Interstitial solutions and compounds

In tire transition-metal monocarbides, such as TiCi j , the metal-rich compound has a large fraction of vacairt octahedral interstitial sites and the diffusion jump for carbon atoms is tlrerefore similar to tlrat for the dilute solution of carbon in the metal. The diffusion coefficient of carbon in the monocarbide shows a relatively constairt activation energy but a decreasing value of the pre-exponential... 

Equation (S.21) is normally used in metallic systems for substitutional phases such as liquid, b.c.c., f.c.c., etc. It can also be used to a limited extent for ceramic systems and useful predictions can be found in the case of quasi-binary and quasi-temary oxide systems (Kaufman and Nesor 1978). However, for phases such as interstitial solutions, ordered intermetallics, ceramic compounds, slags, ionic liquids and aqueous solutions, simple substitutional models are generally not adequate and more appropriate models will be discussed in Sections 5.4 and 5.5. 

Metal borates can be divided into two broad classes hydrated and anhydrous. So-called hydrated borates, which have the general formula aM O bB203 CH2O, contain B-OH groups, sometimes interstitial OH , and may also contain interstitial water. Anhydrous borates do not contain water, OH, or B-OH groups, and have the general formula aMj 0 bB203. As crystaUine compounds, hydrated borates typically crystallize from aqueous solutions under relatively mild conditions. Anhydrous borates nearly always form at... 

Pierre, C. (1985) Isotopic evidence for the dynamic redox cycle of dissolved sulphur compounds between free and interstitial solutions in marine salt pans. Chemical Geology 53, 191-196. 

The presence of carbon in solid solution stabilizes the y iron structure and inhibits the transition to a iron. On slow cooling, therefore, austenite remains stable below the normal transition temperature of 910 °C, and it is only at about 700 °C that the solid solution breaks down and transforms into a mixture of ferrite and cementite. Ferrite is an interstitial solution of carbon in a iron, but the amount of carbon which can be taken up in solution is very small and is limited to about 0 3 atomic per cent. The excess of carbon is therefore thrown out of solution and appears in the definite compound cementite, Fe3C, with a complex orthorhombic structure. The solid is no longer homogeneous, and the characteristic appearance, due to the separation of ferrite and cementite, gives rise to the name pearlite. In this condition the steel is very soft. 

In addition to transport and biologically mediated reaction controls on pore-water composition, interstitial solutes are subject to abiogenic reactions with specific solid phases in the sediment (Suess, 1976, 1979 Emerson, 1976 Hartman et al., 1976 Sayles and Manheim, 1975 Martens et al., 1978). The interaction of particular solid phases with pore-water compositions can be inferred in part by examining the disequilibrium state of pore water with respect to selected compounds likely to be present. 

Alloys are manufactured by combining the component elements in the molten state followed by cooling. If the melt is quenched (cooled rapidly), the distribution of the two types of metal atoms in the solid solution will be random the element in excess is termed the solvent, and the minor component is the solute. Slow cooling may result in a more ordered distribution of the solute atoms. The subject of alloys is not simple, and we shall introduce it only by considering the classes of substitutional and interstitial alloys, and intermetallic compounds. 

Lastly, we turn our attention to the gray area between metallic and ionic solids. An interstitial is a compound that is formed when another atom (typically having about the same electronegativity as the metal) is small enough to occupy some of the interstitial sites in a metallic solid. The elements H, B, C, and N are common examples. For instance, H atoms can occupy the interstitial sites in Pd metal to form compounds having the general formula PdH . Interstitials can also be considered as solid solutions, where the smaller atom serves as the solute and the larger... 

The interstitial solutions are typical of clathrate compounds and they have been considered [l]. Studying water [2,3], urea [4,5], thiourea [6] and hydroquinone [7-9] clathrates we have found that only the latter (in the presence of the limited guest set [10-12]) forms the solutions of this type. 

From the fundamental point of view, vacancies and interstitials in intermetallic compounds present special features and a much more complex behavior, as compared to pure metals. For example, in a pure metal of simple crystal structure, all the bulk lattice sites are equivalent for the formation of a vacancy. On the contrary, in an LRO concentrated alloy, the various sublattices are not equivalent. Moreover, atoms of a given sublattice may have various atomic environments in an LRO alloy as well as in a concentrated solid solution, many types of vacancies can be produced. Also, the vacancy formation energy depends on the extent of off-stoichiometry, and the thermal vacancy concentration may be much larger than in pure metals. 

In this case, the intercalation process is essentially controlled by the Li concentration in the host lattice. As shown in Fig. 13.7b the thermodynamic factor, W, varies linearly with the degree of Li intercalation from 10 to 820 in the range 0.05 < X < 1.5. This x-dependence of W can be associated with the large decrease of the electronic mobility in Li MoOs film. Considering that M0O3 films are oxygen-deficient materials, the model of charge transport in internal defect compounds can be applied [14—18], Defects are Li interstitials, Li, and conductimi electrons, e. It has been shown [17] that, in a solid solution where no internal defect reactions... 

The many possible oxidation states of the actinides up to americium make the chemistry of their compounds rather extensive and complicated. Taking plutonium as an example, it exhibits oxidation states of -E 3, -E 4, +5 and -E 6, four being the most stable oxidation state. These states are all known in solution, for example Pu" as Pu ", and Pu as PuOj. PuOl" is analogous to UO , which is the stable uranium ion in solution. Each oxidation state is characterised by a different colour, for example PuOj is pink, but change of oxidation state and disproportionation can occur very readily between the various states. The chemistry in solution is also complicated by the ease of complex formation. However, plutonium can also form compounds such as oxides, carbides, nitrides and anhydrous halides which do not involve reactions in solution. Hence for example, it forms a violet fluoride, PuFj. and a brown fluoride. Pup4 a monoxide, PuO (probably an interstitial compound), and a stable dioxide, PUO2. The dioxide was the first compound of an artificial element to be separated in a weighable amount and the first to be identified by X-ray diffraction methods. 

By analogy with similar materials in which free elecU ons and electron holes are formed, NiO is called a p-type compound having vacant site Schottky defects, and ZnO is an n-type compound having interstitial Frenkel defects. The concentrations of these defects and their relation to the oxygen pressure in the suiTounding atmosphere can be calculated, for a dilute solution of defects by the application of a mass action equation. The two reactions shown above are represented by the equations... 

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