The Structural Chemistry of Boron, Borospherenes and Borophenes: A Computational Study

Abstract

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