A class of hard materials incorporating borides of transition metals are easily cut yet rival diamond in terms of hardness. In the solids of the three borides: ReB2, TiB2 and OsB2, the boron sublattice is a chair, is planar, and is a boat, respectively. The anion ReB2- has three competing configurations involving the B-B bonding, in contrast to TiB2- and OsB2- which only have one configuration. This led to the determination that the promiscuity of the metal-boron bonds is linked to the ultrahard nature of ReB2. The combined anion photoelectron spectroscopic and theoretical study was able to show that ReB2 is, in fact, an ultrahard boride with promiscuous metal-boron bonding, while TiB2 lacks this promiscuity and is thus only superhard; this model was extended to OsB2 and concludes that it is also superhard.

Hardness of transition metal boride materials

256th National Meeting (2018)

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