Abstract
B 12As 2 is one of a group of boride crystals of R 3 m (space group 166) symmetry. We use a classical valence-force-field (VFF) analysis to account for observed Raman and IR spectra; we fit 20 calculated to 19 observed frequencies with an rms error of 25 cm −1. VFF calculated modes for B 12As 2 are correlated with α-boron modes through eigenvector expansions. This permits identification of modes that remain primarily B 12 cluster modes and helps clarify the effects of crystal forces and of arsenic atom interactions and movements. We attempt to account for IR intensities using displacements of atoms. The agreement between predicted and observed intensities is qualitatively good. Dispersion curves are plotted from Brillouin zone center to edge along two distinct symmetry axes. Acoustic branch speeds are determined to be in the range 0.6–0.9×10 6 cm s −1 for LA and close to 0.4×10 6 cm s −1 for TA modes.
Published Version
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