Recent Results/Neuere Ergebnisse
2019
 |
20th International Symposium on Boron, Borides and Related Materials (ISBB 2019) Sun. Sept. 22, 2019 - Fri. Spet. 27, 2019 TOKI MESSE, Niigata, Niigata, Japan http://www.sogalabo.jp/isbb2019/ |
H. Werheit
New insights in the impact of structural details on the electronic properties of boron carbide
invited contribution
Abstract
H. Werheit
Systematic requirements for getting bulk or surface Raman spectra of solids through the examples boron carbide and some hexaborides
submitted contribution
Abstract
G. Pristáš, H. Werheit, S. Gabáni, S. Shalamberidze, and K. Flachbart
Low-T specific heat anomalies associated with boson peak in isotope-enriched boron carbides B4.3C – B10C
submitted contribution
Abstract
H. Werheit, M.H. Manghnani, A. Hushur
Phonon peculiarities at the high-pressure phase transition of B4.3C boron carbide
Solid State Sciences (2019, submitted)
Abstract
The change from opacity to transparency marks the reversible high-pressure phase transition of boron carbide between 30 and 40 GPa. After related features of the distorted structure and their effect on the electronic properties are at least roughly clarified, object of the present study is the process of structural changes within this phase transition based on the exchange of C atoms between different polar sites of icosahedra. Such minor structural modifications evade detection by X-ray and neutron diffraction, but are accessible in phonon spectra. Peculiarities in connection with the phase transition, suggested at 33(2) GPa, occur in Raman- and IR-active phonons. Interpretation is based on the close relation between the phonon spectra of a-rhombohedral boron and boron carbide and on the shift of B4.3C phonons in differently isotope-enriched samples
H. Werheit
Systematic error in conventionally measured Raman spectra of boron carbide - a general issue in solid state Raman spectroscopy
Review of Scientific Instruments 90, (2019) 043114 ; https://doi.org/10.1063/1.5079525
Abstract
Solid state Raman spectroscopy requires careful attention to the penetration depth of exciting laser radiation. In cases like semiconducting boron carbide and metallic hexaborides, high fundamental absorption above the band gap and reflectivity R ≈ 1 beyond the plasma edge respectively prevent the excitation of bulk phonons largely. Thus, accordingly measured spectra stem preferably from surface scattering. For decades, Raman spectra of boron carbide obtained with different equipment were controversially discussed. Now, common features of icosahedral phonons in FT-Raman spectra of B4.3C and Raman spectra of α–rhombohedral evidence their bulk character, while refuting conventionally measured Raman spectra..
2018
H. Werheit
Assignment of the electronic transitions in B4.3C boron carbide implies a specifically distorted crystal structure
Solid State Sciences 86 (2018) 38
Abstract
The reversible high-pressure phase transition in boron carbide, making this opaque solid glasslike transparent, the visualization of actual structural distortions in the form of atomic arrangements replacing the three-atomic chain in the unit cell, and the electronic DOS calculated for realistic structure models require re-interpretation of the well-known electronic transitions. – The excellent correlation with a selected distorted structure model, proposed by Ektarawong, assuming that the polar C atoms are randomly distributed over all polar sites of the icosahedra, suggests this model to be close to the actual structure. Assignment between electronic properties and structural peculiarities enables solving some open problems, for example, the nature of the high-pressure phase transition. According to Rasim, some boron-rich components replacing a certain share of three-atomic exhibit negative energies; thus they reduce the total energy and make B4.3C the most stable structure.
H. Werheit
Comment on:
Structural Stability of Boron Carbide under Pressure
Proven
by Spectroscopic
Studies up to 73 GPa
(Z. Anorg. Allg.
Chem. 643, 1357 (2017))
Z. Anorg. Allg. Chemie 644 (2018) 353
Abstract
The paper of Chuvachova et al. contains several deficiencies as well as irritating assumptions and conclusions. Their crucial conclusion on the structural stability of boron carbide under pressure is shown to be disproved by their own experimental results.
2017
H.
Werheit, M.H. Manghnani, U.
Kuhlmann, A. Hushur,
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
H. Werheit
Temperature-dependent change of site occupancies in boron carbide
Abstract:
Helmut
Werheit, Murli H. Manghnani, Udo
Kuhlmann, Anwar Hushur,
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.
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
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 = ωp/ωτ
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.
Helmut Werheit
Helmut Werheit and Guido Gerlach
Plasma vibrations in boron carbides
from 100 to 800 K
:
Abstract
Helmut Werheit
:
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
Abstract:
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
Boron carbide (homogeneity range B4.3C – B~11C) exhibits ~ 15 atoms per
elementary
cell and is composed of B12 or B11C icosahedra and linear structure
elements CBC, CBB or B□B
(□, 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 ~300
μV/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.
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,
B4.3C
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
–
LaB6
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
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
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
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)
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.
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.
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.
