ON MICROSTRUCTURE AND ELECTRONIC PROPERTIES OF BORON CARBIDE

Boron carbide (homogeneity range B4.3C - B similar to 11C) exhibits similar to 15 atoms per elementary cell and is composed of B-12 or B11C icosahedra and linear structure elements CBC, CBB or B square B (square, vacancy), whose shares depend on the chemical composition. Their random distribution excludes x-ray diffraction and NMR from structure analysis. Phonon spectroscopy on isotopically pure boron carbide solves this problem. - Experimental investigations established semiconducting character. They contrast with theoretical band structure calculations concluding metallic behavior because of being based on incorrect idealized structures. The actual band scheme (gaps 2.09 and 2.41 eV) exhibiting high-density gap states allows consistently interpreting the experimental results. - Seebeck coefficients of similar to 300 mu V/K up to 2000 K make it a promising candidate for thermoelectric energy conversion. Theoretical speculations on superconductivity up to 36.7 K were not confirmed.