Dreaming up crystals that outdo diamond.

Dreaming up crystals that outdo diamond

After envisioning their dream house and drawing up blueprints, a couple can assemble the dream into a reality. Following a similar course, scientists with a theoretical bent have used a supercomputer to draw up molecular plans for crystalline materials that, if actually made, could surpass the world-record hardness of diamond.

Harder-than-diamond materials would outperform diamond at the jobs it does now, such as protecting the surfaces of drill bits and other cutting tools. The superhard materials might also enable engineers to machine diamonds into intricate shapes -- a feat beyond any existing material -- for electronic and exotic applications.

Using a simple mathematical model of material hardness and supercomputer calculations of physical features of simulated crystals, physicist Marvin L. Cohen and graduate student Amy Y. Liu of the University of California, Berkeley, predict in the Aug. 25 SCIENCE that new solids made of carbon and nitrogen could prove at least as hard as all-carbon diamond.

"We feed in some information about the atoms, and then go through a long computer calculation to get out the information about the solid," Cohen says. but before committing to expensive calculations, Cohen uses a simple equation he developed earlier that describes how a material's compressibility relates to its constituent atoms and the length of the bonds connecting them. The equation indicates that the shorter the distance between the atoms and the more equally, or covalently, the atoms share their bonding, or outermost, electrons, the harder the solid.

Covalent solids made of carbon and nitrogen might be superhard, Cohen says, because carbon-nitrogen bonds are shorter than the carbon-carbon bonds in diamonds. The tougher task involves figuring out which of the many possible crystal structures will yield superhard solids. After ruling out simpler but unstable structures, Cohen and Liu used a supercomputer to analyze a possible carbon-nitrogen structure based on a real crystal made of hexagonally arranged silicon and nitrogen atoms in a 3:4 ratio.

The calculations suggest that a carbon version of the real crystal, once formed, would have enough cohesive energy to remain intact. They also indicate the crystal would be just shy of diamond hardness, though Cohen suspects even harder carbon-nitrogen arrangements exist. Berkeley experimentalist Raymond Jeanloz has begun preliminary attempts to realize the dream crystal by squeezing carbon and nitrogen ingredients at high pressures in a diamond anvil press while heating them with lasers.