H. Werheit

Distribution of carbon atoms on the boron carbide structure elements

Optical absorption and reflectivity measurements in the spectral range of the 1580 cm−1 phonon, the stretching mode of the central atom of the three-atom chain in the unit cell of boron carbide, were simultaneously fitted by suitable model calculations to determine quantitatively the carbon distribution within the unit cell in the homogeneity range. For example, in B4.3C, which is confirmed to be the carbon-rich limit composition, there are 81% CBC and 19% CBB chains, and exactly one carbon atom substitutes an icosahedral site. The concentrations of the CBC and CBB chains change abruptly between 11 and 14 at.% C, while the concentration of the carbon atoms in the icosahedra varies non-monotonically. The reduction in the carbon content towards the boron-rich limit of the homogeneity range is accompanied by an increasing number of unit cells without any chains. This number exceeds even 50% at about 8 at.% C. These results allow us to explain easily the variation in several structure and disorder-dependent physical properties of boron carbide and they disprove certain fundamental model assumptions. The force constants of the CB and BB bonds in the chain were determined. At B4.3C they have the same value of 5.6(1) mdyn A−1; at lower carbon contents they differ.

Correlation between structural defects and electronic properties of icosahedral boron-rich solids

From fine-structure investigations it is well known that in many icosahedral boron-rich solids the occupation densities of specific atomic sites are considerably reduced. Investigations of the electronic properties have proved that the electronic properties of these semiconductors are strongly influenced by high densities of intrinsic states in the band gaps. For -rhombohedral boron and boron carbide, the best investigated icosahedral boron-rich solids, it is shown that the concentrations of structural defects and electronic gap states are quantitatively correlated, and that this way the electron deficiencies theoretically calculated for the valence bands of corresponding idealized structures are compensated. Obviously, the structural defects in these crystals are the necessary consequence of the valence electron deficiency. It is suggested that this correlation holds for the icosahedral boron-rich solids in general.

Boron-rich solids : a chance for high-efficiency high-temperature thermoelectric energy conversion

Abstract Thermoelectric energy conversion is a very reliable way of generating electrical power, for example from solar heat or from waste industrial thermal energy. Boron-rich solids are shown to be very promising candidates for high efficiency thermoelectric energy conversion. The outstanding high melting points and extraordinary chemical stability allow their use under extreme conditions which are not accessible by most other materials. Some boron-rich semiconductors exhibit very favourable transport properties such as high Seebeck coefficients increasing monotonically up to very high temperatures, electrical conductivities with values typical of semiconductors, and very low thermal conductivities.

Raman effect in icosahedral boron-rich solids

Abstract 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 Badgers 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.

On the electronic properties of β-rhombohedral boron interstitially doped with 3d transition metal atoms

Abstract The controlled modification of the p-type character of pure boron-rich solids to n-type by suitable doping is an essential prerequisite for technical applications. Interstitial doping of β-rhombohedral boron with V, Cr, Fe and Ni fulfils this requirement, while Cu and Co do not. Comparative investigations of the Seebeck coefficient, the DC electrical conductivity, the optical absorption and the dynamical conductivity of BV, BCo and BFe lead to the conclusion that donor levels positioned between the conduction band and the uppermost intrinsic electron trapping level and of sufficient density to overcompensate unoccupied valence and gap states are necessary for n-type conductivity. In contrast to Fe, the interstitial accommodation of V atoms seems to reduce the concentration of possible gap states.

Influence of interstitially soluted iron on structural, optical and electrical properties of β-rhombohedral boron

Abstract A systematical investigation of a series of interstitially doped B:Fe solid solutions of the β-rhombohedral boron structure with compositions up to FeB29.5, which is close to the maximum solubility, is presented. At an iron concentration of about 2.5% the conductivity character changes from the p-type behaviour of pure β-rhombohedral boron to n-type. The IR and Raman active phonons change systematically. The damping ‖ c is much stronger than ⊥ c. In the case of the stretching mode of the central boron atom in the unit cell, occupied and unoccupied unit cells can be distinguished. An additional splitting of this vibration coincides with the formation of magnetic clusters and the sign reversal of the Seebeck coefficient. Some low frequency absorption bands are attributed to atoms in the definite voids of the structure. The question of whether they are local vibrational modes or electronic transitions remains unsolved. The iron atoms lead to an electronic level at 0.133 eV above the Jahn-Teller induced split-off valence band of β-rhombohedral boron.

