RExB12C0.33Si3.0 Crystal structure of boron-rich metal borides

fig. 14. crystal structure of rexb12c0.33si3.0 (re=y or dy) viewed along direction close [100]. red, black , blue spheres correspond y/dy, c , si atoms, respectively. vacancies @ y/dy site ignored.

initially these described ternary re-b-si compounds, later carbon included improve structure description resulted in quaternary re-b-c-si composition. rexb12c0.33si3.0 (re=y , gd–lu) have unique crystal structure 2 units – cluster of b12 icosahedra , si8 ethane-like complex – , 1 bonding configuration (b12)3≡si-c≡(b12)3. representative compound of group yxb12c0.33si3.0 (x=0.68). has trigonal crystal structure space group r3m (no. 166) , lattice constants = b = 1.00841(4) nm, c = 1.64714(5) nm, α = β = 90° , γ = 120°.

fig. 15. network of boron icosahedra lying in (001) plane. black, blue , red spheres correspond c, si , y atoms, respectively.

the crystal has layered structure. figure 15 shows network of boron icosahedra spreads parallel (001) plane, connecting 4 neighbors through b1–b1 bonds. c3 , si3 site atoms strengthen network bridging boron icosahedra. contrary other boron-rich icosahedral compounds, boron icosahedra different layers not directly bonded. icosahedra within 1 layer linked through si8 ethane-like clusters (b12)3≡si-c≡(b12)3 bonds, shown in figures 16a , b.

there 8 atomic sites in unit cell: 1 yttrium y, 4 boron b1–b4, 1 carbon c3 , 3 silicon sites si1–si3. atomic coordinates, site occupancy , isotropic displacement factors listed in table va; 68% of y sites randomly occupied , remaining y sites vacant. boron sites , si1 , si2 sites occupied. c3 , si3 sites can occupied either carbon or silicon atoms (mixed occupancy) probability of 50%. separation 0.413 Å, , either c3 or si3 sites, not both, occupied. these sites form si-c pairs, not si-si or c-c pairs. distances between c3 , si3 sites , surrounding sites yxb12c0.33si3.0 summarized in table vb , overall crystal structure shown in figure 14.

salvador et al. reported isotypic terbium compound

tb3–xc2si8(b12)3. parts of crystal structure same described above; however, bonding configuration deduced (b12)3≡c-c≡(b12)3 instead of (b12)3≡si-c≡(b12)3. authors intentionally added carbon grow single crystals whereas previous crystals accidentally contaminated carbon during growth. thus, higher carbon concentration achieved. existence of both bonding schemes of (b12)3≡si-c≡(b12)3 , (b12)3≡c-c≡(b12)3 suggests occpancy of carbon sites of 50–100%. on other hand, (b12)3≡si-si≡(b12)3 bonding scheme unlikely because of short si-si distance, suggesting minimum carbon occupancy @ site 50%. b atoms may replace c atoms @ c3 site, assigned b site. however, carbon occupation more because site tetrahedrally coordinated whereas b occupation of site needs electron complete tetrahedral bonding. thus, carbon indispensable group of compounds.