Unprecedented insights into the local atomic arrangements and band structure of boron carbide
20. April 2018
Karsten Rasim, Reiner Ramlau, Andreas Leithe-Jasper, Takao Mori, Ulrich Burkhardt, Horst Borrmann, Walter Schnelle, Christian Carbogno, Matthias Scheffler, and Yuri Grin
Boron carbide, the simple chemical combination of boron and carbon, is one of the best-known binary ceramic materials. Despite that, a coherent description of its crystal structure and physical properties resembles one of the most challenging problems in materials science.
By combining ab-initio computational studies, precise crystal structure determination from diffraction experiments and state of the art high-resolution transmission-electron microscopy imaging, scientists from the Max-Planck Institute for Chemical Physics of Solids, the Fritz-Haber-Institute of the Max-Planck-Society and the National Institute for Materials Science (NIMS) in Japan, revealed in a concerted investigation hitherto unknown local structure modifications.
For this study cm-sized single crystals of boron carbide were manufactured using the floating-zone melting technique at very high temperatures.
The information obtained from the single crystal structure refinements suggested different and significant deviations from the ideal structural motif of boron carbide which is depicted in Figure 2.
These deviations are due to different local atomic arrangements averaged over multiple unit cells:
(i) carbon atoms in the chain occupy two different positions depending of the presence or absence of boron at the B3 position;
(ii) the chain is asymmetric due to disorder (CBB or CCB, however, the total picture remains symmetric due to the symmetry of the space group);
(iii) the chain contains four instead of three atoms;
(iv) the boron atoms of the chain are located out of the three-fold axis, forming bent chains CBC or rhombi CB2C.
To ultimately shed more light into the local atomic configurations in the real material on the nano-scale, a state of the art high-resolution electron microscopy study was then performed.
In Figure 3 we observe along the crystallographic direction  the icosahedra and, separately, the chains which are projected onto each other. Strong disorder is noticed, since the observed contrast in the regions of icosahedra and chains varies from unit cell to unit cell (Figure 3a) and practically never can be interpreted using the ideal model indicating the omnipresent variations of the ideal crystal structure in real material (Figure 3b).
Independently, theoretical calculations nicely corroborate these substantial (partially hitherto not known) modifications in the region of the CBC chain. They have sizeable negative energies of formation and definitely have a constitutive character for the local crystal structure of boron carbide. This mixture of different local atomic arrangements within the real crystal structure reduces therefor the electron deficiency of the pristine structure CBC+B12, answering the question about the electron precise character of boron carbide and introducing new electronic states within the band gap as can be seen in Figure 4.
This allows now a much better understanding of boron carbide’s physical properties.