Are there bipolarons in icosahedral boron-rich solids?

Abstract

The charge transport of boron carbide, often incorrectly denoted as B(4)C, has been controversially discussed. It is shown that the bipolaron hypothesis is not compatible with numerous experimental results. In particular, the determined real microstructure of boron carbide and its related electronic properties disprove several assumptions, which are fundamental to the bipolaron hypothesis. In contrast, the actual energy band scheme derived mainly from optical investigations is confirmed by careful evaluation of the high-temperature electrical conductivity, and allows a consistent description at most of the experimental results.

Configurational order-disorder induced metal-nonmetal transition inB13C2studied with first-principles superatom-special quasirandom structure method

“…We thus suggest that this could be the main cause for B 13 C 2 to behave as a semiconductor, i.e., due to the inevitable substitutional disorder between boron and carbon atoms. Our results corroborate the suggestions by Balakrishnarajan et al [28], Schmechel [29], and Werheit [30], that semiconducting behavior in B 13 C 2 could arise from substitutional disorder.…”

“…This is because an electron in the splitting states of the B 11 C e (BBC) unit occupies an empty state in the B 12 (CBC) unit, thus compensating the electron deficiency. This observation is in line with the speculations by Balakrishnarajan [28], Schmechel [29], and Werheit [30] that boron/carbon substitutional disorder could lead to localization of electronic states, compensating electron deficiency in B 13 C 2 . Such a compensation of electron deficiency can also take place in a combination of B 12 (CBC) and B 11 C p (CBB) [B 11 C p (BBC)] with a band gap of 1.0 (1.8) eV (not shown).…”

“…According to a study of the bonding nature of B 13 C 2 by Balakrishnarajan et al [28], they suggested that the semiconducting behavior in B 13 C 2 might originate from unavoidable boron/carbon substitutional disorder, leading to the localization of electronic states. This seems to correspond to the statement proposed by Schmechel [29] and Werheit [30] that the electron deficiency in B 13 C 2 can be compensated by defects, e.g., substitutional defects and vacancies, splitting off some valence states into a band gap. However, there are no detailed suggestions or calculations demonstrating how such effects should be realized if present.…”

Coupled polaron-phonon effects on Seebeck coefficient and lattice conductivity of B13C2from first principles

“…The second model is based on structural disorder in B 12 -CBC, where the high density intrinsic defects accompanied by strong distortion compensate for the electron deficiency of ideal metallic behavior and also reduce κ. This structural conjecture is still evolving with the mentioned polymorphism 1,12 , as reported in the phase diagram and diffraction results demonstrating the abundances of phases 1 . The B 11 C-CBC model is believed to be the lowest formation energy and the most relevant structure for x = 0, whereas the B 12 -CBC is the most stable phase for x = 1 1,13,14 .…”

“…Also, the small σ e exponentially increases with temperature (T ) 7,9 . To explain these anomalous properties, two models have been suggested by Emin et al 2,5,10,11 and Werheit et al 4,12 . The first is the small bipolaron model for the nonequivalent B 11 C icosahedra, suggesting localized charge (q) carriers with low mobility, i.e., phonon-assisted polaron hopping.…”