OPTICAL-PROPERTIES OF AMORPHOUS BORON

Amorphous boron samples of different origin were investigated by X-ray diffraction, scanning electron microscopy, i.r. and Raman spectroscop y. Traces of a crystalline beta-rhombohedral boron phase (less than 5- 6%) were found in one specimen, while the other was completely amorpho us. Amorphous boron is known to consist of B-12 icosahedra statistical ly bonded. This situation is different from that in really statistical ly arranged atoms, where the symmetry selection rules of phonons do no t hold, and both the i.r.-active and the Raman-active phonon spectra a re expected to yield the phonon density distribution. In amorphous bor on there is a certain similarity, but neither a far-reaching agreement of these spectra with one another nor with the phonon density determi ned by neutron scattering by Medwick et al. or Delaplane et al. The ma rked short-range and medium-range order of amorphous boron are indicat ed by pronounced maxima in the ranges of the intra-icosahedral vibrati ons and in particular of the two-center B-B bonds known from crystalli ne boron-rich solids. This confirms that the external bonds of the ico sahedra are largely covalently saturated, like in many crystals. Stron g optical excitation of amorphous boron leads to an increasing order, probably caused by electron-phonon interaction.