Icosahedron  Helmut Werheit | Prof. Dr. rer. nat website:   http://www.werheit.mynetcologne.de/
e-mail:   helmut.werheit@koeln.de
  helmut.werheit@uni.duisburg-essen.de
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Recent Results/Neuere Ergebnisse

 

 

2017

isbb2017

H. Werheit, M.H. Manghnani, U. Kuhlmann, A. Hushur,  S. Shalamberidze

On phonons and phonon-base Grüneisen parameters of boron carbide

see below and Solid State Sciences 72 (2017) 80

   

 

H. Werheit, K. Flachbart, G. Pristáš, D. Lotnyk, V. Filipov, U. Kuhlmann, N. Shitsevalova, T. Lundström

 

Influence of dopants, particularly carbon, on beta-rhombohedral boron

 

see below and Semiconductor Science and Technology 32 (2017)  095015

 


H. Werheit

Temperature-dependent change of site occupancies in boron carbide

Abstract:

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Helmut Werheit, Murli H. Manghnani, Udo Kuhlmann, Anwar Hushur,  Sulkhan Shalamberidze

Mode Grüneisen parameters of boron carbide

Solid State Sciences 72 (2017) 80

 

Abstract

IR- and Raman-active phonons of boron carbide and the mode Grüneisen parameters g related are studied concerning their dependence on chemical composition, temperatures between 30 and 800 K and pressures up to ~70 GPa. Most bulk phonons yield g between + 1.5 and  ̶ 1.5: those related to icosahedra yield g = 0.8(3). Surface phonons are distinguished by considerably higher g. Negative g of chain bending modes supports the assumption that the chain center buckles out under pressure. Some striking mode Grüneisen parameters of specific phonons are explained. Pressure-dependent bond lengths suggest the reversible high-pressure phase transition to be second order.


 

 

H. Werheit, K. Flachbart, G. Pristáš, D. Lotnyk, V. Filipov, U. Kuhlmann, N. Shitsevalova, T. Lundström

 

Influence of dopants, particularly carbon, on beta-rhombohedral boron

Semiconductor Science and Technology 32 (2017)  095015
 

Abstract

Owed to the high affinity of carbon to boron, the preparation of carbon-free boron is problematic. Even high-purity (6N) b-rhombohedral boron contains 30 – 60 ppm of C. Hence, carbon affects the boron physical properties published so far more or less significantly. We studied well-defined carbon-doped boron samples based on pure starting material carefully annealed with up to about 1 % C, thus assuring homogeneity. We present and discuss their electrical conductivity, optical absorption, luminescence and phonon spectra. Earlier attempts of other authors to determine the conductivity of C-doped boron are revised. Our results allow estimating the effects of oxygen and iron doping on the electrical conductivity using results taken from literature. Discontinuities at low T impair the electronic properties.

.


 

Helmut Werheit

Comment on the paper: New Ground-State Crystal Structure of Elemental Boron

Phys. Rev. Lett. 118, 089601 (2017)

 


 

 

2016

 

 

Helmut Werheit

Boron carbide: Consistency of components, lattice parameters, fine structure and chemical composition makes the complex structure reasonable

Solid State Sciences 60 (2016) 45-54

                              

Abstract

The complex, highly distorted structure of boron carbide is composed of B12 and B11C icosahedra and CBC, CBB and B□B linear elements, whose concentration depends on the chemical composition each. These concentrations are shown to be consistent with lattice parameters, fine structure data and chemical composition. The respective impacts on lattice parameters are estimated and discussed. Considering the contributions of the different structural components to the energy of the overall structure makes the structure and its variation within the homogeneity range reasonable; in particular that of B4.3C representing the carbon-rich limit of the homogeneity range. Replacing in B4.3C virtually the B□B components by CBC yields the hypothetical moderately distorted structure (B11C)CBC. The reduction of lattice parameters related is compatible with recently reported uncommonly prepared single crystals, whose compositions deviate from B4.3C.


 

 

 

Anwar Hushur, Murli H. Manghnani, Helmut Werheit, Przemyslaw Dera and Quentin Williams

High-pressure phase transition makes B4.3C boron carbide a wide-gap semiconductor

J. Phys: Condensed Matter 28 (2016) 045403

                                    

Abstract

Single-crystal B4.3C boron carbide is investigated through the pressure-dependence and interrelation 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 natB4.3C 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.


