Binary nitrides, group

In the last decade, numerous compounds of these types have been the subject of detailed CVD studies, demonstrating their potential for the deposition of the corresponding binary materials. Most of the work has concentrated on binary nitrides and phosphides, while the deposition of binary MSb films has been studied to a far lesser extent. The lack of potential precursors has been the major problem for the deposition of group 13-antimonide films for many years. Only a very few group 13-Sb compounds have been known until we and Wells established general synthetic pathways as was shown in Sections 2 and 3. Consequently, detailed investigations concerning their potential to serve for the deposition of the desired materials... 

Alkaline earth-containing ternary nitrides make up the second largest group of ternary phases. Because the alkaline earth metals form stable binary nitrides, most alkaline earth containing ternary nitrides have been synthesized by the reaction of a binary nitride with a metal or by the reaction of two binary nitrides. This synthesis has resulted in a number of new ternary nitrides with a variety of structures. For example, the reaction of calcium nitride with Group 14 or 15 metals or metalloids forms a series of structurally related ternary nitrides with the anti-perovskite type structure. In Ca3MN (M = P, As, Sb, Bi, Ge, Sn, Pb) (Figure 8.5) the... 

Because of the evident structmal similarities between transition metal carbides and transition metal nitrides the carbon atoms in group 4 and 5 carbides can be replaced completely by nitrogen without changing the structme of the binary phases. So far only one distinct ternary phase Cr2 (C,N)2 has been reported. Intersolubihty between the binary nitrides and carbides in the group 6 carbonitride systems Cr-C-N and Mo-C-N is not complete because of the differences in the crystal structmes of the carbide and nitride phases. 

Cesium also forms no binary nitride. Its azide forms with the KHF, structure [85], and its low-temperature amide (below 50 C ) [95] forms with a tetragonal distortion of the CsCl structure. The high-temperature form (above 35 C) of CsNH, has the cubic CsCl structure with a rotationally-disordered NH, group. The hydrogen atoms have not been located for the amides. CsTaN, has been reported with a disordered structure resembling (i-cristobalite [82], Yellow CsOsO, N has the BaS04 structure [96]. 

Air-sensitive Ca3CrN3 can be made from reaction of the binary nitrides at 1350 C in sealed Mo tubes [131], The structure contains planar CrN, groups and CaNj square pyramids N atoms center Ca5Cr octahedra. The anion-centered description of the structure begins from the rocksalt structure viewed down a 4-fold axis. If one removes 1 /4 of the octahedra in a layer and then shears the layers, one obtains a layer of the Ca3CrN, structure. The layers are joined by edge-sharing as shown in Fig. 17. 

Figure 26. The crystal structures of y -Fc4N and Ti3AlN (space group Pm3m). In the binary nitride the nitrogen atoms occupy one quarter of the octahedral voids formed by the cubic close-packed iron atoms. In the ternary compound the nitrogen atoms fill those octahedral voids which are formed solely by the titanium atoms.

Ternary nitrides have recently been extensively studied, because they may display a wider variety of useful properties compared to the binary nitrides. The majority of reported ternary nitrides contain an electropositive element along with a transition metal or main-group metal [245]. The electropositive element (e.g., Ca, Sr, Ba, Mg) is included to increase the stabUity of the nitride, via the inductive effect [246]. 

The binary compounds of the Group 13 metals with the elements of Group 15 (N, P, As, Sb, Bi) are stmcturally less diverse than the chalcogenides just considered but they have achieved considerable technological application as III-V semiconductors isoelectronic with Si and Ge (cf. BN isoelectronic with C, p. 207). Their stmctures are summarized in Table 7.10 all adopt the cubic ZnS stmcture except the nitrides of Al, Ga and In which are probably more ionic (less covalent or metallic) than the others. Thallium does not form simple compounds... 

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. ... 

The binary borides (p. 145), carbides (p. 299), and nitrides (p. 418) have already been discussed. Suffice it to note here that the chromium atom is too small to allow the ready insertion of carbon into its lattice, and its carbide is consequently more reactive than those of its predecessors. As for the hydrides, only CrH is known which is consistent with the general trend in this part of the periodic table that hydrides become less stable across the d block and down each group. 

Silicon-containing ceramics include the oxide materials, silica and the silicates the binary compounds of silicon with non-metals, principally silicon carbide and silicon nitride silicon oxynitride and the sialons main group and transition metal silicides, and, finally, elemental silicon itself. There is a vigorous research activity throughout the world on the preparation of all of these classes of solid silicon compounds by the newer preparative techniques. In this report, we will focus on silicon carbide and silicon nitride. 

Most nonmetallic elements will react with the group IA and IIA metals to give binary compounds. Heating the metals with nitrogen or phosphorus gives nitrides and phosphides of the metals. 

Figure 7.15 (a) Enthalpy of formation of ternary oxides and nitrides from their binary constituent compounds as a function of the ratio of ionic potential [16]. Reprinted with permission from [16] Copyright (1997) American Chemical Society, (b) Gibbs energy of the oxide-sulfide equilibrium for group 1 and 2 metals at 1773 K as a function of the optical basicity of the metal. 

In the group, the most familiar members are the oxides, halides, hydrides (including the hydrocarbons), nitrides, sulfides, and carbides. Many methods are available for the preparation of binary compounds, and the most general ones will be illustrated by exercises. 

The crystal structures adopted by the binary carbides and nitrides are similar to those found in noble metals. The resemblance is not coincidental, and has been explained using Engel-Brewer valence bond theory [5]. Briefly, the main group elements C and N increase the metal s effective s-p electron count, so that structures and chemical properties of the early transition metals resemble those of the Group 8 metals. This idea was first introduced by Levy and Boudart [6] who noted that tungsten carbide had platinum-like properties. 

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. 

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