Transition metals borides

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. 

Although boron forms borides with many elements, only the borides of the transition metals have been investigated extensively for their CVD characteristics. Boron forms stable borides with the transition metals, and the most refractory of these and those with the greatest potential interest are the borides of the elements of Groups IVa (Ti, Zr, Hf), Va (V, Nb, Ta) and, to a lesser degree. Via (Cr, Mo, W) (see Table... 

Pierson, H., A Survey of the Chemical Vapor Deposition of Refractory Transition Metal Borides, in Chemical Vapor Deposited Coatings, pp. 27-45, Am. Ceram. Soc. (1981)... 

Selected Chemical Properties of Transition-Metal Borides... 

Figure 1. Formation of ternary borides MreMj3B2 and different structure types (Mre = rare-earth element, M-p = transition-metal element). , CeCo3B2 type ErIr3B2 type O, URujBj type El, Ndo7,Rh3 29B2 type IS, YOS3B2 type B, Laofi3Rh3B2 type , compound formation observed, but structure type unknown. Refs a , b , c , d e , f g , h , i , j ", k , 1 , m , r, s - , t u = 45 see also ref. 62.

Binary and ternary structure types with isolated B atoms are listed in Table 1. In the metal borides of the formula (My, Mi ),B or T,(B, E) (M-p, M - = transition metals, E = nonmetal), the influence of the radius ratio as well as the... 

Among metal borides of the formula MjM B or (Mj, M/r)2B, the competing structural units are (a) the antiprism and (b) the trigonal metal prism. In many cases the CUAI2 structure with BMg-antiprismatic B coordination is adopted in close resemblance to transition-metal silicides, but no boron-carbon substitution is ob-served - " . 

The formation of higher transition-metal borides depends on the competition and the statistical weight of the d and states of the metal atoms. Consequently, the acceptor metals Ni, Pd and Pt are expected to form metal-rich borides only i.e., besides the known PtBo.7 (anti-NiAs), Pt forms two borides, Pt 2B and Pt 4B, which... 

Because they exhibit interplay of magnetic and superconducting properties, the formation and crystal chemistry of MRgMy4B4 compounds have been examined. Ternary rare-earth and actinide (Th, U, Pu)-transition metal borides of the approxi-... 

Figure 2. Formation of ternary borides MREMT4B4 (Mre = rare-earth element, Mj = transition metal) and different structure types. B, CeCo4B4 type B, LURU4B4 type a, NdCo4B4 type H, YOS4B4 type 8, Sm,4.eFe4B4 type MRcRe4B4 type , LuRh4B4type. Refs b, c , d", e, f , g , h, il l ...

In this method " - the melt eontains boric oxide and the metal oxide in a suitable electrolyte, usually an alkali or alkaline-earth halide or fluoroborate. The cell is operated at 700-1000 C depending on electrolyte composition. To limit corrosion, the container serving as cathode is made of mild steel or of the metal whose boride is sought. The anode is graphite or Fe. Numerous borides are prepared in this way, e.g., alkaline-earth and rare-earth hexaborides " and transition-metal borides, e.g, TiBj NijB, NiB and TaB... 

This method is used extensively in the laboratory because it is particularly suitable for preparing borides of rare or expensive metals, e.g., the transition-metal-rich borides CrB, Cr3B4, CrB2 (except Cr3B2 and Cr4B), the diborides ScB2, TiB2 the rare-earth hexaborides, dodecaborides and MB -type borides. 

This method is the simplest and cheapest for making borides. It is used to prepare transition-metal borides and alkaline-earth metal hexaborides. 

The reduction of a transition-metal oxide and boron oxide by an electropositive metal such as Al, Mg or an alkali metal has been used as a pathway to titanium, iron, chromium, tungsten and alkali-earth borides . ... 

These methods deal with specific cases. The list of examples is not exhaustive. The low-T (200-300°C) decomposition of the transition-metal borohydrides M(BH4> , e.g., leads to titanium, zirconium, halfnium, uranium and thorium borides . Alternatively, the uranium diboride may be obtained by reacting uranium hydride with diborane in hydrogen at 200-400°C. 

The compressibility of group-IVA and -VIA transition-metal boride powders is measured by the dimensions and weights of the blanks, by measuring the stroke of the punches with a cathetometer, or alternatively by electrical conductivity (based upon the metallic conductivity of most borides). The process of densiheation by pressing is defined by ... 

Tables 1 to 5 summarize the sintering eharaeteristies of borides of groups III A to VIA transition metals.

The small atoms at the center of the first row of the Periodic Table (B, C, N, O, and to a lesser extent Al, Si, and P) can fit into the interstices of aggregates of larger transition metal atoms to form boride, carbide, and nitride compounds. These compounds are both hard and moderately good electronic conductors. Therefore, they are commonly known as hard metals (Schwarzkopf and Kieffer, 1953). 

The prototype hard metals are the compounds of six of the transition metals Ti, Zr, and Hf, as well as V, Nb, and Ta. Their carbides all have the NaCl crystal structure, as do their nitrides except for Ta. The NaCi structure consists of close-packed planes of metal atoms stacked in the fee pattern with the metalloids (C, N) located in the octahedral holes. The borides have the A1B2 structure in which close-packed planes of metal atoms are stacked in the simple hexagonal pattern with all of the trigonal prismatic holes occupied by boron atoms. Thus the structures are based on the highest possible atomic packing densities consistent with the atomic sizes. 

The hardest of the transition-metal borides are the diborides. Their characteristic crystal structure (Figure 10.6) consists of plane layers of close-packed metal atoms separated by plane openly-patterned layers of boron atoms ( chicken-wire pattern). If the metal atoms in the hexagonal close-packed layer have a spacing, d, then the boron atoms have a spacing of d/V3. 

Borides of Some Transition Metals. J. chem. Physics 20, 1050 (1952). 

Considering other families of similar compounds, the contributions given by Guillermet and Frisk (1992), Guillermet and Grimvall (1991) (cohesive and thermodynamic properties, atomic average volumes, etc. of nitrides, borides, etc. of transition metals) are other examples of systematic descriptions of selected groups of phases and of the use of special interpolation and extrapolation procedures to predict specific properties. 

Parth6, E. and Chabot, B. (1984) Crystal structures and crystal chemistry of ternary rare earth-transition metal borides, silicides and homologues. In Handbook on the Physics and Chemistry of Rare Earths, ed. Gschneidner Jr., K.A. and Eyring, L. (North-Holland, Amsterdam), Vol. 6, p. 113. 

---