Nitrides covalent

Unlike the interstitial nitrides, the covalent nitrides are not metallic compounds. The differences in electronegativity and atomic size between the nitrogen and the other element are small and their electronic bonding is essentially covalent. In this respect, they are similar to the covalent carbides. They include the nitrides of Group mb (B, Al, Ga, In, Tl) and those of silicon and phosphorus. Of these, only three are considered refractory boron nitride, silicon nitride, and aluminum nitride. These are reviewed in Chs. 12 and 13. 

The atomic and crystalline structure of the three covalent nitrides, aluminum, boron, and silicon nitrides, is less complex than that of the interstitial nitrides. Their bonding is essentially covalent. 

The three covalent nitrides have the following common features and are in many ways similar to the covalent carbides reviewed in Ch. 9  

In this chapter, each nitride is listed alphabetically with its basic properties, its major CVD reactions and processes, and its present and potential applications. 

Aluminum nitride is a highly stable covalent compound with the unusual combination of high thermal conductivity (comparable to that of metals) and high electrical insulation (comparable to the 

Color White when pure, tan or gray with impurities 

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The nitrides reviewed here are those which are commonly produced by CVD. They are similar in many respects to the carbides reviewed in Ch. 9. They are hard and wear-resistant and have high melting points and good chemical resistance. They include several of the refractory-metal (interstitial) nitrides and three covalent nitrides those of aluminum, boron, and silicon. Most are important industrial materials and have a number of major applications in cutting and grinding tools, wear surfaces, semiconductors, and others. Their development is proceeding at a rapid pace and CVD is a major factor in their growth. 

The "nitrides" of most nonmetals exist. By calling the compounds nitrides, it is indicated that the other element has an electronegativity that is lower than that of nitrogen. Therefore, N02, NF3, N2F2, and the like would not be considered "nitrides" because the other element is the more electronegative. This leaves quite a number of compounds such as HN3, S4N4, (CN)2, that are covalent nitrides. Chemically, these compounds are quite different, and as will be shown later, methods for synthesizing them vary enormously. 

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... 

Until recently, the synthesis of ionic/covalent nitrides was relatively unexplored except for the pioneering work of Juza on ternary lithium nitrides.11 However, within the last decade, several groups have begun to explore ternary nitride systems, many of which have relied on the inductive effect. The inductive effect is based on the donation of electron density from an electropositive element to an adjacent metal-nitrogen bond, thereby increasing the covalency and stability of that bond and of the nitride material itself. The success of this method is illustrated by the fact that almost all of the known ionic/covalent ternary nitrides contain electropositive elements. Only recently has a small number of transition metal ternary nitrides been synthesized in the absence of the inductive effect at moderate temperatures, by taking advantage of low temperature techniques, such as the ammonolysis of oxide precursors and metathesis reactions.6,12-17... 

We distinguish three kinds of nitrides, i.e. ionic nitrides (e g. Th3N2), covalent nitrides (e.g. BN) and intermediate forms (e g. VN). These intermediate forms are extremely inert, very hard and they have high melting points. VN has a Mohs hardness of 9-10 and a melting point of 2570 °C. Furthermore, the intermediate form conducts electricity since the electronic band structure of the metal is maintained when N atoms are placed in the cavities of the crystal lattice... 

There are various covalent nitrides (BN, S4N4, P3N5, etc.), and their properties vary greatly depending on the element with which it is combined. Such substances are therefore discussed under the appropriate element. 

The nitrides (see Nitrides Transition Metal Solid-state Chemistry) can be divided ronghly into ionic nitrides (e.g. alkali and alkaline-earth nitrides), covalent nitrides (e.g. 

For the development of new high-performance materials it is particularly interesting to search for new covalent nitrides with highly cross-linked structures. The binary nitrides P3N5 and Si3N4 are now well known but what about ternary nitrides in the system Si-P-N ... 

As already mentioned, synthesis of multinary silicon boron nitrides or carbon nitrides cannot be achieved via the well-known powder route, including mixing, milling, and sintering of binary nitride/carbide powders, because the interdiffiision of the covalent nitrides, and carbides, proceeds... 

Nitrides are the binary salts of metals and nitrogen, containing the anion. While the nitrides of alkali metals and some alkaline earth metals are ionic, most other metal nitrides are covalent. Nitrides of silicon and selenium are covalent and polymeric. Many complex and polynuclear metal nitrides are known. The formulas and CAS numbers for some nitrides are presented in Table 57.1. 

Transition metals form nitrides that are different from the ionic or covalent nitrides, the stoichiometries are, for example, ZrN, W2N, Mn4N. They are often called interstitial nitrides. They are very hard and their formation is the basis of surface hardening by exposing hot metal to ammonia gas (see case hardening). 

Nitride compounds can also be categorized as binary, ternary, quartemary, and multinary based on the number of elements included. Binary covalent nitrides, such as GaN and AIN, are not easily considered as host lattices for phosphors due to the fact that they lack suitable crystal sites for activators. Ternary, quartemary, and multinary covalent nitride compounds, typically silicon-based nitrides, have distinctive and rigid crystal stmctures. They have suitable crystal sites for activators and have a versatile stmcture, which allows the doped RE ions to exhibit useful photoluminescence [33]. 

This chapter is a review of the general characteristics of the refixictory nitrides and their classification. Like the refiactory carbides (see Ch. 2), the refractory nitrides can be divided into two major types the interstitial nitrides reviewed in Chs. 10 and 11 and the covalent nitrides, reviewed in Chs. 12 and 13. 

In the five categories listed above, only some of the interstitial and covalent nitrides qualify as refractory, i.e., the nitrides of the elements of Groups IV and V and the covalent nitrides of boron, aluminum, and silicon. These elements are shown in bold type in Table 9.1. Unlike the carbides of Group VI elements, the Group VI nitrides are not refractory and consequently are not considered in any depth in this book. 

The importance of the atomic radius will become evident as the structure of interstitial and covalent nitrides is reviewed in Chs. 10 and 12. Generally speaking, when the difference in radii of the two elements is large, interstitial nitrides are formed (i.e., TiN) when it is small, covalent nitrides are formed (i.e., Si3N4). 

The third factor governing the structure of nitrides is the nature of the bond between the nitrogen atom and the other element forming the compound. As mentioned in Ch. 2, the bond is the force of attraction that holds together the atoms of a molecule.l The bonds in refractory carbides can be ionic (saltlike nitrides), covalent (covalent nitrides), or a combination of metallic, covalent, and ionic (interstitial nitrides) (for a discussion of electronic bonding, see Ch. 2, Sec. 5.0). 

As mentioned in Ch. 9, the refractory nitrides consist of two structurally different types generally known as interstitial and covalent nitrides. This chapter provides a general review of the structural characteristics and composition of the interstitial nitrides and follows the outline of Ch. 3, Interstitial Carbides Structure and Composition. Some of these interstitial nitrides, titanium nitride in particular, are major industrial materials. 

As stated in Ch. 9, the refiactory nitrides ccmsist of two structurally different types (a) die interstitial nitrides of die early transition metals (reviewed in Chs. 10 and 11), and the covalent nitrides of wdiich three are... 

Like the covalent carbides, the covalent nitrides have a relatively simple crystal structure and an atomic bonding which is less complex than the interstitial nitrides. The bonding is mostly covalent by the sharing of electrons and is achieved by the hybridization of the respective electron orbitals. 

The location in the Periodic Table and the atomic number of the four elements forming the refractory covalent nitrides is as follows ... 

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