A stick-and-ball structure of atoms arranged in linked pentagons, and its projection onto three orthogonal planes.

The central pore in a pentadiamond crystal (pictured) makes the material much less dense than diamond — although of similar hardness. Credit: Yasumaru Fujii, Mina Maruyama, Nguyen Thanh Cuong, Susumu Okada

Materials science

This material is almost as hard as diamond — but as light as graphite

A theoretical form of carbon called pentadiamond becomes thicker when stretched.

A new addition to the carbon family, pentadiamond, is predicted to be light like graphite, hard like diamond and semiconducting like silicon.

Carbon is an extremely versatile element: it has hundreds of hypothetical forms and several observed varieties, including diamond and graphite. In diamond, every carbon atom connects to four others in a 3D cubic lattice that is strong and hard. But in graphite, each atom bonds only to three others, forming flat sheets of hexagons.

Yasumaru Fujii and his colleagues at the University of Tsukuba, Japan, calculated that carbon could also take a form they call pentadiamond. Its carbon atoms are organized in pentagons, in which some atoms are bonded to four others, and others to three. The pentagons are arranged in a cubic lattice similar to diamond’s. But diamond has carbon atoms inside each cube of the lattice, whereas pentadiamond’s rings occupy only a cube’s edges and corners — leaving a pore in the middle.

This porosity means that pentadiamond’s predicted density is low, on a par with graphite’s. The material should also be a semiconductor and exhibit the unusual property of becoming thicker when stretched.