 

 

2015

 

 

Helmut Werheit,

Comment on:

Is Linear Group X–Y–Z in Boron Carbide the Weakest Link in the Structure?” by S. V. Konovalikhin and V. I. Ponomarev (Russian Journal of Physical Chemistry A, 2015, Vol. 89, No. 10, pp. 1850–1853)

submitted to:

Russian Journal of Physical Chemistry A
                                                            
                          

 Abstract

The characterization of the boron carbide investigated in the above-mentioned paper and some of the conclusions made on it by the authors are critically appraised with regard to reliable results obtained earlier by other scientists.

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Helmut Werheit, Volodimir Filipov and Natalya Shitsevalova

Confined Raman Scattering –

Easy Access to the Surface Phonons of Specific Crystalline Solids

ZAAC - Journal of Inorganic and General Chemistry/Zeitschrift für Anorganische und Allgemeine Chemie

 Z. Anorg. Allg. Chem. 641 (2015) 1835
                                                                                      

Abstract

An easily feasible method to measure surface phonons of specific crystalline solids is described. Special experimental conditions allow confining Raman scattering to the surface region of crystals, while the contribution of bulk scattering to the spectra is largely excluded. Such conditions exist for example in the case of hexaborides, using FT Raman spectroscopy with a commercial spectrometer with ND:YAG laser excitation We show results obtained on LaB6, SmB6 and boron carbide. – The surface phonon spectra of LaB6 are analyzed by comparing them with bulk spectra and discussed in comparison with results of other experimental methods and theoretical calculations.


 

 

 

 

SIPS

 

Helmut Werheit

Structural Defects: Essential elements of icosahedral boron-rich solids.

2015 Sustainable Industrial Processing Summit

Volume 8: Composite, Quasi-crystals and Nanomaterials

Edited By F. Kongoli, M. Pech-Canul, A. Kalemtas, GH. Werheit, FLOGEN 2015

2015 SIPS 15, 18, 161 FS, page 159

 Abstract:

Owing to their distinguished outstanding properties, Boron and boron-rich compounds are promising for various technical applications. Icosahedra as their common structural key features suggest that physical characteristics are related. The complex structures of icosahedral boron-rich solids vary from  a-rhombohedral boron with ~12 to YB66 –type crystals with ~1584 atoms per elementary cell. Their idealized structures are characterized by considerable electron deficiencies leading to the incorrect theoretical prediction of metallic behavior, opposite to experimental findings. In the real structures, high concentrations of incompletely or even unoccupied regular sites compensate these electron deficiencies, thus making them semiconductors. Hollow spaces between the icosahedra allow accommodation of foreign atoms, thus enabling tailoring individual properties. – In the examples b-rhombohedral boron and boron carbide, structural defects, their determination and the correlation with the electronic properties are described.

.


 

Helmut Werheit, Stefan Hoffmann, Guido Gerlach, Andreas Leithe-Jasper, Takaho Tanaka

Phase Transition and Isotope-induced Phonon Softening in natB4.3C Boron Carbide at Moderate Temperatures

2015 Sustainable Industrial Processing Summit

Volume 8: Composite, Quasi-crystals and Nanomaterials

Edited By F. Kongoli, M. Pech-Canul, A. Kalemtas, GH. Werheit, FLOGEN 2015

2015 SIPS 15, 18, 281 FS, page 183

 

 Abstract:

A recent theoretical structure model of carbon-rich boron carbide (Yao et al.) assumes a continuous phase transition accompanied by loss of inversion symmetry near 790K and a first-order transition at 717 K, breaking the 3-fold rotational symmetry. We checked this model by experiment, performing DSC measurements on single crystal B4.3C. A clear anomaly at 712 K, close to the theoretically predicted phase transition, has exothermic character, opposite to the endothermic one predicted. Phonon splitting between 700 and 800 K indicates structural changes. The IR-active vibration of the bending mode of the C-B-C chains between 100 and 800 K shows additional anomalies between 400 and 500 K: the distribution of isotopes on the B(3) site changes drastically, accompanied by a considerable lattice softening.

 


 

 

 

 

2014

 

Helmut Werheit and Guido Gerlach

Dynamical conductivity of boron carbide: heavily damped plasma vibrations

J. Phys.: Condens. Matter 26 (2014) 425801

Abstract:

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 = ωpτ 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 to m*/me ~ 2.5. The mean free path of the free holes has the order of the cell parameters.


