Interstitial carbides and nitrides

A few more details about these systems are summarized in the following data as taken from the compilation by Okamoto (2000). 

Ti-C In this system one intermediate phase is formed with a homogeneity range extending, depending on temperature, between 32 and 50 at.% C, in short named TiC (or, better, TiC x) and having a NaCl-type structure. 

Nb-C Two intermediate phases NbC (37.7-49 at.% C), NaCl-type and —Nb2C (29-33.5 at.% C), NiAs-type. 

Ti-N The system is complex with several intermediate phases among them TiN (28 to 50 at.% N), NaCl-type. 

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Periodic Table to justify the leaving of such slighted elements for a more advanced course taken by students with special interests in inorganic chemistry. Similarly, I have passed over many interesting but somewhat exotic compounds of the more usual elements (for example, the nitrides of sulfur, metallic nitrosyl compounds, heteropoly acids, and interstitial carbides and nitrides). 

Bundle (548) has pointed out that resonating, hybrid sp orbitals arc probably active in bonding in interstitial carbides and nitrides. 

Compounds with the sodium chloride structure range from the essentially ionic halides and hydrides of the alkali metals and the monoxides and monosulphides of Mg and the alkaline-earths, through ionic-covalent compounds such as transition-metal monoxides to the semi-metallic compounds of B subgroup metals such as PbTe, InSb, and SnAs, and the interstitial carbides and nitrides (Table 6.1). Unique and different distorted forms of the structure are adopted by the Group IIIB... 

Table 10.6 Bond Energy and Melting Point of Interstitial Carbides and Nitrides...

A discussion on the electrical properties of interstitial carbides and nitrides is given in Ch. 4, Sec. 3.0. The electrical properties of these materials are shown in Table 1 l,4.WO0][ii][i3]... 

On the other hand, not all the interstitial carbides and nitrides have large-volume applications in fact, some have never developed much beyond the laboratory stage and only tungsten carbide, titanium carbide, chromium carbide, and titanium nitride have found large-scale applications at this time. 

Another example for an investigation of multinuclear transition metal clusters is the SOS-DFPT-IGLO study of the C shift tensors for interstitial carbides enclosed in carbonyl clusters. The interstitial shifts are important, both as a proof for the existence of an interstitial atom, and as a potential probe of electronic structure. Table 2 compares computed and experimental shifts. For the two rhodium clusters, it has been possible to compare not only isotropic shifts but the entire shift tensors, as an independent solid-state NMR study given the first tensor data for a number of interstitial carbides and nitrides. The overall agreement between computation and experiment is good, both for the isotropic shifts and for the available tensors. The largest deviation (43 ppm) was found... 

The carbides and nitrides of the elements Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Th, and U are considered to be typical interstitial compounds. Their compositions frequently correspond to one of the approximate formulas M2X or MX. As a rule, they are nonstoichiometric compounds with compositions ranging within certain limits. This fact, the limitation to a... 

When discussing metal alloys (Section 4.3), we saw that atoms of non-metallic elements such as H, B, C, and N can be inserted into the interstices (tetrahedral and octahedral holes) of a lattice of metal atoms to form metal-like compounds that are usually nonstoichiometric and have considerable technological importance. These interstitial compounds are commonly referred to as metal hydrides, borides, carbides, or nitrides, but the implication that they contain the anions H, B3, C4, or N3- is misleading. To clarify this point, we consider first the properties of truly ionic hydrides, carbides, and nitrides. 

Why do carbides and nitrides exhibit the properties that make them so useful in industrial applications It is well accepted that these properties are related to the strength of interatomic bonding.2 In transition metal carbides and nitrides, bonding is believed to have both covalent and ionic contributions.3 The carbon or nitrogen atoms occupy interstitial sites in the metal lattice and are believed to promote strong metal-to-nonmetal and metal-to-metal bonds.1 More detailed bonding explanations require... 

The monometallic carbides and nitrides often adopt simple crystal structures (Fig. 1 ) with the metal atoms arranged in cubic close-packed (ccp), hexagonal close-packed (hep) or simple hexagonal (hex) arrays. The nonmetallic elements, C, N, and O, occupy interstitial spaces between metal atoms, and for this reason the materials are also known as interstitial alloys. 

Tungsten carbide — WC, belongs to a class of Group IV B-VIB transition metal carbides and nitrides, often referred to as interstitial alloys, in which the carbon and nitrogen atoms occupy the interstitial lattice positions of the metal [i]. These compounds possess properties known from group VIII B precious metals like platinum and palladium [ii]. Thus, they show remarkable catalytic activities, attributed to a distinct electronic structure induced by the presence of carbon or nitrogen in the metal lattice. Tungsten carbide resembles platinum in its electrocatalytic oxidation activity (- electrocatalysis) and is therefore often considered as an inexpensive anode electrocatalyst for fuel cell [iii] and -> biofuel cell [iv] application. 

In crystal structure the metallic carbides and nitrides are almost all interstitial compounds in which the carbon or nitrogen atoms are found in the octahedral interstices of the metal lattice. The borides and silicides as well as a few carbides (e.g. Cr3C2) possess more complex structures. 

The metallic carbides and nitrides are for the most part interstitial compounds and form mixed crystals with one another... 

A. .. A etc. contacts between the layers (see later). The reason for the great importance of the most closely packed structures is that in many halides, oxides, and sulphides the anions are appreciably larger than the metal atoms (ions) and are arranged in one of the types of closest packing. The smaller metal ions occupy the interstices between the c.p. anions. In another large group of compounds, the interstitial borides, carbides, and nitrides, the non-metal atoms occupy Interstices between c.p. metal atoms. 

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... 

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