Interstitial structure

Similar considerations may be made with reference to the other simple close-packed structure, that is to the hexagonal Mg-type structure. In this case two basic derived structures can be considered the NiAs type with occupied octahedral holes and the wurtzite (ZnS) type with one set of occupied tetrahedral holes (compare with the data given with an origin shift in 7.4.2.3.2). For a few more comments about interstices and interstitial structures see 3.8.4. See Fig. 3.35. 

The renal medulla is the middle portion of the kidney and consists of the loops of Henle, vasa recta, and collecting ducts. Medullary blood flow (about 6% of total renal blood flow) is considerably lower than cortical flow. However, by virtue of its countercurrent arrangement between tubular and vascular components, the medulla may be exposed to high concentrations of toxicants within tubular and interstitial structures. 

These metals have an atomic radius below 1.3 A. Iron (1.16 A), chromium (1.17 A) and manganese (1.17 A) form carbides with properties intermediate between the salt-like and the interstitial. Structurally FcgC, MugC and NigC have C atoms inside the trigonal prisms formed by the metal atoms. They are easily decomposed by acids and water. In CrgC2 the carbon atoms form chains in the solid. 

Interstitial structures are not possible for the larger P and As atoms, and apart from a few cases such as LaP, PrP, and GeP with the NaCl structure there is usually little similarity between nitrides and phosphides (or arsenides). Compare, for example, the formulae of the nitrides and phosphides of Mo and W M02N, W2N, MoN, and WN, but M03P, W3P, MoP, WP, M0P2, and... 

Figure 1 Investigated molecular interstitial structures in ice Ih. Arrows indicate crystallographic directions (c.f. Ref. 1). a) Capped trigonal (Tc) intestitial b) Uncapped trigonal (Tu) interstitial c) Bond center (Be) interstitial)...

L.P. Davila, Computer modeling studies of the interstitial structure of selected silica polymorphs, M.S. Thesis, University of California, Davis, 1998. 

An interstitial atom in an antibonding (AB) site is bonded to its nearest neighbour lattice atom. This location is often found in H complexes involving a donor atom and results in the relaxation of the local lattice bonding. There also exists a special interstitial structure, the di-interstitial configuration. Incidentally, Fig. 2.5 shows the ternary symmetry of the sphalerite lattice along a <111 > direction. This is analogous with the wurtzite structure, where... 

To these may be added a fourth class (4) the interstitial structures, which resemble alloy systems in many of their properties, although they are structures in which one of the components is a non-metallic element. 

This completes our discussion of the three principal classes into which we have divided the alloy systems. As a summary, a condensed survey of the chief structural characteristics of these classes is given in table 13.10. The remaining class of alloys to be considered is that embracing the interstitial structures. These systems, however, differ materially from the alloy systems so far discussed, and a description of them is accordingly deferred until later in the chapter ( 13 37). 

The interstitial structures comprise the compounds of certain metallic elements, notably the transition metals and those of the lanthanide and actinide series, with the four non-metallic elements hydrogen, boron, carbon and nitrogen. In chapter 8 we discussed the structures of a number of hydrides, borides, carbides and nitrides of the most electropositive metals, and these we found to be typical salt-like compounds with a definite composition and with physical properties entirely different from those of the constituent elements they are generally transparent to light and poor conductors of electricity. The systems now to be considered are strikingly different. They resemble... 

These observations provide the basis for a classification of interstitial structures, originally proposed by Hagg, in which we consider as of one type all those structures produced by the occupation of... 

Although the geometrical picture of interstitial structures presented above provides a convenient basis for their classification, and is, indeed, the origin of the very name by which they are known, it must not be assumed that these structures can be regarded simply as interstitial solid solutions in the crystal structure of the parent metal. There are many reasons for this view, among which the following may be mentioned ... 

It is not sufficient, however, to consider only the bonding of the non-metal atoms in the interstitial structures. The argument outlined above explains why these atoms are octahedrally co-ordinated, but the same co-ordination could be achieved in a 126, 6 structure, with the metal... 

The interstitial structures of by far the greatest technical importance are those which occur in the iron-carbon system, and the application of X-ray analysis to this system has resulted in a great extension of our understanding of the properties of carbon steels, and in a considerable simplification in the description of their behaviour. We cannot give here a detailed account of all the work in this field but certain features are of general interest and may be briefly discussed. 

The crystal chemistry of the iron-carbon system is especially complex on account of the relatively small size of the iron atom, resulting in a carbon iron radius ratio of about o 6o, which is so close to the critical value 0 59 discussed above that both interstitial structures and structures of greater complexity may be expected. Added to this is the further complication that iron is dimorphous. Below about 910 °C, and from about 1400 °C to the melting point, the structure is cubic body centred, and is known as a iron. Between these two temperatures a cubic close-packed structure, termed y iron, is formed. The ferromagnetism of iron... 

In addition to serving as markers of renal dysfunction, it is now evident that the filtration of abnormal amounts and/or types of proteins influences the progression of renal disease by promoting secondary injury to tubular epithelial cells and interstitial structures. For example, the upregulation of various cytokines in tubular epithelial cells may contribute to the development of interstitial fibrosis and cell cycle activation leading to tubular cell proliferation and/or apoptosis [52-54],... 

Wo might here deal briefly with the structure of the well-known interstitial compound iron carbide, or cementite. The largest interatomic distance in iron is 2-52A, which figure may be taken for the atomic diameter of iron, while that of carbon is 1-54A, so that the diameter ratio is 1 54/2 52 — (Mil i.e. the diameter ratio is greater than 0 59. The relatively small size of the iron atom therefore induces the formation of an interstitial structure which is more complex than the simple cubic or hexagonal types described above. ... 

The lymphatic endothelium has a number of similarities with vascular endothelium. It forms a continuous lining and has typical cytoskeletal fibers such as microtubules, intermediate fibers, and actin in both fiber bundle form and mafric form. There are numerous caveolae, Weibel-Palade bodies, but lymphatic endothelium has fewer interendothehal adhesion complexes and a discontinuous basement membrane. The residues of the basement membrane are attached to interstitial collagen via anchoring filaments [Leak and Burke, 1968] that provide relatively hrm attachment of the endothelium to interstitial structures. 

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