 

Logo HAWAII 

 

 

 

 

Helmut Werheit

Boron Carbide: On Structural Details and Electronic Properties  (invited talk)

(see: Ceramic Engineering and Science Proceedings 2014" (CESP;  ICACC – S4 – 012 – 2014); Vol. 35, Issue 4, Adv. in Ceramic Armor X , Jerry C. LaSalvia (Editor ))  in connection with J. Phys.: Condens. Matter 26 (2014) 425801)

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 Abstract:  

 


 

 

 

Helmut Werheit and Guido Gerlach

Plasma vibrations in boron carbides from 100 to 800 K

(see: J. Phys.: Condens. Matter 26 (2014) 425801)

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:  

 

 

Abstract

 


 

 

Helmut Werheit

Van Hove singularities of some icosahedral boron-rich solids by differential reflectivity spectra 

Solid State Sciences (in press);

online available at: http://dx.doi.org/10.1016/j.solidstatesciences.2014.12.017

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:  

Abstract

 


 

 

H. Werheit, V. Filipov, U. Kuhlmann, T. Dose and T. Lundström

Phase Change in β-Rhombohedral Boron at the n-type/p-type Transition

Solid State Sciences (in press)       

online available at:http://dx.doi.org/10.1016/j.solidstatesciences.2014.11.010

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Abstract:

 


 

icacc 

 

 

H. Werheit

On microstructure and electronic properties of boron carbide (Invited Talk)

Proceedings of the ICACC'14, Daytona Beach, Florida, Jnauary 26 - 31, 2014

Ceramic Engineering and Science Proceedings" (CESP) Volume 35, Issue 4, Advances in Ceramic Armor X , Jerry C. LaSalvia (Editor ), 2014

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Abstract:  

 


 

H Werheit, V Filipov, N Shitsevalova, M Armbrüster, U Schwarz, A. Ievdokimova, V Muratov, V N Gurin and M M Korsukova

Raman scattering in rare earths tetraborides 

Solid State Sciences 31 (2014) 24-32

Abstract:

The Raman spectra of single crystalline RE (rare earth) tetraborides REB4 (RE = Y, La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu) are measured and analyzed with respect to the dependence of the phonon frequencies on the rare earth metal. Phonons representing octahedral B6 units are identified by comparison to the according phonon modes of hexaborides. Their relative force parameters are estimated.

 


 

2012

 

 

H. Werheit, S. Shalamberidze

Advanced microstructure of boron carbide
J. Phys.: Condens. Matter 24 (2012) 385406 

Abstract:

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 B4.3C 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)0.66(CBB)0.16 (B□B)0.18, deviating fundamentally from (B12)CBC, which is 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 natBXC make evident that boron isotopes are not randomly distributed in the structure. However, doping with 2% silicon bring about random distribution. 

 


 

H. Werheit, U. Kuhlmann

Is the established structure of α-rhombohedral boron correct? -
Comparative study of IR-active phonons with B6O, B
4.3C and β-rhombohedral boron.
J. Phys.: Condens. Matter 24 (2012) 305401

 

 Abstract:

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, B4.3C and β-rhombohedral boron makes evident that – aside from the B12 icosahedra – the α-rhombohedral boron structure contains single boron atoms additionally. We assume these single atoms to replace the so far 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 ancient 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.

 


 

H. Werheit, V. Filipov, N Shitsevalova , M. Armbrüster, U. Schwarz

Isotopic phonon effects in LaB6
LaB
6 does not possess cubic symmetry and shows a non-random isotope distribution 

J. Phys.: Condens. Matter  24 (2012) 385405

  

Abstract:

The isotopic phonon effects in LaB6 are investigated on the Raman spectra of a series of high-quality single crystals with systematically varied contents of 10B and 11B isotopes. A detailed group theoretical analysis enlightens the correlation between distortions of the B6 octahedra and the splitting or broadening of phonon modes. The group-theoretical analysis of the spectra of isotopically pure LaB6 makes evident that LaB6 does not have cubic symmetry as assumed so far. A further symmetry reduction of the B6 octahedra occurs in isotopically mixed crystals. There, the distribution of isotopes is not random as commonly assumed; the probability of associating equal isotopes increases with their mass decreasing.

