Li in BeaN2 is reached at the composition BeLiN for example, there cannot be a compound BeLi4N2 with tetrahedrally coordinated metal atoms. 

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