Sc0.83.E2.80.93xB10.0.E2.80.93yC0.17.2BySi0.083.E2.80.93z Crystal structure of boron-rich metal borides

fig. 23. b10 polyhedron in sc0.83–xb10.0–yc0.17+ysi0.083–z crystal structure.

sc0.83–xb10.0–yc0.17+ysi0.083–z (x = 0.030, y = 0.36 , z = 0.026) has cubic crystal structure space group f43m (no. 216) , lattice constant = 2.03085(5) nm. compound identified scb15c0.8 (phase in sc-b-c phase diagram of figure 17). small amount of si added floating zone crystal growth , phase quaternary compound. rare cubic structure has 26 sites in unit cell: 3 sc sites, 2 si sites, 1 c site , 20 b sites; 4 out of 20 b sites boron-carbon mixed-occupancy sites. atomic coordinates, site occupancies , isotropic displacement factors listed in table viii.

in unit cell, there 3 independent icosahedra, i1, i2 , i3, , b10 polyhedron formed b1–b4, b5–b8, b9–b13 , b14–b17 sites, respectively. b10 polyhedron has not been observed , shown in figure 23. icosahedron i2 has boron-carbon mixed-occupancy site b,c6 occupancy b/c=0.58/0.42. remaining 3 boron-carbon mixed-occupancy sites bridge sites; c , si sites bridge sites.

more 1000 atoms available in unit cell, built large structure units such 2 supertetrahedra t(1) , t(2) , 1 superoctahedron o(1). shown in figure 24a, t(1) consists of 4 icosahedra i(1) have no direct bonding bridged 4 b , c20 atoms. these atoms form tetrahedron centered si2 sites. supertetrahedron t(2) consists of 4 icosahedra i(2) same shown in figure 18b; mixed-occupancy sites b , c6 directly bond each other. superoctahedron o(1) consists of 6 icosahedra i(3) , bridge sites b, c18, c1 , si1; here si1 , c1 exhibit tetrahedral arrangement @ center of o(1). interestingly, b10 polyhedra arrange octahedrally, without central atom, shown in figure 24c b , c19 atoms bridge b10 polyhedra form octahedral supercluster of b10 polyhedra.

fig. 25. boron framework structure of sc0.83–xb10.0–yc0.17+ysi0.083–z depicted supertetrahedra t(1) , t(2), superoctahedron o(1) , superoctahedron based on b10 polyhedron. vertexes of each superpolyhedron adjusted center of constituent icosahedra, real volumes of these superpolyhedra larger appear in picture.

using these large polyhedra, crystal structure of sc0.83–xb10.0–yc0.17+ysi0.083–z can described shown in figure 25. owing crystal symmetry, tetrahedral coordination between these superstructure units again key factor. supertetrahedron t(1) lies @ body center , @ edge center of unit cell. superoctahedra o(1) locate @ body center (0.25, 0.25, 0.25) of quarter of unit cell. coordinate tetrahedrally around t(1) forming giant tetrahedron. supertetrahedra t(2) located @ symmetry-related positions (0.25, 0.25, 0.75); form giant tetrahedron surrounding t(1). edges of both giant tetrahedra orthogonally cross each other @ centers; @ edge centers, each b10 polyhedron bridges super-structure clusters t(1), t(2) , o(1). superoctahedron built of b10 polyhedra located @ each cubic face center.

scandium atoms reside in voids of boron framework. 4 sc1 atoms form tetrahedral arrangement inside b10 polyhedron-based superoctahedron. sc2 atoms sit between b10 polyhedron-based superoctahedron , o(1) superoctahedron. 3 sc3 atoms form triangle , surrounded 3 b10 polyhedra, supertetrahedron t(1) , superoctahedron o(1).

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