Beamlines | I15 Case Study
New material in the creation
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
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.
Image to the left: Figure 2, The new CaB6structure is comprised of large 24-atom boron units
The research team used a series of experimental combinations and a special computational calculation method, a ‘evolutionary search’, for the determination of the stable compound structure. The advantages of this method are that no experimentally obtained values are needed to start with. During the optimisation process all computed beneficial atomic positions and spacing are passed on, so that the calculation of the whole crystal structure can evolve efficiently. However even by using this advanced methodical approach, systems of 20 and more atoms are increasingly difficult to deal with. Only a few stable structures of the size of the newly found material have been successfully solved and confirmed experimentally so far.
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)