Main Group Element Carbides

The occurrence of the binary borides of the alkaline, alkaline earth, aluminum, and transition elements has been collected in Table 1, together with boron compounds of the right main group elements (carbides, etc.). Only relatively well-established phases have been included. Noncorroborated and/or badly characterized borides lacking precise composition and structure data are not included. The reader is referred to other sources for references. There are no binary borides among the Cu, Zn, Ga, and Ge group elements with the exception of a noncorroborated early report on diborides in the Ag-B and Au-B systems. Two silicon borides have been established, namely, SiB3 4 and SiBe. 

III. POLYMERIC ROUTES TO MAIN GROUP ELEMENT CARBIDES... 

For carbides and nitrides containing more than one transition metal, or a transition metal and another element, the compositions and structures are more varied and complex. Plate 1.3 shows examples where the secondary component is a metal or main group element. Figure 1.2 shows examples where one of the secondary components is an alkali or alkaline earth... 

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. 

Other methides are the cubic ternary carbides of the type A3MC31 where A is mostly a rare earth or transition element (e.g., Sc, Y, La—Na, Gd—Lu) and M is a metallic or semimetallic main group element (e.g., Al, Ge, In, Tl, Sn, Pb). These perovskite carbides are typically hydrolyzed by dilute HC1 to give —84-97 (wt-%) methane and 3-16% saturated and unsaturated higher hydrocarbons. 

There are several families of ternary carbide phases with transition metals and main group elements. Several of them are of structural interest such as the H-phases, the filled /i-manganese type carbides and the x-carbides. The r]-phases are formed in carbon-deficient hardmetals and cause embrittlement. 

Until now, polymeric routes to transition metal nitrides or carbides have not been as numerously reported in the literature as those of the main group elements. They have been developed in most cases to produce thin coatings on various substrates or fibers. 

All precursors are amorphous up to calcination temperatures of around 600°C. At higher temperatures, in most cases powders with extremely small crystallite sizes of around 20-40 nm are formed (Fig. 7). A further increase in calcination temperature promotes crystal growth. With aluminum nitride, a white powder with a low oxygen and carbon content is obtained [97]. Other main group element precursors exhibit fairly different behaviors Mg and Ca precursors yield metal cyanamide [99]. Calcination of the transition element precursors (Fig. 8) results in the formation of nitrides, carbonitrides, or carbides. For the titanium-containing precursors, TiN/TiC solid solutions can be obtained [96] the quantity of carbon strongly depends on the calcination atmosphere applied (argon, 31 wt% ammonia, 5.1 wt%). 

Metals, both pure and alloyed, consist of atoms held together by the delocalized electrons that overcome the mutual repulsion between the ion cores. Many main-group elements and all the transition and inner transition elements are metals. They also include alloys—combinations of metallic elements or metallic and nonmetallic elements (such as in steel, which is an alloy of primarily Fe and C). Some commercial steels, such as many tool steels, contain ceramics. These are the carbides (e.g., FeaC and WgC) that produce the hardening and enhance wear resistance, but also make it more brittle. The delocalized electrons give metals many of their characteristic properties (e.g., good thermal and electrical conductivity). It is because of their bonding that many metals have close packed structures and deform plastically at room temperature. 

We shall discuss in more detail only the synthesis of the exposed and interstitial carbides, since this class of clusters is one of the most investigated and well developed. Moreover, their syntheses require methods which have been, or can likely be, extended to other main group elements. 

In both metal lattices and closo polyhedral metal clusters there are cavities present, whose dimensions are a function of the number of vertices, the shape of the polyhedral moiety, and the interatomic separation. Partial occupation of these sites within the metal lattice by main group elements results in the formation of interstitial alloys such as, for example, metal hydrides, carbides, and nitrides. Occupation of the cavity within a molecular metal cluster gives rise to interstitial clusters. Although close topological relationships are sometimes found between interstitial alloys and interstitial clusters, the greater degree of freedom of a molecular assembly of metals, in comparison to a three dimensional infinite array of metals, increases the possible number of interstitially lodged elements... 

M5 and M6E are typically encapsulated structures and similarly to M4E species are accessible only when E is a first row element. Larger main group elements are not appropriate for these cavities. Selected examples of clusters with stoichiometries M4E, M5E, and M6E are shown in Table 3.2. The most important family of clusters with interstitial or encapsulated atoms are by far the carbides. 

Cluster of the series M5E often adopt a square-based pyramidal geometry with the main group element in the center of the square base. Such compounds often have seven skeletal electron pairs and are considered to be m do-octahedral molecules. The main group element often acquires a position slightly below the square of metal atoms. The position of the carbon atom in carbides apparently depends upon the charge of the complex. Thus in the compound Fe5(CO)i5C the carbon atom lies 0.09 A below the square base meanwhile in the anion [Fe5(CO)i4C] such deviation grows up to 0.18 A. In the series of compounds Ru5(CO)i5 x (PPh3),C these distances are 0.11,0.19, and 0.23 A for X = 0, 1, and 2 respectively. 

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