Correlation between structural defects and electronic properties of icosahedral boron-rich solids

Abstract

From fine-structure investigations it is well known that in many icosahedral boron-rich solids the occupation densities of specific atomic sites are considerably reduced. Investigations of the electronic properties have proved that the electronic properties of these semiconductors are strongly influenced by high densities of intrinsic states in the band gaps. For -rhombohedral boron and boron carbide, the best investigated icosahedral boron-rich solids, it is shown that the concentrations of structural defects and electronic gap states are quantitatively correlated, and that this way the electron deficiencies theoretically calculated for the valence bands of corresponding idealized structures are compensated. Obviously, the structural defects in these crystals are the necessary consequence of the valence electron deficiency. It is suggested that this correlation holds for the icosahedral boron-rich solids in general.

Raman effect in icosahedral boron-rich solids

“…groups of icosahedral boron-rich structures. The determining role of the Jahn-Teller effect and other structural distortions in the electronic properties of these solids was shown by Schmechel and Werheit [9] who considered the splitting of electronic states calculated by Fujimori and Kimura [10].…”

Electronic structures and mechanical properties of boron and boron-rich crystals (Part I)

“…The icosahedron-based structure was first identified for boron carbide, which is formally referred to as B 12 C 3 or more shortly as B 4 C [33,34]. The true structures became more complicated as the study progressed [35][36][37][38][39][40][41]. The composition varies in some range, typically from B 12 C 3 to B 13 C 2 , but more C-poor compounds exist [42].…”

“…For boron solids, the electronic conduction mechanism is very complicated. For β-rhombohedral boron and boron carbides, there are many models and discussions to account for their unusual behaviours [11,14,27,41,64]. That the electronic transport is conveyed by the hopping of a carrier between defect sites is the only point on which most of researchers can agree.…”

“…But, in the author opinion, the Jahn-Teller effect itself is too general to account for special features of boron solids. From a good correlation between the disagreement of the energy gap and the concentration of defects, Schmechel and Werheit have put forth an idea that the insulating properties of β-rhombohedral boron and boron carbides are due to the localization of the relevant bands by the intrinsic defects [27,41]. Though discussion of this issue based on DFT calculations is deferred to in subsection Cause of defects, this is the most reasonable account for the discrepancy about the gap structure.…”

Evidence for multiple polytypes of semiconducting boron carbide (C2B10) from electronic structure

“…Generally these different polytypes have been associated with changing composition such as increasing the boron concentration [3,4,6,10,[24][25][26][27][28][29][30][31][32][33][34] or replacing carbon with other main group elements such as phosphorus [35,36], aluminium [37][38][39][40][41] and other main group elements. We have identified different polytypes of the semiconducting boron carbides with large direct band gaps (materials formed from para-carborane and dimeric phosphorus bridged ortho-carborane (C 2 B 10 H 10 PCl) 2 ) and those with smaller band gaps (materials formed from ortho-carborane and meta-carborane).…”