New Material

Researchers from the University of Oxford and Diamond Light Source have discovered a new material, a Calcium hexaboride (CaB₆) compound crystallising in a previously unknown crystal structure.  Published in Physical Review Letters these findings can pave the way to customised boron-based intermetallics. The ordinary CaB₆ is a semiconductor with an amazing hardness and high melting temperature governed by a high chemical stability. It has been investigated for a long time due to its relevance for many industrial productions processes, like the manufacturing of boron-alloyed ultrahigh-strength steels.

This new research suggests that by choosing the right metal and synthesis conditions or dopants it may be possible to achieve new electronic, magnetic or even superconducting properties of such MB₆ compounds.

The Oxford team - a collaboration between a group from the Department of Materials and a group from Earth Sciences - achieved these results by imposing high pressure up to 44 GPa and laser heating up to 2.000 K on a ‘normal’ CaB₆ crystal. In doing so they created this novel substance which probably has metallic properties. The measurements were taken at Diamond’s Extreme Conditions beamline, I15, which is specially suited for such studies of material properties under extreme conditions.

Image to the right: Figure 1, The known cubic structure of CaB₆ is made out of boron octahedron

The starting point of the deformation process was the well-known cubic structure of CaB₆, which consists of octahedrons, formed by six boron atoms, around the larger Ca atoms in the centre of the cube (Figure 1). By increasing the pressure the structure distorts and finally fuses into a bonded 24-atom unit of boron above a pressure of 30 GPa after the laser heating is switched on (Figure 2). Although the new CaB₆ phase, which looks like a dented version of the ‘standard’ structure, was formed under such high pressure, the researchers succeeded in quenching it down to ambient pressure, a prerequisite for a potential use or application.

Pressure-Driven Evolution of the Covalent Network in CaB₆
A. N. Kolmogorov, S. Shah, E. R. Margine, A. K. Kleppe, and A. P. Jephcoat
Physical Review Letters Volume 109, Issue 7 (2012)
DOI: 10.1103/PhysRevLett.109.075501