ScBSi has a tetragonal crystal structure with space group P422 (No. 92) or P422 and lattice constants of a, b = 1.03081(2) and c = 1.42589(3) nm; it is
isotypic to the α-AlB structure type. There are 28 atomic sites in the unit cell, which are assigned to 3 scandium atoms, 24 boron atoms and one silicon atom. Atomic coordinates, site occupancies and isotropic displacement factors are listed in table VI.
The boron framework of ScBSi is based on one B icosahedron and one B unit. This unit can be observed in β-tetragonal boron and is a modification of the B unit of α-AlB (or B unit in early reports). The B unit is a twinned icosahedron made from B13 to B22 sites with two vacant sites and one B atom (B23) bridging both sides of the unit. The twinned icosahedron is shown in figure 18a. B23 was treated as an isolated atom in the early reports; it is bonded to each twinned icosahedra through B18 and to another icosahedron through B5 site. If the twinned icosahedra were independent without twinning then B23 would be a bridge site linking three icosahedra. However, because of twinning, B23 shifts closer to the twinned icosahedra than another icosahedron; thus B23 is currently treated as a member of the twinned icosahedra. In ScBSi, the two B24 sites which correspond to the vacant sites in the B unit are partially occupied; thus, the unit should be referred to as a B cluster which is occupied by about 20.6 boron atoms. Scandium atoms occupy 3 of 5 Al sites of α-AlB, that is Sc1, Sc2 and Sc3 correspond to Al4, Al1 and Al2 sites of α-AlB, respectively. The Al3 and Al5 sites are empty for ScBSi, and the Si site links two B units. This phase also exists without silicon.
Figure 19a shows the network of boron icosahedra in the boron framework of ScBSi. In this network, 4 icosahedra form a supertetrahedron (figure 18b); its one edge is parallel to the a-axis, and the icosahedra on this edge make up a chain along the a-axis. The opposite edge of the supertetrahedron is parallel to the b-axis and the icosahedra on this edge form a chain along the b-axis. As shown in figure 19, there are wide tunnels surrounded by the icosahedron arrangement along the a- and b-axes. The tunnels are filled by the B units which strongly bond to the surrounding icosahedra; the connection of the B units is helical and it runs along the c-axis as shown in figure 19b. Scandium atoms occupy the voids in the boron network as shown in figure 19c, and the Si atoms bridge the B units.