Harder than diamond: Rhenium diboride

In the recent issue of Science, researchers from UCLA (Chung et al) report on an ambient pressure synthesis (using arc melting) of a compound, namely, rhenium diboride, which is superhard. Apparently, the material leaves scratch marks on the surface of diamond. Here is the abstract of the paper:

• The quest to create superhard materials rarely strays from the use of high-pressure synthetic methods, which typically require gigapascals of applied pressure. We report that rhenium diboride (ReB2), synthesized in bulk quantities via arc-melting under ambient pressure, rivals materials produced with high-pressure methods. Microindentation measurements on ReB2 indicated an average hardness of 48 gigapascals under an applied load of 0.49 newton, and scratch marks left on a diamond surface confirmed its superhard nature. Its incompressibility along the c axis was equal in magnitude to the linear incompressibility of diamond. In situ high-pressure x-ray diffraction measurements yielded a bulk modulus of 360 gigapascals, and radial diffraction indicated that ReB2 is able to support a remarkably high differential stress. This combination of properties suggests that this material may find applications in cutting when the formation of carbides prevents the use of traditional materials such as diamond.

The commentary at NewScientist (via /.) indicates the cost benefits of the ambience pressure synthesis, as well as the physics behind the design of the new material, namely, the optimization of high valence-electron density coupled with strong directionality (covalent) of bonds. Take a look!