Optical properties of amorphous boron

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Abstract

Amorphous boron samples of different origin were investigated by X-ray diffraction, scanning electron microscopy, i.r. and Raman spectroscopy. Traces of a crystalline β-rhombohedral boron phase (less than 5–6%) were found in one specimen, while the other was completely amorphous. Amorphous boron is known to consist of B12 icosahedra statistically bonded. This situation is different from that in really statistically arranged atoms, where the symmetry selection rules of phonons do not hold, and both the i.r.-active and the Raman-active phonon spectra are expected to yield the phonon density distribution. In amorphous boron there is a certain similarity, but neither a far-reaching agreement of these spectra with one another nor with the phonon density determined by neutron scattering by Medwick et al. or Delaplane et al. The marked short-range and medium-range order of amorphous boron are indicated by pronounced maxima in the ranges of the intra-icosahedral vibrations and in particular of the two-center B-B bonds known from crystalline boron-rich solids. This confirms that the external bonds of the icosahedra are largely covalently saturated, like in many crystals. Strong optical excitation of amorphous boron leads to an increasing order, probably caused by electron-phonon interaction.

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