Substitutional Metal Borides

Table 2. Structure Types, Boron Coordination and Representatives of Metal Borides (Substitutional Types and CaCu5 Structure Family)... 

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 - " . 

Line compounds. These are phases where sublattice occupation is restricted by particular combinations of atomic size, electronegativity, etc., and there is a well-defined stoichiometry with respect to the components. Many examples occur in transition metal borides and silicides, III-V compounds and a number of carbides. Although such phases are considered to be stoichiometric in the relevant binary systems, they can have partial or complete solubility of other components with preferential substitution for one of the binary elements. This can be demonstrated for the case of a compound such as the orthorhombic Cr2B-type boride which exists in a number or refractory metal-boride phase diagrams. Mixing then occurs by substitution on the metal sublattice. 

TaB -MoSi composite This composite started to shrink at 1530°C, but at 1680°C no further movement of the rams was measured. The X-ray diffraction pattern of TBM composite showed the presence of hexagonal TaB and traces of MoSij and quartz. The TaB lattice parameters were the same as those reported in the PDF card 65-1462, no evidence of cell shrinkage could be observed, in contrast to the previously presented hexagonal transition metals borides containing MoSi or TaSij. Indeed, the atomic radii of tantalum and molybdenum are very close, 1.43 and 1.36 A, respectively hence, even if Mo substituted in Ta sites in the TaB lattice, it would be very difficult to detect a peak shift by this technique. 

Despite the occurrence of binary AIB2 borides (see also Fig. 2), no ternary representatives are known (Mn, Mo)B2 found from isothermal sections is a stabilized high-T phase by conversion to lower T by a statistical ( ) metal-metal substitution. Both MnB2 and M0B2 are high-T compounds stable above 1075°C and 1517°C respectively WB2 is claimed but is either metastable or impurity stabilized. Similar examples are observed with (W, Pd>2B5 (M02B5 type) as well as (Mo, Rh),, (B3 and (W, Ni), B3 (Mo,., 83 type). The phase relations in the B-rich section of the Mo(W)-B binaries, however, are not known precisely. 

The structures of many binary and ternary scandium borides and carbides may be deducted from other structure types by a multiple substitution of larger atoms by B-B or C-C pairs or by the inclusion of B or C atoms into the cavities of the parent structure types. The same is valid for the structures of scandium intermetallics with the 6B elements. The Ti7Si2, NiAs, ErAgSc2 and Al2Mg04 structure types can be obtained by an inclusion of cations of metals into the octahedral and tetrahedral cavities of different packings (usually close ones) of 6B anions. Frequently the inclusion of atoms causes a distortion of the parent close-packed structure. 

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