On the microstructure of boron carbide

Abstract The isotope distribution in natural ans isotope enriched boron and in natural carbon makes different micro-compositions of the structure elements in boron carbide distinguishable by their lattice vibrations. The phonon band of the stretching mode of the central atom yields the C-B-C and C-B-B chain concentrations. Based on the chemical analysis the portions of B12 and B11C icosahedra were derived, too. The structure is not uniform at any composition. B4.3C is most homogenous but contains 19% C-B-B besides of 81% C-B-C chains. The minimum content of B11C icosahedra (60 %) is at B6.5C. The force constant in the chains at B4.3C is 5.6 mdyne A−1. Present structure conceptions are largely disproved; related models of physical properties must be revised.

Raman effect of boron carbide (B4.3C to B10.37C)

Abstract Careful FT-Raman investigations were performed on boron carbide with chemical compositions between B 4.3 C at the carbon-rich limit of the homogeneity range and B 10.37 C not far from the boron-rich limit yielding highly resolved and reliable spectra. Contrary to the results of other authors (Tallant et al. ), it is evident that the symmetry selection rules of IR and Raman active phonons hold in the case of boron carbide in spite of the considerable structure distortions. Hence boron carbide remains essentially crystalline, at least close to the carbon-rich limit. The most prominent Raman doublet at 270/320 and 869/928 cm −1 bands are attributed to a total symmetrical vibration of pentagonal pyramids of icosahedra relative to the end atoms of the chain; the Raman band at 1065 cm −1 belongs to the inter-icosahedral BB bond. From the resonance frequencies, the force constants of the intra-icosahedral bond (0.4 mdyne A −1 ), of the icosahedron-chain bond (2.2 mdyne A −1 ) and of the intericosahedral BB bond (2.4 mdyne A −1 ) were determined. They agree quite well with the theoretical calculations of Beckel et al. Towards the boron-rich limit of the homogeneity range the Raman spectra are considerably influenced by electronic transitions.

Raman scattering and isotopic phonon effects in dodecaborides

The Raman spectra of numerous dodecaborides have been measured on high-quality single crystals at ambient conditions with high spectral resolution and signal-to-noise ratio. Besides the strong Raman-active modes, numerous Raman-inactive modes occur in the spectra, indicating distortions of the structures. Ab initio calculation of the phonon spectra on ZrB(12) excellently agrees with the experimental results. Force constants are theoretically calculated and force parameters are estimated from the Raman frequencies. The influence of the surface range on the Raman spectra is evident. The different isotopic effects (virtual crystal approximation, the polarization effect and the effect of isotopic disorder) on the phonon frequencies are determined, separated and discussed.

On the insertion of carbon atoms in B12 icosahedra and the structural anisotropy of β-rhombohedral boron and boron carbide

Abstract X-Ray diffraction of β-rhombohedral boron doped with up to 1 at.% carbon and a model calculation of the anisotropic distortion of a single icosahedron were carried out. The carbon atoms substitute for boron, but only at polar sites of the B 12 icosahedra. Compared with a boron atom, the carbon atom is shifted by 0.115 A (6.5%) towards the centre of the icosahedron. The application of the model calculation on boron carbide using structural data of other authors, partly confirmed by our own measurements, shows that the distortion of the unit cell of boron carbide parallel to c exceeds that of the icosahedron by far, while the distortions are similar for the a axis. These are double the corresponding distortion in β-rhombohedral boron. The effect of elementary cells of boron carbide without chains on the distortion of the structure is evident; the effect of an electron-phonon interaction seems possible. The anisotropy of the structural modifications is immediately correlated with the changes in the concentrations of structural elements.