Diborides structure

Figure 3. Comparison of boron net-type diboride structures, ThMoB4, YCrB4, Y2ReBg, ErNiB4, Er4NiB,3, and AIB2 projected along [001].

Thermodynamic considerations postulate BjH to be a better boron source than BCl, in CVD of TaB2Using reaction (f) at < 1200 K deposits with extremely small crystal sizes are obtained on graphite substrates . They contain amorphous B at deposition temperatures < 873 K and are substoichiometric in B above this T. Carbon from the substrate substitutes for B, thereby stabilizing the diboride structure at high deposition T... 

For the late TM diborides the 5(M, M ) 0.15, which is comparable with the corresponding values in elementary metals. Thus, the bonding picture in these diborides can smoothly vary from covalent delocalized bonding inside the boron sheets combined with ionic one between these sheets and metals to the covalent bonding between M and B as well as between metal atoms in all three spatial directions, which explains remarkable chemical flexibility of these diboride structure type. The weakening of the B-B bonds within the sheet for the late transition metals can explain the appearance of distorted boron sheets, observed in the crystal structures of some 4d-metal diborides. Enhanced interaction between metal atoms in two and three directions is likely related with the... 

For the larger rare-earth metals ternary diborides are formed, having tKe CeCr2B6 structure. Here puckered B nets are incompletely developed, with Bj groups in the form of loosely attached 6 -network fragments (B—B 210 pm). There is a resemblance to the structures of W3lrB6 and W2C0B2 (see Fig. 4). 

A rather definite range in r, is associated with each structural type, but there is a broad zone of overlap (1.58 10 pm < r < 1-98 10 pm) in which more than one structural type is observed. Particularly large atoms (r, > 1-97 10 pm) form only hexaborides and the smallest atoms (r, < l.SS 10 pm) only diborides. Metal atoms for which 1.60 10 pm < -82 10 pm form borides in four structural types. 

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. 

The crystal structures of these rare metal diborides are similar to Figure... 

The molecular structure of the mixed acetylacetonate-acetate B(MeCOCHCOMe)(02CMe)2 consists of discrete units having approximate C2v symmetry. This compound is notable for its method of preparation, having been obtained from a bizarre reaction of vanadium diboride with acetic acid.74... 

The Lifshitz parameter z as a function of x in the case of the A1 and Sc substitutions for Mg in the Mgi xAlxB2, and Mgi xScxB2 systems and for the C for B substitution in the MgB2.xCx system has been calculated by R. De Coss et al. by band structure calculations described elsewhere [204] therefore it has been possible to convert the variation of the critical temperature as a function of the number density of substituted ions x to the variation of Tc versus the universal reduced Lifshitz parameter z for all doped magnesium diborides. 

Five phases of titanium boride have been reported. TiB2 [12405-65-35], TL,B [12505-68-9], TiB [12007-08-8], TL,B5 [12447-59-5], and TiB12 [51311-04-7]. The most important of these is the diboride, TiB2, which has a hexagonal structure and lattice parameters of a = 302.8 pm and c = 322.8 pm. Titanium diboride is a gray crystalline solid. It is not attacked by cold concentrated hydrochloric or sulfuric acids, but dissolves slowly at boiling temperatures. It dissolves more readily in nitric acid/hydrogen peroxide or nitric acid/sulfuric acid mixtures. It also decomposes upon fusion with alkali hydroxides, carbonates, or bisulfates. 

ZS-7. [Advanced Refractory Tech.] Zirconium diboride for oxidation-resistant ctxnposites, burble absorber of neutrons, elec, contacts, mdten metal crucibles, refractory roughener, cutting tool composites, structural ceramics, wear conqxxients, metal matrix omi-posites. 

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. 

A. C. Switendick, Electronic Structure and Charge Density of Zirconium Diboride, in Boron-rich Solids , eds. D. Emin,... 

Intentionally, the neutron-radiation treatment is nsed to obtain structural disorder in magnesium diboride, MgB2, which is an interesting high-temperature superconducting material. The temperature... 

---