Interstitial nitrides

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... 

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]. 

The vacancy is very mobile in many semiconductors. In Si, its activation energy for diffusion ranges from 0.18 to 0.45 eV depending on its charge state, that is, on the position of the Fenni level. Wlrile the equilibrium concentration of vacancies is rather low, many processing steps inject vacancies into the bulk ion implantation, electron irradiation, etching, the deposition of some thin films on the surface, such as Al contacts or nitride layers etc. Such non-equilibrium situations can greatly affect the mobility of impurities as vacancies flood the sample and trap interstitials. 

The many possible oxidation states of the actinides up to americium make the chemistry of their compounds rather extensive and complicated. Taking plutonium as an example, it exhibits oxidation states of -E 3, -E 4, +5 and -E 6, four being the most stable oxidation state. These states are all known in solution, for example Pu" as Pu ", and Pu as PuOj. PuOl" is analogous to UO , which is the stable uranium ion in solution. Each oxidation state is characterised by a different colour, for example PuOj is pink, but change of oxidation state and disproportionation can occur very readily between the various states. The chemistry in solution is also complicated by the ease of complex formation. However, plutonium can also form compounds such as oxides, carbides, nitrides and anhydrous halides which do not involve reactions in solution. Hence for example, it forms a violet fluoride, PuFj. and a brown fluoride. Pup4 a monoxide, PuO (probably an interstitial compound), and a stable dioxide, PUO2. The dioxide was the first compound of an artificial element to be separated in a weighable amount and the first to be identified by X-ray diffraction methods. 

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. 

Nitrogen forms binary compounds with almost all elements of the periodic table and for many elements several stoichiometries are observed, e.g. MnN, Mn Ns, Mn3N2, MniN, Mn4N and Mn tN (9.2 < jc < 25.3). Nitrides are frequently classified into 4 groups salt-like , covalent, diamond-like and metallic (or interstitial ). The remarks on p. 64 concerning the limitations of such classifications are relevant here. The two main methods of preparation are by direct reaction of the metal with Ni or NH3 (often at high temperatures) and the thermal decomposition of metal amides, e.g. ... 

When iron surfaces are exposed to ammonia at high temperatures, nitriding —the incorporation of nitrogen into the iron lattice—occurs. The atomic radius of iron is 124 pm. (a) Is the alloy interstitial or substitutional Justify your answer. 

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. 

Interstitial nitrides are crystalline compounds of a host metal and nitrogen, where the nitrogen atom occupies specific interstitial... 

The interstitial nitrides have several important characteristics in common with the interstitial carbides. 

More so than the carbides, the interstitial nitrides are susceptible to the presence of even minute amounts of impurities such as hydrogen and particularly oxygen which tend to distort the structure. To avoid such harmful contamination, it is necessary to maintain a deposition system that is completely free of oxygen and hydrogen. 

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. 

CVD titanium nitride (TiN) is the most important interstitial-nitride coating from an application standpoint. It is used extensively to provide wear resistance and as a diffusion barrier and antireflection coating in semiconductor devices. 1 °]... 

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... 

In the face-centred cubic structure there are four atoms per unit cell, 8x1/8 cube comers and 6x1/2 face centres. There are also four octahedral holes, one body centre and 12 x 1/4 on each cube edge. When all of the holes are filled the overall composition is thus 1 1, metal to interstitial. In the same metal structure there are eight cube comers where tetrahedral sites occur at the 1/4, 1/4, 1/4 positions. When these are all filled there is a 1 2 metal to interstititial ratio. The transition metals can therefore form monocarbides, nitrides and oxides with the octahedrally coordinated interstitial atoms, and dihydrides with the tetrahedral coordination of the hydrogen atoms. 

Nitrides. Among the elements of the 15th group, the particular behaviour of nitrogen is notable. Several are the analogies with carbon in the formation, for instance, of interstitial compounds. A number of these phases, such as the refractory solid solution MeN. phases, have been described in 3.8.4 ff. 

At present the iron-based alloys diffusion saturation by nitrogen is widely used in industry for the increase of strength, hardness, corrosion resistance of metal production. Inexhaustible and unrealized potentialities of nitriding are opened when applying it in combination with cold working [1-3], It is connected with one of important factors, which affects diffusion processes and phase formation and determines surface layer structure, mechanical and corrosion properties, like crystal defects and stresses [4, 5], The topical question in this direction is clarification of mechanisms of interstitial atoms diffusion and phase formation in cold worked iron and iron-based alloys under nitriding. 

Niobium metal absorbs nitrogen, similar to hydrogen, forming interstitial solid solution. The absorption occurs at 300°C and the solubility of nitrogen in the metal is directly proportional to the square root of the partial pressure of nitrogen. The reaction is exothermic and the composition of such interstitial solid solution varies with the temperature. When the metal is heated with nitrogen at temperatures between 700 to 1,100°C, the product is niobium nitride, Nb2N or (NbNo.s) [12033-43-1]. When heated with ammonia at these temperatures, niobium forms this nitride. Another niobium nitride exists, NbN [24621-21-4], with a face-centered cubic crystalline structure. 

However, its reactions with heavy metals at elevated temperatures form hard refractory interstitial nitrides of nonstoichiometric compositions. 

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... 

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