Abstract
We have resolved long-standing discrepancies between the theoretical and experimental crystal structures of boron carbide . Theoretical studies predict that should be stoichiometric and have the highest symmetry of the boron carbides. Experimentally, is a semiconductor and many defect states have been reported, particularly in the CBC chain. Reconciling the disordered states of the chain, the chemical composition, and the lowest-energy state is problematic. We have solved this problem by constructing a structural model where approximately three-quarters of the unit cells contain (CBC) and one-quarter of them contain . This structural model explains many experimental results, such as the large thermal factors in x-ray diffraction and the broadening of the Raman spectra, without introducing unstable CBB chains. The model also solves the energy-gap problem. We show that there are many arrangements of these two types of unit cells, which are energetically almost degenerate. This demonstrates that boron carbides are well described by a geometrically frustrated system, similar to that proposed for -rhombohedral boron.
- Received 12 March 2014
- Revised 12 August 2014
DOI:https://doi.org/10.1103/PhysRevB.90.064109
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