Nonstoichiometric nitrides

Anderson et al. (1969), Colquhoun et al. (1975) and McColm et al. (1977) prepared samples containing unstable and highly nonstoichiometric nitride-carbides of cerium, praseodymium and lanthanum with a NaCl structure by high-temperature reactions between RC2 and RN, RN and C, as well as R and HCN etc. The methods employed to produce alloys in the lanthanum-nitrogen-carbon, cerium-nitrogen-carbon and praseodymium nitrogen-carbon systems only lead to nonequilibrium phases. 

I 1 Combustion Synthesis of Nitrides for Development of Ceramic Materials of New Generation of a lower composition and nonstoichiometric nitrides. 

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

In this chapter we describe the results of theoretical studies of the electronic structure and properties of refractory nitrides, as compared with the properties of binary carbides. We shall consider here only those compounds with ideal crystal lattices, without any defects or impurities. The electronic structure and chemical bonding in nonstoichiometric nitrides will be discussed in Chapter 4. 

There are only a few reports in which the appearance of a second nitrogen peak on the lower BE side of the N l5 line, with an energy separation between 0.9-1.4 eV is mentioned [45, 47, 54], In [54] the peak found at 395.8 eV correlates with the Ti 2p component for TiNiO,. In [47] the peak at 395.4 eV was assigned to a nonstoichiometric nitride. Only Miyagi et al. [45] explicitly related the low-BE shoulder to super stoichiometric TiN(+, in agreement with findings just described,... 

Nitrides. Uranium nitrides are weU known and are used in the nuclear fuel cycle. There are three nitrides of exact stoichiometry, uranium nitride [2565843-9], UN U2N3 [12033-85-1/ and U4N2 [12266-20-5]. In addition to these, nonstoichiometric complexes, U2N3, where the N/U ratio ranges... 

The binaiy hydrides (p. 64), borides (p. 145), carbides (p. 299) and nitrides (p. 417) are hard, refractory, nonstoichiometric materials with metallic conductivities. They have already been discussed in relation to comparable compounds of other metals in earlier chapters. 

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... 

This structure is commonly adopted by oxides, nitrides halides, and sulfides MX, including the nonstoichiometric 3d transition-metal oxides TiO, VO, MnO, FeO, CoO, and NiO. 

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

Heating the metal with ammonia at elevated temperatures (at about 700°C) yields nitrides of nonstoichiometric compositions. With nitric oxide, uranium is oxidized at about 400°C, forming triuranium octaoxide, UsOs-... 

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. 

In the Sm2Fei7Xy (X = C and/or N) materials, the Tc value and EMD are dominated by the content of X, which cause expansion of the Fe-Fe interatomic distance and enhance magnetic interactions. The carbide Sm2Fe17Cy consists of nonstoichiometric compounds with a wide range of carbon content and for y < 1 are stable at temperatures ( 1300 K) sufficient for sintering. On the other hand, Sm2Fe17N3, completely decomposes at such temperatures because no nitride with x < 1 is obtained. Similar effects on Tc and EMD are produced when Co substitutes for Fe, e.g. (Sm2Fe1 Co )l7C>,. 

Carbides and nitrides can be prepared in many ways (chemical vapour deposition, physical vapour deposition, precipitation of salts containing metal, carbon and oxygen followed by reduction and annealing, reaction of a metal or its oxides with a gas or with solid carbon). Carbides and nitrides are often nonstoichiometric with complex phase diagrams.4-9 The compounds sometimes contain multiple phases and impurities, notably oxygen. This can lead to even more complex compounds, like oxycarbides, carbonitrides or oxycarbonitrides. 

Because intermetallic systems undoubtedly display certain special features that follow from their metallic binding forces, considerable importance attached to the growing evidence that the chalcogenides, the essentially ionic oxides, the nitrides, and other representative binary compounds of the transition metals were, not infrequently, both variable and irrational in composition. Schenck and Ding-mann s equilibrium study of the iron-oxygen system (39) was notable in this connection They showed that stoichiometric ferrous oxide, FeOi 000, the oxide of an important and typical valence state, did not exist. It lay outside the broad existence field of a nonstoichiometric phase. It is, perhaps, still not certain... 

At elevated temperatures it combines with most nonmetals. With oxygen it gives V205 contaminated with lower oxides, and with nitrogen the interstitial nitride VN. Arsenides, silicides, carbides, and other such compounds, many of which are interstitial and nonstoichiometric, are also obtained by direct reaction of the elements. 

The phenomenon of superconductivity is common in several particular types of compounds. Thus more than two dozen binary compounds with the fee sodium chloride (NaCl) stracture are superconducting. The carbides AC and nitrides AN, such as NbN with Tc = 17 K, have the highest transition temperatures of this group, and the metallic A atoms with values above 10 K were Nb, Mo, Ta, W, and Zr. The NaCl-type superconductors are compositionally stoichiometric but not structurally so. hi other words, these compounds have a small to moderate concentration of vacancies in the lattice. For example, YS has 10% vacancies, which means that its chemical formula should properly be written 0,980.9. Nonstoichiometric NaCl-type compounds such as Tai.oCo.ye also exist. Ordinarily the vacancies are random, but sometimes they are ordered. 

NaCl-type phase with nonstoichiometric composition and y-MojN-type phase, are formed. The WC-type region and the other region can be separated simply by r lr, eg., in the region of larger than 0.53, only the WC-type phase is formed, in which there exists not only simple nitrides and carbides, such as WN, MoN, OsC and RuC, but also solid-solution compounds, such as MoN-NbN, MoN-TiN, TiN-CoN and TiN-NiN systems. It is of interest to note that Tio 7C00 3N and Tip yNif, 3N have the WC-structure while the end members, TiN, CoN and NiN do not take the WC structure. 

Hydrides, nitride and carbides are known for some of the elements. Some have simple stoichiometry and structure, such as TiN and TiC with the rocksalt structure. Many are nonstoichiometric with metallic properties, and some can be regarded as interstitial compounds with the nonmetal atom occupying sites between metallic atoms in the normal elemental structure. 

At high pressures (34CiPa), UN transforms from cubic rock salt structure [256]) to hexagonal symmetry. At 10 MPa, IJN, is known to form with the fluorite structure Ut = 5.31 A) [257]. UjN, has the bixbyite structure [257]. UN is paramagnetic with a susceptibility maximum at 53 K. The higher nitrides are nonstoichiometric and have antiferromagnetic ordering temperatures from 94 K to 8 K (UN,.55 to UN,., ) [258],... 

The binary systems actually and potentially important as nuclear fuel include oxides, carbides, nitrides, phosphides, and sulfides of uranium, plutonium, and thorium. An increasing amount of detailed information is becoming available on the phase equilibria of these compounds, but the relations existing between the composition (especially nonstoichiometric) and the vapor pressure (or activity) of each component are known only for a limited number of systems. 

Many of the oxides, carbides, and nitrides with the NaCl structure tend to be nonstoichiometric. Titanium monoxide exists over the range Tio.ssO to TiO, while FeO never occurs it is always nonstoichiometric with a composition ranging from Feo.goO to Feo.geO. As a consequence of these vacancies, the transition metal exists in two valence states, causing the oxide to exhibit semiconductor properties (as for NiO). 

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