Interstitial mononitrides

Table 10.5 shows the density of the interstitial mononitrides (for materials closest to stoichiometry), the density of the host metals and the difference between the two in percent.OlWI l... 

The electronic configuration of the interstitial mononitrides, including the band structure, the density of states, and other bonding considerations, have been the object of much research and is now relatively well defined. A schematic representation of the bonding orbitals of TiN... 

Bond Energy and Melting Points. The comparison between the melting points of interstitial mononitrides and their host metals and the differences in the bond energy of these nitrides is shown in Table 10.6. This provides a qualitative view of the M-M and M-N bonds. xhe... 

Unlike the interstitial monocarbides, MC,, where C is never >1, the interstitial mononitrides, MN,, can have a composition where x >1. In substoichiometric compositions (x < 1), the sublattice of nitrogen is predominantly deficient while at hyperstoichiometric compositions (x > 1), the metal lattice is predominantly deficient. The lattice parameter is at a maximum at stoichiometry. Even at stoichiometry, a substantial fiaction of both nitrogen and metal sites are usually vacant. 

Titanium mononitride S-TiN c (0.42 < x < 1.0) has wide solubility from 10.93 to 22.63% of interstitial nitrogen [ 11. The TiN is stable golden compound of face-centered cubic (Bl) structure. Strictly speaking, stoichiometric composition TiNi.o cannot exist under atmospheric pressure, but substoichiometric composition TiNo.97 is stable. For this reason, the atomic vacancies [2] in sublattices of Ti and N reduce height of Fermi level based on valence electron concentration (VEC) of transition metal compounds [3], resulting in stabilization of band structure. It was experimentally reported that the VEC value of the transition metal compounds is stable at 8.8 under atmospheric pressure [4]. In case of titanium nitride, TiNo.ge is the most stable composition. 

Another structure is the simple hexagonal structure (hex) such as that of tungsten mononitride (8WN) where the metal atom layers form a sequence of layers AA or BB. Such structures are not close-packed and do not form octahedral sites the available interstitial sites are trigonal prisms (see Fig, 3.13 of Ch. 3). This structure cannot form if the ratio of the nitrogen/ metal atomic radii is small, as is the case in the Zr-N and Hf-N systems. 

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