Structure of icosahedral borides by Raman spectroscopy

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Abstract
We have obtained Raman spectra of icosahedral boron‐rich solids. The spectra of α‐rhombohedral boron, boron arsenide, and boron phosphide are consistent with highly‐ordered materials. Polarization studies have resulted in symmetry assignments for most of the Raman bands of α‐rhombohedral boron. In contrast, the Raman spectra of the boron carbides reveal local substitutional disorder. They also change progressively as a function of carbon content. A structural model for the boron carbides has been developed to explain the Raman and infrared absorption spectra, x‐ray data, and electrical and thermal transport properties. Raman spectra of boron carbide samples enriched in 10B, 11B, and 13C reveal details of the atomic motions. The vibrational frequencies and exceptionally narrow linewidths of certain Raman modes are discussed in terms of a ‘‘strong’’ bond model. In this model certain vibrational modes involving relatively stiff bonds between chain atoms, chain and icosahedral atoms, and atoms on different icosahedra are decoupled from the boride lattice by weak, intraicosahedral bonds.

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