Phonon peculiarities at the high-pressure phase transition of B4.3C boron carbide

摘要

The change from opacity to transparency marks the reversible high-pressure phase transition of boron carbide between 30 and 40 GPa. After related features of the distorted structure and their effect on the electronic properties are at least roughly clarified, object of the present study is the process of structural changes within this phase transition based on the exchange of C atoms between different polar sites of icosahedra. Such minor structural modifications evade detection by X-ray and neutron diffraction, but are accessible in phonon spectra. Peculiarities in connection with the phase transition, suggested at 33(2) GPa, occur in Raman- and IR-active phonons. Interpretation is based on the close relation between the phonon spectra of alpha-rhombohedral boron and boron carbide and on the shift of B4.3C phonons in differently isotope-enriched samples.