 

 


 

 

H. Werheit, V. Filipov, U. Kuhlmann, U. Schwarz, M. Armbrüster, M. Antadze

Isotopic phonon effects in β-rhombohedral boron - non-statistical isotope distribution
J. Phys.: Condens. Matter  24 (2012) 175401 

see also: http://iopscience.iop.org/0953-8984/labtalk-article/49210

Abstract:

Based on the spectra of IR- and Raman-active phonons, the isotopic phonon effects in β-rhombohedral boron are analyzed in polycrystalline 10B and 11B enriched samples of different origin and high purity natB single crystals. Intra- and inter-icosahedral B-B vibrations are harmonic hence meeting the virtual crystal approximation (VCA). Deviations from the phonon shift expected according to the VCA are attributed to the anharmonic share of lattice vibrations. In the case of icosahedral vibrations, the agreement with calculations on α-rhombohedral boron by Shirai and Katayama-Yoshida is quite satisfactory. Phonon shifts due to isotopic disorder in natB are separated and determined. Some phonon frequencies are sensitive to impurities. – The isotopic phonon effects yield valuable specific information on the nature of the different phonon modes. The occupation of regular boron sites by isotopes deviates significantly from the random distribution.


2011

 

Logo Istanbul 

 

11 - 17 September 2011, Istanbul, Turkey

Proceedings of the ISBB 2011: Special Issue of Solid State Sciences Vol. 14 , Issues 11 - 12 (2012)

 

 

H. Werheit

Isotopic phonon effects in boron-rich solids   (invited) 

Solid State Sciences 14 (2012) 1559-1566

Abstract

The isotopic phonon effects in b-rhombohedral boron, boron carbide (B4.3C - B10C) and LuB12 and ZrB12 dodecaborides are analysed. Phonons based on simple movements of atoms fit the virtual crystal approximation, while complex modes deviate more or less significantly. In boron carbide these deviations are evidently mainly due to anharmonic effects, in agreement with theory. Hence the analysis of isotopic phonon shifts makes anharmonic effects measurable. Polarisation effects, stated by theory, appear to be comparably small in boron-rich structures. Phonon shifts due to isotopic disorder are determined. e The isotopic phonon effects yield valuable specific information on the nature of the different phonon modes. 



 

 

S. Hoffmann and H. Werheit

Phase transition in β-rhombohedral boron at 550 K

Solid State Sciences 14 (2012) 1572-1577

 

Abstract

Differential scanning calorimetry (DSC) of high-purity, single crystal b-rhombohedral boron (melting point 2365 K) evidences a structural phase transition close to 550 K, which is obviously responsible for discontinuities of numerous physical properties in this temperature region. These can be consistently interpreted assuming site hopping of boron atoms between regular, partially occupied crystallographic sites in the crystal structure. This site hopping is influenced by optical excitation.

 



 

 

 

H. Werheit, V. Filipov, U. Schwarz and M. Armbrüster

Excitation-dependent Raman spectra in hexaborides

Solid State Sciences 14 (2012) 1567-1571

 

Abstract

Excitation-dependent Raman spectra of hexaborides are systematically analyzed. Excitation energies below the plasma edge yield the spectra of surface phonons. The most prominent features in the accordingly obtained surface spectrum of LaB6 are due to the metal atoms in the uppermost atomic layer of the [001] face and to the layer of distorted octahedra immediately below.

 

 

 


 

H. Werheit and U. Kuhlmann

On the question of superconductivity in boron carbide: Examination by low-temperature MIR/FIR absorption spectra  

 

(Detailed paper see J. Phys.: Condens. Matter 23 (2011) 435501 - see below) 

 



 

H. Werheit and U. Kuhlmann

Superconductivity in boron carbide? Clarification by low-temperature MIR/FIR spectra 

J. Phys.: Condens. Matter 23 (2011) 435501

  

Abstract

The electronic structure and phonon density of B13B2 boron carbide calculated by Calandra et al. defines this compound as metallic, and the authors predict superconductivity with TC’s up to 36.7 K. Their results are affected by the same deficiencies like former band structure calculations based on hypothetical crystal structures deviating significantly from the real ones. We present optical MIR/FIR spectra of boron carbide with compositions between B4.3C and B10.37C, evidencing semiconducting behaviour at least down to 30 K. There is no indication of superconductivity. – The spectra yield new information on numerous localized gap states close to the valence band edge .



 

H Werheit

On the exceptional negative Seebeck effect in 3d-doped β-rhombohedral boron

Solid State Sciences 13 (2011) 1786 - 1796

Abstract.

