Interband Transitions and Optical Phonons of B48Al3C2

The optical absorption edge of B48Al3C2 has been measured. It is decomposed into several interband transitions between 0.97 and 2.5 eV. Additionally, several gap-state-related transitions with energies between 0.45 and 0.96 eV were determined. They are attributed to defects in the structure, whose character is not yet known. The spectra of IR- and Ramanactive phonons and their resonance frequencies are presented. According to other icosahedral boron-rich solids, the phonons can roughly be separated into vibrations of icosahedra and those of single atoms. The vibrations of the tetrahedrally bonded Al1 atom are discussed in analogy to the tetrahedral XY4 molecule.