Udo Kuhlmann

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.

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.

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.

Optical and electronic properties of the orthorhombic MgAIB14-type borides

Abstract A systematic investigation of the orthorhombic borides LiAlB 14 , MgAlB 14 and ErAlB 14 was carried out. In all cases several indirect allowed optical interband transitions with phonon emission were derived from transmission measurements in the absorption edge range. For LiAlB 14 the temperature dependence between 22 and 293 K was determined. The results are compatible with the interband photoconductivity and the electroabsorption. Despite the identical basic structures of the icosahedral boron network, the interband transition energies of the orthorhombic borides depend on the chemical composition. As for other icosahedral boron-rich solids, there are edge tails with considerable absorption levels extending to lower energies, at least in MgAlB 14 and ErAlB 14 , obviously evoked by the metal atoms. ErAlB 14 is a one-dimensional conductor or a one-dimensional high-conductivity semiconductor with a distinct plasma edge close to 2000 cm −1 . The IR phonon spectra in the spectral range between 2000 and 1200 cm −1 show the vibrations of the light single atoms. The phonon spectrum at 1200 cm −1 or less, belonging essentially to the intra-icosahedral vibrations, is closely related to the α-rhombohedral boron structural group. Phonon quenching as a result of electron-phonon interaction on the icosahedra occurs as the atomic number of the metal atoms is increased. The carrier type depends on the transfer of electrons from the metal atoms to the structure. In the case of MgAlB 14 the Seebeck coefficient is of the order of −6500 μV K −1 making it of interest for thermoelectric applications.

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.

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.

Solid solutions of silicon in boron-carbide-type crystals

Abstract The optical absorption spectrum of silicon-doped boron carbide in the spectral range of the absorption edge and its low-energy tail, obtained from transmission measurements between 0.25 and 4 eV, is compared with that of undoped boron carbide (B 4.3 C) with comparably low distortions. Silicon is proved to be an effective dopant of boron carbide, because the edge absorption spectrum and the plasma edge are considerably changed. However, the conduction remains p-type for the sample investigated. The IR phonon spectrum of silicon-doped boron carbide is similar to that of boron-rich boron carbide, which contains a considerable amount of chainless unit cells. It is shown that the Si atoms occupy these cells, forming a two-atom chain. From a comparison of the Raman spectrum with those of similar structures and theoretical calculations, it can be estimated that the force field constant in such chains varies linearly with the distance between the atoms.

On the reliability of the Raman spectra of boron-rich solids

The problem of qualitatively different Raman spectra of boron-rich solids obtained by conventional and FT (Fourier-transform) Raman spectroscopy has been solved. Because of the high absorption coefficient in the spectral range of the fundamental absorption the penetration depth at the energy of the 514.5 nm line of the Ar laser (2.41 eV) frequently used in conventional Raman spectrometers is so small that the spectra are essentially determined by the oxidized sample surfaces or by the strongly distorted Beilby layers. The photon energy of the Nd:YAG laser (1.16 eV) used in FT-Raman spectrometers is beyond the absorption edge, and therefore scattering essentially takes place within the bulk of the sample. Hence, FT-Raman spectra are much more bulk-determined than conventional Raman spectra.

Optical properties of amorphous boron

Abstract Amorphous boron samples of different origin were investigated by X-ray diffraction, scanning electron microscopy, i.r. and Raman spectroscopy. Traces of a crystalline β-rhombohedral boron phase (less than 5–6%) were found in one specimen, while the other was completely amorphous. Amorphous boron is known to consist of B 12 icosahedra statistically bonded. This situation is different from that in really statistically arranged atoms, where the symmetry selection rules of phonons do not hold, and both the i.r.-active and the Raman-active phonon spectra are expected to yield the phonon density distribution. In amorphous boron there is a certain similarity, but neither a far-reaching agreement of these spectra with one another nor with the phonon density determined by neutron scattering by Medwick et al. or Delaplane et al . The marked short-range and medium-range order of amorphous boron are indicated by pronounced maxima in the ranges of the intra-icosahedral vibrations and in particular of the two-center B-B bonds known from crystalline boron-rich solids. This confirms that the external bonds of the icosahedra are largely covalently saturated, like in many crystals. Strong optical excitation of amorphous boron leads to an increasing order, probably caused by electron-phonon interaction.