Literatura académica sobre el tema "Icosahedral B12 units"

Boron carbide phases exist over a widely varying compositional range B12+xC3-x(0.06 <x< 1.7). One idealized structure corresponds to the B13C2composition (space groupR-3m) and contains one icosahedral B12unit and one linear C—B—C chain. The B12units are composed of crystallographically distinct B atoms BP(polar, B1) and BEq(equatorial, B2). Boron icosahedra are interconnected by C atomsviatheir BEqatoms, forming layers parallel to (001), while the B12units of the adjacent layers are linked through intericosahedral BP—BPbonds. The unique B atom (BC) connects the two C atoms of adjacent layers, forming a C—B—C chain along [001]. Depending on the carbon concentration, the carbon and BPsites exhibit mixed B/C occupancies to varying degrees; besides, the BCsite shows partial occupancy. The decrease in carbon content was reported to be realizedviaan increasing number of chainless unit cells. On the basis of X-ray single-crystal refinement, we have concluded that the unit cell of the given boron-rich crystal contains following structural units: [B12] and [B11C] icosahedra (about 96 and 4%, respectively) and C—B—C chains (87%). Besides, there is a fraction of unit cells (13%) with the B atom located against the triangular face of a neighboring icosahedron formed by BEq(B2) thus rendering the formula B0.87(B0.98C0.02)12(B0.13C0.87)2for the current boron carbide crystal.

Single crystals of a novel sodium–magnesium boride silicide, Na3MgB37Si9 [a = 10.1630 (3) Å, c = 16.5742 (6) Å, space group R\overline{3}m (No. 166)], were synthesized by heating a mixture of Na, Si and crystalline B with B2O3 flux in Mg vapor at 1373 K. The Mg atoms in the title compound are located at an interstitial site of the Dy2.1B37Si9-type structure with an occupancy of 0.5. The (001) layers of B12 icosahedra stack along the c-axis direction with shifting in the [–a/3, b/3, c/3] direction. A three-dimensional framework structure of the layers is formed via B—Si bonds and {Si8} units of [Si]3—Si—Si—[Si]3.

Boron carbide (BC, B15-xCx B4C) has a unique combination of properties. This makes it a material for priority applications for a wide range of engineering solutions. The high melting point and heat resistance of the compound contribute to its use in refractory conditions. Due to its extreme abrasion resistance, B4C is used as an abrasive powder and coating. Due to its high hardness and low density, B15-xCx has ballistic characteristics. It is usually used in nuclear programs as an absorbent of neutron radiation Boron carbide ceramics (B15-xCx or BC) may lose strength and toughness due to the amorphization effect under high shear stresses. Aluminum dodecaboride AlB12 or B12Al, as well as boron carbide B12 [(CCC) x (CBC) 1-x] have common structural units B12 family of boron-icosahedral structures. The bond between icosahedrons is mainly due to atoms (Al, Si, O) or chains (CMC), where M is Al, Si, B, C. Doping BC powder with a small amount of AlB12, in cases of shock-shear stress, triggers the mechanism of "micro-cracking". Micro cracks and pores are formed in ceramics. The breakdown voltage decreases. AlB12 synthesis is associated with known difficulties. On the other hand. The production of metal-ceramic materials for several decades is associated with the interaction of liquid aluminum and boron nitride. The calculation of this reaction shows that it is exothermic. Avoiding oxidation in vacuum, the reaction occurs through the formation of aluminum nitride and aluminum dodeca-boride. In contrast to the liquid state, the process continues until the end, at conditional temperatures of evaporation of aluminum with slight changes in vacuum. The reaction product is a mixture of nanosized AlN/AlB12 powders with a weight ratio of 3/1 ready for baking without grinding. The acid-base properties of the nanosized powder mixture AlN + AlB12, the products of the interaction BN + Al in vacuum, which are used optionally, emit separate in pure phases of aluminum nitride and aluminum dodeca-boride. The yield of AlB12 is ~ 25%, boron reaches ~ 100%. The average particle size of the AlB12 powders according to TEM and ACS X-rays (area of coherent X-rays scattering), L (nm) is LTEM=110-150nm, LACS=51-70nm. The average specific surface area of the powder according to BET, TEM and ACS, SBET.m2/g=21,0-15,0; STEM.m2/g=21,4-15,4; SACS.m2/g=46,1-33,6; (at 1460 and 1640K, respectively).

ABSTRACTThe stoichiometry of boron suboxide (B6O1-x) synthesized at high pressure lies closer to the nominal composition (x = 0) than materials obtained at atmospheric pressure. The crystallinity of materials obtained in the presence of molten B2O3 is also higher than for sintered powders. Further, for syntheses at 1700–1800 °C between 4 and approximately 5–6 GPa, the well-crystallized particles are dominated by large (up to ∼40 μm in diameter) icosahedral multiply-twinned particles. This unusual morphology is obtained by Mackay packing; i.e., by assembly of successive shells of icosahedral B12 units around a central icosahedral nucleus. The result is a multiply-twinned particle in which each of the 20 elements has the R 3 m space group of the α-B structure.

Structural complexity is one of the characteristics of boron chemistry. The 3D solids formed from icosahedral B12 units is thought of as most favorable arrangement for boron allotropes. However, recent discoveries in this area have extended these perceptions. The H atom removal from polyhedral boranes flattens the clusters. As a number of boron atoms increases, spherical borospherenes become competitive in stability to planar clusters. With further increase in boron content, consequences vary depending upon the synthetic techniques. Depending upon the metal templates, most stable forms are extended either one-dimensional tubular structures or two-dimensional phases, named as borophenes. The three-dimensional solids are formed of icosahedral B12 fragments. We study these structural varieties computationally, developing correlations among them, based on density functional theory and the bonding principles known in chemistry. Though τ-boron is built up of similar fragments as β-rhombohedral boron, the computations reported that τ-boron is more stable than other allotropic forms. The symmetry of the unit cell is reduced from rhombohedron in β-boron to orthorhombic in τ-boron. We use a fragment molecular approach to provide a structural overview and an explanation for the varying electron requirement of the structural fragments of τ-boron allotrope. We have found the τ-B106 is less stable than β-B106 by 13.8 meV/atom. The change in unit cell symmetry shows slight changes in its electronic structure as well. The stability of τ-B105 is reduced due to rotation of B28-B-B28 chain and electronic requirement of the B57 units become less as compared to β-B105. Thus, a greater number of boron atoms would have partial occupancies in τ-Boron and the possibility of several polymorphs with almost equivalent stability would also be greater compared to β-boron. We have designed a few model structures for τ-boron starting from ideal τ-B105 by varying the number of partial occupancies and compared the relative cohesive energy per atom with the previously reported τ-B106 structure. In general, all these structures are reminiscent of β-boron where the extra occupancies, vacancies and the symmetry of the constituent fragments should depend on the rate of cooling of the boron melt