β Scribed by H. Werheit; C. Janowitz; R. Schmechel; T. Tanaka; Y. Ishizawa
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The complex dielectric functions of -rhombohedral boron, of boron carbide with compositions between B 4.23 C and B 10.37 C and YB 66 , were measured between about 2.5 and 9.5 eV. Similarities of the electronic structures probably due to the icosahedra are indicated by obvious qualitative similarities of the spectra. Numerous critical points have been determined and confirm the dense sequence of energy bands. For -rhombohedral boron the critical points are compatible with the combined density of states maxima derived from cluster calculations by K. Shirai and H. Nakamatsu (1994, in ''Proc. 11th Int. Symp. Boron, Borides and Related Comp, Tsukuba, 1993,'' JJAP Series 10, p. 70). Otherwise no satisfactory correlation with theoretical band structure calculations is possible. In the case of boron carbide the critical points are the same within the whole homogeneity range, while the values of the dielectric function at higher energies indicate composition-dependent densities of states, which are correlated with the density of B 12 icosahedra.
π SIMILAR VOLUMES
For -rhombohedral boron and boron carbide, the hitherto best-investigated icoasahedral boron-rich solids, the concentrations of structural defects and electronic gap states are quantitatively correlated. In this way the theoretically determined valence electron de5ciencies are exactly compensated, a
Optical conductivities and dielectric functions derived from the IR-optical reflectivity spectra exhibit clear similarities between icosahedral boron-rich structures and Al-based isosahedral quasicyrstals. This suggests a close qualitative relationship of their electronic structures. Accordingly ico