Initially these were described as ternary RE-B-Si compounds, but later carbon was included to improve the structure description that resulted in a quaternary RE-B-C-Si composition. REBCSi (RE=Y and Gd–Lu) have a unique crystal structure with two units – a cluster of B icosahedra and a Si ethane-like complex – and one bonding configuration (B)≡Si-C≡(B). A representative compound of this group is YBCSi (x=0.68). It has a trigonal crystal structure with space group Rm (No. 166) and lattice constants a = b = 1.00841(4) nm, c = 1.64714(5) nm, α = β = 90° and γ = 120°.
The crystal has layered structure. Figure 15 shows a network of boron icosahedra that spreads parallel to the (001) plane, connecting with four neighbors through B1–B1 bonds. The C3 and Si3 site atoms strengthen the network by bridging the boron icosahedra. Contrary to other boron-rich icosahedral compounds, the boron icosahedra from different layers are not directly bonded. The icosahedra within one layer are linked through Si ethane-like clusters with (B)≡Si-C≡(B) bonds, as shown in figures 16a and b.
There are eight atomic sites in the unit cell: one yttrium Y, four boron B1–B4, one carbon C3 and three silicon sites Si1–Si3. Atomic coordinates, site occupancy and isotropic displacement factors are listed in table Va; 68% of the Y sites are randomly occupied and remaining Y sites are vacant. All boron sites and Si1 and Si2 sites are fully occupied. The C3 and Si3 sites can be occupied by either carbon or silicon atoms (mixed occupancy) with a probability of about 50%. Their separation is only 0.413 Å, and thus either the C3 or Si3 sites, but not both, are occupied. These sites form Si-C pairs, but not Si-Si or C-C pairs. The distances between the C3 and Si3 sites and the surrounding sites for YBCSi are summarized in table Vb and the overall crystal structure is shown in figure 14.
Salvador et al.  reported an isotypic terbium compound
TbCSi(B). Most parts of the crystal structure are the same as those described above; however, its bonding configuration is deduced as (B)≡C-C≡(B) instead of (B)≡Si-C≡(B). The authors intentionally added carbon to grow single crystals whereas the previous crystals were accidentally contaminated by carbon during their growth. Thus, higher carbon concentration was achieved. Existence of both bonding schemes of (B)≡Si-C≡(B) and (B)≡C-C≡(B) suggests the occupancy of the carbon sites of 50–100%. On the other hand, (B)≡Si-Si≡(B) bonding scheme is unlikely because of too short Si-Si distance, suggesting that the minimum carbon occupancy at the site is 50%. Some B atoms may replace C atoms at the C3 site, as previously assigned to the B site. However, the carbon occupation is more likely because the site is tetrahedrally coordinated whereas the B occupation of the site needs an extra electron to complete tetrahedral bonding. Thus, carbon is indispensable for this group of compounds.