Very small amount of carbon is sufficient to stabilize "ScBC". This compound has a broad composition range, namely ScBC with x ≤ 2.2 and y ≤ 0.44. ScBC has a hexagonal crystal structure with space group P6mmm (No. 199) and lattice constants a, b = 1.45501(15) nm and c = 0.84543(16) nm.
There are 19 atomic sites in the unit cell, which are assigned to one scandium site Sc, 14 boron sites B1–B14 having 100% occupancy, two boron-carbon mixed-occupancy sites B/C15 and B/C16, and two partial-occupancy boron sites B17 and B18. Atomic coordinates, site occupancies and isotropic displacement factors are listed in table VII. Although a very small amount of carbon (less than 2 wt%!) plays an important role in the phase stability, carbon does not have its own sites but shares with boron two interstitial sites B/C15 and B/C16.
There are two inequivalent B icosahedra, I1 and I2, which are constructed by the B1–B5 and B8–B12 sites, respectively. A "tube" is another characteristic structure unit of ScBC. It extends along the c-axis and consists of B13, B14, B17 and B18 sites where B13 and B14 form 6-membered rings. B17 and B18 sites also form 6-membered rings; however, their mutual distances (0.985 Å for B17 and 0.955 Å for B18) are too short for a simultaneous occupation of the neighboring sites. Therefore, boron atoms occupy 2nd neighbor site forming a triangle. The occupancies of B17 and B18 sites should be 50%, but the structure analysis suggests larger values. The crystal structure viewed along the a-axis is shown in figure 20, which suggests that the ScBC is a layered material. Two layers, respectively constructed by the icosahedra I1 and I2, alternatively stack along the c-axis. However, the ScBC crystal is not layered. For example, during arc-melting, ScBC needle crystals violently grow along the c-axis – this never happens in layered compounds. The crystal structure viewed along the c-axis is shown in figure 21a. The icosahedra I1 and I2 form a ring centered by the "tube" shown in figure 21b, which probably governs the properties of the ScBC crystal. B/C15 and B/C16 mixed-occupancy sites interconnect the rings. A structural similarity can be seen between ScBC and BeB.
Figures 22a and b present HRTEM lattice images and electron diffraction patterns taken along the [0001] and [110] crystalline directions, respectively. The HRTEM lattice image of figure 22a reproduces well the (a, b) plane of the crystal structure shown in figure 21a, with the clearly visible rings membered by icosahedra I1 and I2 and centered by the "tube". Figure 22b proves that ScBC does not have layered character but its c-axis direction is built up by the ring-like structure and tubular structures.