Interstitial doping with 3d-elements impact the electronic properties of β-rhombohedral boron significantly; in some cases high negative Seebeck coefficients occur. The explanation by simple charge transfer is excluded. We attribute these effects to the correlation among the diluted 3d atoms generating a specific system of electronic states in the band gap. The interaction with Ogitsu’s antiferromagnetic Ising model, describing the incomplete occupancy of regular B sites, is probable. Details depend on the kind of dopants, occupied sites, and occupancies. Very low V, Cr, and Ni contents evoke extraordinary high negative Seebeck coefficients at ambient conditions; e.g. VB190: S =  ̶ 450 μVK-1 at 270 K,  a shift of more than  ̶ 1000 μVK-1 compared with pure boron. Structural and electronic properties of B:V, B:Fe, B:Cu and some other 3d transition metal solid solutions in β-rhombohedral boron are discussed.

 


  

H Werheit, V Filipov, K Shirai, H. Dekura, N Shitsevalova, U. Schwarz, M Armbrüster

Raman scattering and isotopic phonon effects in dodecaborides

J. Phys.: Condens. Matter 23 (2011) 065403

Abstract.

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 of 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 ZrB12 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, polarization effect and effect of isotopic disorder) on the phonon frequencies are determined, separated and discussed.

 


 

2010

 

Helmut Werheit,  Heinz W. Rotter, Sulkhan Shalamberidze, Andreas Leithe-Jasper and Takaho Tanaka

Gap-state related photoluminescence in boron carbide

Published online: 21 Oct. 2010: Phys. Status Solidi B, 1–5 (2010) / DOI 10.1002/pssb.201046342

Phys. Status Solidi B 248, No. 5, 1275-1279 (2011)

 

Abstract.

The luminescence spectra of B4.3C, B6.5C and B10C boron carbide with different isotope enrichments (10B, 11B, 12C, 13C) yield electron transition energies from gap states into the valence band and between localized levels in the gap with 0.25 meV resolution. The results well agree with the actual band scheme derived from optical absorption, electrical properties and interband luminescence. Within the whole homogeneity range, the transition energies are independent of chemical composition and isotopes.


 

 

H Werheit, U Kuhlmann, H W Rotter and S O Shalamberidze

Isotopic effects on the phonon modes in boron carbide

J. Phys.: Condens. Matter 22 (2010) 395401

Abstract.

The effect of isotopes (10B–11B; 12C–13C) on the infrared- and Raman-active phonons of boron carbide has been investigated. For B isotopes, the contributions of the virtual crystal approximation, polarization vector and isotopical disorder are separated. Boron and carbon isotope effects are largely opposite to one another and indicate the share of the particular atoms in the atomic assemblies vibrating in specific phonon modes. Some infrared-active phonons behave as expected for monatomic boron crystals.

 

 


 

H Werheit, V Filipov, U Schwarz, M Armbrüster, A Leithe-Jasper, T Tanaka and S O Shalamberidze

On Surface Raman scattering and luminescence radiation of boron carbide

J. Phys.: Condens. Matter 22 (2010) 045401

Abstract.

The discrepancy between Raman spectra of boron carbide obtained by Fourier transform Raman and conventional Raman spectrometry is systematically investigated. While at photon energies below the exciton energy (1.560 eV), Raman scattering of bulk phonons of boron carbide occurs, photon energies exceeding the fundamental absorption edge (2.09 eV) evoke additional patterns, which may essentially be attributed to luminescence or to the excitation of Raman-active processes in the surface region. The reason for this is the very high fundamental absorption in boron carbide inducing a very small penetration depth of the exciting laser radiation. Raman excitations essentially restricted to the boron carbide surface region yield spectra which considerably differ from bulk phonon ones, thus indicating structural modifications.

 

 


 

H Werheit, V Filipov, U Kuhlmann, U Schwarz, M Armbrüster, A Leithe-Jasper, T Tanaka, I Higashi, T Lundström, V N Gurin and M M Korsukova

Raman effect in icosahedral boron-rich solids

Sci. Technol. Adv. Mater. 11 (2010) 023001

Free download: http://iopscience.iop.org/1468-6996/11/2/023001

Abstract.  

We present Raman spectra of numerous icosahedral boron-rich solids having the structure of a-rhombohedral, b-rhombohedral, a-tetragonal, b-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 -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 -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.