[en] B₁₂ icosahedra or related structure elements determine the different modifications of elementary boron and numerous boron-rich compounds from α-rhombohedral boron with 12 to YB₆₆ type with about 1584 atoms per unit cell. Typical are well-defined high density intrinsic defects: Jahn-Teller distorted icosahedra, vacancies, incomplete occupancies, statistical occupancies and antisite defects. The correlation between intrinsic point defects and electron deficiencies solves the discrepancy between theoretically predicted metal and experimentally proved semiconducting character. The electron deficiencies generate split-off valence states, which are decisive for the electronic transport, a superposition of band-type and hopping-type conduction. Their share depends on actual conditions like temperature or pre-excitation. The theoretical model of bipolaron hopping is incompatible with numerous experiments. Technical application of the typically p-type icosahedral boron-rich solids requires suitable n-type counterparts; doping and other possibilities are discussed.

[en] The energy band structure of β-rhombohedral boron is characterized by multiple electron and hole traps in the band gap. Fractional photoconductivity glow experiments were performed between 170 and 360 K for obtaining further information on the various traps in this semiconductor. The results can easily be interpreted within the actual band scheme and confirm the previous assumption of a cascade-like recombination of optically excited electrons with an activation energy of about 0.12 eV. The result support the assumption of optically induced structural changes regarding the partly occupied sites B(13) and B(16) to B(20).

[en] Reliable measurement of the Seebeck coefficient requires painstaking precautions. Unfortunately, these requirements are not met even in commercially available equipment, and therefore the obtained data often contain systematic errors. We describe equipment for reliable Seebeck measurements and present results on YB₆₆ and B_4.3C boron carbide as an example.

[en] The established structure of α-rhombohedral boron, based on one B₁₂ icosahedron per unit cell only, is put in question. A careful evaluation of the IR-active phonons in comparison with B₆O, B_4.3C and β-rhombohedral boron makes it evident that-aside from the B₁₂ icosahedra-the α-rhombohedral boron structure also contains single boron atoms. We assume these single atoms replace the currently assumed inter-icosahedral three-centre-bonds by covalently saturating the outward directing bonds of the equatorial atoms of the three adjacent icosahedra. Indeed, the implied structure formula B₁₂B₂ is not supported by previous density measurements. The IR-phonon spectra of the related structures are correlated and merely shifted relative to one another; significant features depend quantitatively on the actual structure, but they can be easily allocated. (paper)

[en] The rhombohedral elementary cell of the complex boron carbide structure is composed of B₁₂ or B₁₁C icosahedra and CBC, CBB or B□B (□, vacancy) linear arrangements, whose shares vary depending on the actual chemical compound. The evaluation of the IR phonon spectra of isotopically pure boron carbide yields the quantitative concentrations of these components within the homogeneity range. The structure formula of B_4.3C at the carbon-rich limit of the homogeneity range is (B₁₁C) (CBC)_0.91 (B□B)_0.09 (□, vacancy); and the actual structure formula of B₁₃C₂ is (B₁₂)_0.5(B₁₁C)_0.5(CBC)_0.65(CBB)_0.16 (B□B)_0.19, and deviates fundamentally from (B₁₂)CBC, predicted by theory to be the energetically most favourable structure of boron carbide. In reality, it is the most distorted structure in the homogeneity range. The spectra of ^natB_xC make it evident that boron isotopes are not randomly distributed in the structure. However, doping with 2% silicon brings about a random distribution.

[en] The FIR reflectivity spectra of boron carbide, measured down to ω ∼10 cm⁻¹ between 100 and 800 K, are essentially determined by heavily damped plasma vibrations. The spectra are fitted applying the classical Drude-Lorentz theory of free carriers. The fitting Parameter Π = ω_p/ω_τ yields the carrier densities, which are immediately correlated with the concentration of structural defects in the homogeneity range. This correlation is proved for band-type and hopping conductivity. The effective mass of free holes in the valence band is estimated at m*/m_e ∼ 2.5. The mean free path of the free holes has the order of the cell parameters. (paper)

[en] Single-crystal B_4_._3C boron carbide is investigated through the pressure-dependence and inter-relation of atomic distances, optical properties and Raman-active phonons up to ∼70 GPa. The anomalous pressure evolution of the gap width to higher energies is striking. This is obtained from observations of transparency, which most rapidly increases around 55 GPa. Full visible optical transparency is approached at pressures of  >60 GPa indicating that the band gap reaches ∼3.5 eV; at high pressure, boron carbide is a wide-gap semiconductor. The reason is that the high concentration of structural defects controlling the electronic properties of boron carbide at ambient conditions initially decreases and finally vanishes at high pressures. The structural parameters and Raman-active phonons indicate a pressure-dependent phase transition in single-crystal "n"a"tB_4_._3C boron carbide near 40 GPa, likely related to structural changes in the C–B–C chains, while the basic icosahedral structure appears to be less affected. (paper)

[en] We present Raman spectra of numerous icosahedral boron-rich solids having the structure of α-rhombohedral, β-rhombohedral, α-tetragonal, β-tetragonal, YB₆₆, orthorhombic or amorphous boron. The spectra were newly measured and, in some cases, compared with reported data and discussed. We emphasize the importance of a high signal-to-noise ratio in the Raman spectra for detecting weak effects evoked by the modification of compounds, accommodation of interstitial atoms and other structural defects. Vibrations of the icosahedra, occurring in all the spectra, are interpreted using the description of modes in α-rhombohedral boron by Beckel et al. The Raman spectrum of boron carbide is largely clarified. Relative intra- and inter-icosahedral bonding forces are estimated for the different structural groups and for vanadium-doped β-rhombohedral boron. The validity of Badger's rule is demonstrated for the force constants of inter-icosahedral B-B bonds, whereas the agreement is less satisfactory for the intra-icosahedral B-B bonds. (topical review)