The strangeness of boron as an element is that it generally behaves differently than other members of its family. Instead of acting like a metal typical of group 13, it forms structures that are rarely observed in those elements. It appears that boron tries to adopt forms that are typically seen in carbon compounds, but it lacks the necessary number of electrons to do so.
My question pertains to a structure that is commonly found in "metal-rich" borides which have formulas between MB2 and MB4. Many of these compounds adopt a layered structure in which boron forms a graphene-like hexagonal layer, and the metal atoms are sandwiched in between the hexagonal layers. In terms of hardness, these compounds (like AlB2, TiB2, and CrB4), lie close to or just above the superhard cutoff.
My question is the following; since boron cannot form graphene-like layers because it is electron deficient when compared to carbon, how many electrons does boron need from the metal atoms to be able to form these hexagonal sheets in metal borides?
Hi Edgar,
that is a great question. Empirically between two and six electrons are needed, since both MgB2 and CrB2 exist in the AlB2-type structure, while MnB2 is only stable at high-temperature. However, things are more complicated than that: Ionization energy, atomic radius and electron configuration affect if a metal-rich borides exists or not. If metals with large/small radii are used, a combination of 5-, 6- and 7-member rings are formed in the B-layer (see ScFeB4 for example).
In general, most metal-rich borides contain boron-centered trigonal metal-prisms and it seems that the formation of such prisms requires metal atoms with accessible d-orbitals. However, there is no explaination for this observation yet.
I should add that we cannot assume full charge transfer from the metal to the boron layer. If there is any charge tranfer, it is a partial charge transfer at best. The largest contribution to the metal-boron bonds is in fact covalent. Which is why the valence electron range for AlB2-type borides is quite large.
Yeah, I have seen stoichiometries ranging from MB2 to MB4.
One of the reasons why I am interested in that question is because these MB2-MB4 compounds are not only very hard but some (or all of them) are also electrically conductive.
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