Interstitial structures classification

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 ... 

The long-term goal in the science of thermochemical conversion of a solid fuel is to develop comprehensive computer codes, herein referred to as a bed model or CFSD (computational fluid-solid dynamics). Firstly, this CFSD code must be able to simulate basic conversion concepts, with respect to the mode, movement, composition and configuration of the fuel bed. The conversion concept has a great effect on the behaviour of the thermochemical conversion process variables, such as the molecular composition and mass flow of conversion gas. Secondly, the bed model must also consider the fuel-bed structure on both micro- and macro-scale. This classification refers to three structures, namely interstitial gas phase, intraparticle gas phase, and intraparticle solid phase. Commonly, a packed bed is referred to as a two-phase system. 

Nitrides can be sub-divided into ionic, covalent and interstitial types.An alternate general classification of nitrides, based on bonding classification, as ionic, covalent and metallic has also been applied. Ionic or salt-like nitrides are formed by electropositive elements such as Li, Mg, Ca, Sr, Ba, Cu, Zn, Cd and Hg and possess formulae which correspond to those expected on the basis of the combination of the metal ion with ions. A range of covalent nitrides are known and are exhibited by less electropositive elements such as B, S, P, C and Si. Interstitial nitrides are formed by some transition metals and refer to compounds which can be described in terms of the occupancy of interstitial sites in close packed metallic structures by nitrogen atoms. Oxygen can also be accommodated within these structures and a range of oxynitrides are known to... 

As for hydrides, borides, and carbides, different types of nitrides are possible depending on the type of metallic element. The classifications of nitrides are similarly referred to as ionic (salt-like), covalent, and interstitial. However, it should be noted that there is a transition of bond types. Within the covalent classification, nitrides are known that have a diamond or graphite structure. Principally, these are the boron nitrides that were discussed in Chapter 8. 

This chapter is a general review of the structural characteristics of the refractory carbides, their classification, and general features. These materials can be divided into two major types the interstitial carbides reviewed in Chs. 3 to 6, and the covalent carbides, reviewed in Chs. 7 and 8. 

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