Interstitial metal nitrides

The crystal stmeture and stoichiometry of these materials is determined from two contributions, geometric and electronic. The geometric factor is an empirical one (8) simple interstitial carbides, nitrides, borides, and hydrides are formed for small ratios of nonmetal to metal radii, eg, < 0.59. 

The crystal structure and stoichiometry of these materials is determined from two contributions, geometric and electronic. The geometric factor is an empirical one (8) simple interstitial carbides, nitrides, borides, and hydrides are formed for small ratios of nonmetal to metal radii, eg, rx / rM < 0.59. When this ratio is larger than 0.59, as in the Group 7—10 metals, the structure becomes more complex to compensate for the loss of metal—metal interactions. Although there are minor exceptions, the H gg rule provides a useful basis for predicting structure. 

The stmeture of transition metal carbides are closely related to those of the transition metal nitrides. However, transition metal carbides feature generally simpler stmeture elements as compared to the nitrides. In carbides, the metal atoms are arranged in such a way that they form close-packed arrangements of metal layers with a hexagonal (h) or cubic (c) stacking sequence or with a mixtme of these (see Nitrides Transition Metal Solid-state Chemistry). The carbon atoms in these phases occupy the octahedral interstitial sites. A crystallochemical rule claims that the phases of pure h type can have a maximum carbon content of [C]/[T] = 1/2 and the c type phases a maximum carbon content of [C]/[T] = 1 hence in stractures with layer sequences comprising h and c stractme elements the maximum nonmetal content follows suit. 

The metal nitrides of the transition metals of the subgroups of the IVth, Vth and Vlth groups are very similar to the corresponding carbides in their. structures (interstitial compounds of the MN type) and their properties such as hardness, melting point and electrical conductivity (see Table 5.6-3). Tungsten and molybdenum nitrides are exceptions with different structures and decompose with nitrogen loss above 800°C. 

Metallic nitrides, sometimes termed interstitial compounds, are formed from combinations of N with transition metals of groups IVA, VA, and VIA. As the name implies, they exhibit electrical conductivity and most of the general characteristics associated with standard metals. They are also refractory and hard, and usually depart from the ideal stoichiometry ratios displayed above (see 17.3.9, Table 1). They readily form solid solutions with carbides and oxides, which gives rise to problems when it is necessary to obtain nitrides in pure form. Included in this category are numerous ternary nitrides of a transition metal with a group B metal. 

LijN BejNj MgsN2 CajNj etc. Transition-metal nitrides Interstitial and more complex structures HgaNj BN AIN GaN InN Si3N4 Ge3N4 Molecular nitrides of non-metals... 

We have included not only pure nitride materials but also many amides, azides, oxynitrides (but not nitrates and nitrites), nitride chalcogenides, and nitride halides. Specifically excluded are polymeric compounds (e.g. SxNy), cluster compounds (e.g. Sc2C12N), coordination compounds (e.g. [Pd(NS3)2]), ternary azides and amides, as well as ternary interstitial transition-metal nitrides. We have attempted to include all important structures and all type compounds. This review is also unique in that we attempt to evaluate the... 

Another view of the FeMoco compares it to bulk metals with interstitial carbide, nitride, or oxide in the lattice. It is difficult to draw clear analogies between the band electronic structure of solids (through which oxide, nitride, and carbide materials are understood) and the localized interactions in the FeMoco, although theoretical calculations should give progress along these lines. 

Transition-metal nitrides where nitrogen is present as an interstitial are prepared by the reaction of the metals with NH3 around 1470 K. BN is obtained by heating boron with NH3 at white heat. Metal nitrides are also prepared by the reaction of metal chlorides with NHj. For example, MN and WN films are prepared by the reaction of NH3 with AICI3 and WCl respectively. Recently, nitrides of Ti, Zr, Hf and lanthanides have been prepared by the reaction of lithium nitride with the anhydrous metal chlorides [8] ... 

This chapter is a review of the properties and general characteristics of the interstitial nitrides formed by the metals of Group IV (titanium, zirconium, and hafiiium) and Group V (vanadiiun, niobium, and tantalum). As mentioned in Ch. 10, these six nitrides are the only refiactory transition-metal nitrides. Th have similar properties and characteristics and, of the six, titanium nitride has the greatest importance from an application standpoint. 

The transition metals often form interstitial, nonstoichiometric nitrides in which nitrogen atoms occupy holes in the close-packed metal lattices. 

The heats of formation of the more common nitrides are shown in Table 9.1. All of the transition metal nitrides are close-packed interstitial structures and in some cases a wide variety of nitrides are formed depending on the number of octahedral holes that are filled. Manganese, for instance, forms... 

The same situation is met in R-M-N ternary nitrides in which the nature of the M element determines the dominating type of bond involved in the material. This is illustrated by the fact that with lithium (or barium) as a cationic element, the R-M-N corresponding nitride is essentially ionic in character, whereas with silicon, more covalent nitrido-silicates are formed. In addition, metallic nitrided alloys exist, with nitrogen located as an interstitial element in octahedral voids of the metal atom lattice. The presence of insertion nitrogen (as well as carbon) in such compounds is sometimes necessary for their existence, and can strongly modify the physical properties. 

Those elements active as NH3 synthesis catalysts are found in groups IVA through VIII, which form interstitial nitrides. Interstitial nitrides have expanded metal lattices in which nitrogen occupies interstital positions. They are referred to as metallic nitrides because of their resemblance to the metals [29]. 

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