Transition Element Carbides

Table 8.3 Stoichiometries of some transition element carbides...

V. POLYMERIC ROUTES TO TRANSITION ELEMENT CARBIDES AND NITRIDES... 

Transition element carbides and nitrides are applied as cutting tools because of their extreme hardness and wear resistance. In some cases nitrides and carbides (e.g., of titanium) form solid solutions over the entire compositional range other transition metal nitrides and carbides exhibit fairly different structures and are not completely soluble. Carbon contents within the range of few percentage points usually do not influence the mechanical properties of transition metal nitrides, and vice versa. Hence, completely carbon-free nitrides or nitrogen-free carbides are not required, especially for the titanium compounds. 

Jones, D.W., McColm, I.J., Yerkess, J., Clark, N.J., Carbon Species in the Crystal Structures of Uranium-Transition-Element Carbides, UMC2 , 7. Solid State Chem., 74, 304-313 (1988) (Crys. Stmcture, Experimental, 27)... 

Attempts to classify carbides according to structure or bond type meet the same difficulties as were encountered with hydrides (p. 64) and borides (p. 145) and for the same reasons. The general trends in properties of the three groups of compounds are, however, broadly similar, being most polar (ionic) for the electropositive metals, most covalent (molecular) for the electronegative non-metals and somewhat complex (interstitial) for the elements in the centre of the d block. There are also several elements with poorly characterized, unstable, or non-existent carbides, namely the later transition elements (Groups 11 and 12), the platinum metals, and the post transition-metal elements in Group 13. 

The lanthanoids also form metal-rich carbides of stoichiometry M3C in which individual C atoms occupy at random one-third of the octahedral Cl sites in a NaCl-like structure. Several of the actinoids (e.g. Th, U, Pu) form monocarbides, MC, in which all the octahedral Cl sites in the NaCl structure are occupied and this stoichiometry is also observed for several other carbides of the early transition elements, e.g. M = Ti, Zr, Hf V, Nb, Ta Mo, W. These... 

Silicides of groups I and 2 are generally much more reactive than those of the transition elements (cf. borides and carbides). Hydrogen and/or silanes are typical products e.g. ... 

Carbides produced by CVD include the refractory-metal carbides and two important non-metallic carbides boron carbide and silicon carbide. The refractory-metal carbides consist of those of the nine transition elements of Groups IVa, Va, and Via and the 4th, 5th, and 6th Periods as shown below in Table 9.1. 

Borides, in contrast to carbides and nitrides, are characterized by an unusual structural complexity for both metal-rich and B-rich compositions. This complexity has its origin in the tendency of B atoms to form one- two-, or three-dimensional covalent arrangements and to show uncommon coordination numbers because of their large size (rg = 0.88 10 pm) and their electronic structure (deficiency in valence electrons). The structures of the transition-element borides are well established " . 

Perspectives for fabrication of improved oxygen electrodes at a low cost have been offered by non-noble, transition metal catalysts, although their intrinsic catalytic activity and stability are lower in comparison with those of Pt and Pt-alloys. The vast majority of these materials comprise (1) macrocyclic metal transition complexes of the N4-type having Fe or Co as the central metal ion, i.e., porphyrins, phthalocyanines, and tetraazaannulenes [6-8] (2) transition metal carbides, nitrides, and oxides (e.g., FeCjc, TaOjcNy, MnOx) and (3) transition metal chalcogenide cluster compounds based on Chevrel phases, and Ru-based cluster/amorphous systems that contain chalcogen elements, mostly selenium. 

Hagg, G. 1931. Regularity in crystal structure in hydrides, borides, carbides and nitrides of transition elements. Z. Physik. Chem. 12B 33-56. 

The transition elements Nb, Zr, Ti, Ta are able to reduce to metals the actinide carbides (these last obtained by carboreduction of their oxides in vacuum). The actinide... 

With purely ionic compounds, appropriate ionic radii must evidently be compared. Complications arise, however, with compounds formed by a metal with a non-metallic element, having partly covalent bonds. Though the values of covalent radii are available as well,152 153 the precise nature of the chemical bond in any particular chemical compound is usually not known. It is yet unclear whether the tabular values can be used to predict the mobility of the components, for example, in the crystal lattices of transition metal carbides, borides or silicides. 

Table 1. Heat and free energy of formation from the elements of transition metal carbides (Mmol-1) calculated from data in Ref. 22.

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. 

Many binary transition metal carbides, especially the 5-phases of group 4 elements, exist over a broad range of composition with an upper limit of the carbon to metal ratio near 1. Practically all solid-state properties show a gradual change with the [C]/[T] ratio (T = transition metal). Only some carbides such as tungsten monocarbide, WC, and the chromium carbides have a very narrow homogeneity region. 

The stmeture of transition metal carbides are closely related to those of the transition metal nitrides. However, transition metal carbides feature generally simpler stmeture elements as compared to the nitrides. In carbides, the metal atoms are arranged in such a way that they form close-packed arrangements of metal layers with a hexagonal (h) or cubic (c) stacking sequence or with a mixtme of these (see Nitrides Transition Metal Solid-state Chemistry). The carbon atoms in these phases occupy the octahedral interstitial sites. A crystallochemical rule claims that the phases of pure h type can have a maximum carbon content of [C]/[T] = 1/2 and the c type phases a maximum carbon content of [C]/[T] = 1 hence in stractures with layer sequences comprising h and c stractme elements the maximum nonmetal content follows suit. 

Transition metal carbides can be prepared by direct combination of the elements, preferably as powder mixtures, at elevated temperatures. If the heat generated by the reaction... 

The hydrid( S, borides, carbides and nitrides of the transitional elements have metallic properties. Only atoms with small covalent radii are capable of occupying the interstices in relatively close-packed arrangements ... 

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. 

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