Focus: Levitating Liquid Boron
Probing the innards of a material to determine its structure is no problem for physicists, but they run into trouble if that material is a liquid which attacks any container. That difficulty was recently overcome by ingeniously eliminating the need for a container. Now, in the 20 July PRL, a team sheds some light on the structure of liquid boron, by performing the first successful x-ray diffraction on this complex material’s liquid form–as it floats on a cushion of gas.
As long ago as 1913, physicists learned that passing x rays through crystalline substances gives a remarkably well defined diffraction pattern which tells them the positions of individual atoms in the crystal. Boron is one of the most complicated of all crystals; in its most common form, the unit cell contains 12 atoms arranged at the corners of an icosahedron, a regular solid with 20 faces. In 1995, researchers simulated the structure of solid and liquid boron on a computer and concluded that these icosahedra were destroyed upon melting. This result contradicted some indirect evidence that this unusual material maintains some short-range order in the liquid state. The presence of icosahedra would affect properties like the electrical conductivity of liquid boron. But directly probing very hot liquids with x rays is extremely difficult because they invariably become contaminated by melting their containers.
Liquid boron is particularly uncooperative: not only does it melt at about 2300 K, but it also chemically attacks any known container. Last year, however, a team of Illinois researchers from Argonne National Laboratory in Argonne and Containerless Research, Inc. in Evanston obtained a diffraction pattern for molten aluminum oxide by levitating it in a flow of air. Later, they used the same technique to probe supercooled liquid silicon and yttrium oxide. Now they have repeated their success with liquid boron.
The researchers, led by David Price of Argonne and Shankar Krishnan of Containerless Research, levitated a small piece of pure boron in jets of argon gas in a closed chamber at the Brookhaven National Laboratory x-ray beam facility in Upton, NY. They then aimed a 270 watt laser beam at the crystal to melt it. They were able to hold the molten drop stable for long enough to take x-ray diffraction data at different temperatures, both above and below the melting point. They found that at short length scales, the structure of liquid boron is similar to its nonicosahedral solid forms, a comparison that the computer simulations did not address. The similarity is surprising, says Krishnan, because as a solid, boron is an insulator, while as a liquid, it behaves like a metal. But the team was unable to conclusively determine whether icosahedra remain intact above the melting temperature.
John Copley, of the National Institute of Standards and Technology in Maryland, says that experimentally, the work is a “tour de force” because of the tremendous difficulty in working with liquid boron. However, it is not the final word on its structure. As he points out, the authors still cannot say whether liquid boron contains icosahedra.
Meher Antia is a freelance science writer.