Electron Deformation Density in Rhombohedral α-Boron

Abstract

a-boron is the most simple structure of all boron modifications having one B 12 icosahedron per (rhombohedral) unit cell. The conventional, free atom crystal structure refinement with R = 6.2% indicates considerable charge redistribution into covalent bonding. In a high order-low order and multipole refinement the R value could be reduced to 1.19%. The ensuing deformation difference density maps reveal bonding between the boron atoms, in the icosahedra and between the icosahedra. Elektronen-Deformationsdichte in rhomboedrischem a-BorInhaltsu È bersicht. a-Bor ist die einfachste von den drei bekannten Bor Phasen, mit nur einer ikosaedrischen B 12 Einheit pro (rhomboedrischer) Elementarzelle. Eine konventionelle Strukturanalyse und Verfeinerung ergab einen R-Wert von nur 6.2%. Dies deutete auf eine erhebliche ElektronenUmverteilung in die Bor±Bor-Bindungen hin. Mit einer ¹high order-low orderª und Multipol Verfeinerung konnte der R-Wert auf 1.19% reduziert werden. Die berechneten Differenz-Deformations-Elektronendichten zeigten deutliche kovalente Bindungsanteile zwischen den Bor-Atomen, sowohl in den Ikosaedern als auch zwischen den Ikosaedern.

Ionic conductivity and the formation of cubic CaH2 in the LiBH4–Ca(BH4)2 composite

“…Its formation along with the formation of CaH 2 cannot be fully excluded. It must, however, be noted that elementary boron is considered very difficult to obtain, and is generally not observed as a reaction product of thermal decomposition of borohydrides below temperatures around 250°C [70,71]. Therefore, we do not consider it very likely that the majority of the boron atoms that become available during the reaction that forms CaH 2 go into the formation of elemental boron.…”

Synthesis of α- and β-Rhombohedral Boron Powders via Gas Phase Thermal Dissociation of Boron Trichloride by Hydrogen

“…The crystal structure of boron is proven as a-rhombohedral through the comparison of lattice parameters with those in literature. [20] As shown in Figure 3(b), SiO 2 phase was removed completely after leaching, and only a-boron were clearly present, inferring that the formation of a-boron is possible at 1173 K (900°C) on a quartz surface.…”

High-pressure high-temperature synthesis and structure of α-tetragonal boron

“…By contrast, the B1-B1 distance (2.148 Å, table 3, figure 6) between the icosahedra is much longer than in the model by Hoard et al (1.837-1.858 Å [2]). Such a long B-B bond was observed, for instance, in α-rhombohedral boron [25] where the intericosahedral bond length B2-B2 is 2.0135 Å. The distance between the icosahedra and boron atom B5 in the position 2b of the unit cell increases from 1.601-1.648 Å [2] to 1.714 Å in our case (table 3, figure 6).…”