Rare metal diborides

The rare metal Rh lies adjacent to Os on the Periodic Table. Like Os, it forms a hexagonal diboride that is even harder. Its hardness is about 4800kg/mm2 (Chung et al., 2007), while its VED is about 0.477 elec./A3. 

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

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). 

Among the tetraborides, UB4 has the smallest volume and hence the smallest effective radius. Thus an actinide element having a metallic radius of 1.59 A (Pu) or smaller forms a diboride, while those having larger radii do not. As in the rare-earth series, the actinides able to form MB4, MBg and MB,2 borides form also MB2 diborides (Table 1). 

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. 

SiC improve their mechanical properties [58]. As mentioned above, some transition metals (Hf) and rare earths (Dy, Eu, Gd, Er) are efficient neutron absorbers, leading to quite high absorption efficiency. As such, they have for a long time been considered as potential neutron absorbers [59]. Among different compounds (diborides, tet-raborides, hexaborides), the metal diborides present the best behavior under irradiation [60]. It is noted that the more the structure is closed (that is, from hexaborides to diborides), the better is the helium retention (Table 15.5). However, most of the materials cracked this requiring efficient containment with strong mechanical resistance. 

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