Icosahedral Boron-rich Solids Research Articles

Carbon contamination of elemental beta-boron promoted a stable boron carbide by spark plasma sintering

The effect of carbon contamination on the elemental beta boron (β-B) from the graphite components of spark plasma sintering (SPS) was investigated. The X-ray diffraction, analytical electron microscopy (AEM) and Raman spectroscopy witnessed boron carbide (B13+xC2-x where x=0.05–0.35), when SPS temperature was above 1750 °C. The degree of carbon contamination became severe with increasing SPS temperature to 1800 °C and 1850 °C and formed a single-phase rhombohedral boron carbide with about 12 wt% of carbon. Our observations indicate that the carbon gets kinetically trapped into β-B to form boron carbide with a primitive unit cell consisting mainly of icosahedral B11C and three-atom chain of CBB configurations. After 6 months of natural aging, it has been revealed that boron carbide formed at and below sintering temperatures of 1800 °C is not a thermodynamically stable phase, and transformed reversibly to β-B structure. X-ray photoelectron spectroscopy ascertained that there are no longer B-C bonds and the existence of more CC bonds in a 6 months aged specimen, suggesting carbon is no longer strongly bonded to boron, but precipitates as graphitic structure. Further examination using atom probe tomography and AEM confirmed the existence of carbon nanoclusters within grains and high concentration of carbon segregating onto grain boundaries. The phase transformation of boron carbide to mostly boron resulted in a significant loss of hardness and modulus in a bulk SPS specimen. This study underscores the importance of solid solution and local bonding environment in icosahedral boron-rich solids for phase stability and sustained properties.

Intrinsic dense twinning via release of native strain

Icosahedral boron-rich solids exhibit outstanding mechanical properties, but the crystal structure of this prominent class of materials has long remained enigmatic. Here, we report on a surprising discovery of an unprecedented twinning induced structural stabilization that creates highly nanotwinned crystal structures that are stabler than the prevailing single crystal structure comprising complex multi-atom structural units. This phenomenon is showcased via a symmetry guided optimization process that produces a series of increasingly nanotwinned B4C structures with progressively lower energy below that of the single crystal, and this behavior also occurs in multiple other boron-rich solids. These findings unveil a distinct paradigm of defect (twinning) induced structural stabilizing mechanism that reduces energy via release of native strains built in the complex structural units of the single crystal, creating an exceptional category of materials that comprise multiple domains of intrinsic dense twinning in the crystal structures.

High-pressure deformation and amorphization in boron carbide

Icosahedral boron-rich solids fall second in hardness to diamondlike structures and have been the subject of intense investigations over the past two decades, as they possess low density, high thermal, and mechanical stability at high temperatures, and superior industrial manufacturability. A common deleterious feature called “presssure-induced amorphization,” limits their performance in high-velocity projectile applications. This article discusses spectral characteristics of amorphized states of boron carbide, a common icosahedral boron-rich ceramic, with the goal of understanding the mechanistic layout of pressure-induced amorphization. Mystery has surrounded the appearance of new peaks in Raman spectrum of pressure-induced amorphized boron carbide, but to date, no convincing explanation exists on their origin. Shock studies of boron carbide have proposed phase transformation at high pressures, but to date, no conclusive evidence has been corroborative to prove the existence of new high-pressure phases. We propose a new rationale toward deciphering the amorphization phenomenon in boron carbide centered on a thermodynamic approach to explain atomic interactions in amorphous islands. Quantum mechanical simulations are utilized to understand the impact of stresses on Raman spectra, while results from molecular dynamics (MD) simulations of volumetric compression are used to understand thermodynamic aspects of amorphization. Atomic-level nonbonded interactions from the MD potential are utilized to demonstrate origins of the residual pressure. Combining these efforts, the present study deciphers the connection between deformation behavior of boron carbide at high pressure and its mysterious amorphous Raman spectrum. The approach highlights the importance of meticulously incorporating multiscale modeling considerations in determining accurate material behavior of ultrahard materials.

Effect of temperature and configurational disorder on the electronic band gap of boron carbide from first principles

The overestimation, rather than the usual underestimation, of the electronic band gap at 0 K of boron carbide with the ideally stoichiometric composition of ${\mathrm{B}}_{4}\mathrm{C}$, represented by ${\mathrm{B}}_{11}{\mathrm{C}}^{p}$(CBC), in density functional theory calculations is one of the outstanding controversial issues in the field of icosahedral boron-rich solids. Using a first-principles approach, we explore the effect of temperature and configurational disorder on the electronic band gap of ${\mathrm{B}}_{4}\mathrm{C}$. Ab initio molecular dynamics simulations are performed to account for the effects of vibrational disorder. The results reveal that the volumetric thermal expansion as well as the thermally induced configurational disorder of icosahedral ${\mathrm{C}}^{p}$ atoms residing in the ${\mathrm{B}}_{11}{\mathrm{C}}^{p}$ icosahedra have a minimal impact on the band gap of ${\mathrm{B}}_{4}\mathrm{C}$, while a major decrease of the band gap is caused by explicit atomic displacements, induced by lattice vibrations. At 298 K, the band gap of ${\mathrm{B}}_{4}\mathrm{C}$ is overestimated, as compared to the experimental value, by approximately 31%. However, configurational disorder induced by introducing a small fraction of ${\mathrm{B}}_{12}$(CBC) and ${\mathrm{B}}_{12}({\mathrm{B}}_{4}$) into a matrix of ${\mathrm{B}}_{11}{\mathrm{C}}^{p}$(CBC) to make the composition of boron carbide approximately ${\mathrm{B}}_{4.3}\mathrm{C}$, claimed to be the carbon-rich limit of the material in experiment, leads to a smaller band gap due to the appearance of midgap states. These results can explain at least a part of the previous discrepancies between theory and experiments for the band gap of boron carbide.

Configurational order-disorder induced metal-nonmetal transition inB13C2studied with first-principles superatom-special quasirandom structure method

Due to a large discrepancy between theory and experiment, the electronic character of crystalline boron carbide B$_{13}$C$_{2}$ has been a controversial topic in the field of icosahedral boron-rich solids. We demonstrate that this discrepancy is removed when configurational disorder is accurately considered in the theoretical calculations. We find that while ordered ground state B$_{13}$C$_{2}$ is metallic, configurationally disordered B$_{13}$C$_{2}$, modeled with a superatom-special quasirandom structure method, goes through a metal to non-metal transition as the degree of disorder is increased with increasing temperature. Specifically, one of the chain-end carbon atoms in the CBC chains substitutes a neighboring equatorial boron atom in a B$_{12}$ icosahedron bonded to it, giving rise to a B$_{11}$C$^{e}$(BBC) unit. The atomic configuration of the substitutionally disordered B$_{13}$C$_{2}$ thus tends to be dominated by a mixture between B$_{12}$(CBC) and B$_{11}$C$^{e}$(BBC). Due to splitting of valence states in B$_{11}$C$^{e}$(BBC), the electron deficiency in B$_{12}$(CBC) is gradually compensated.

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

Differential reflectivity spectra of some icosahedral boron rich solids, β-rhombohedral boron, boron carbide and YB66-type crystals, were measured. The derivatives yield the van Hove singularities, which are compared with results obtained by other experimental methods.

Electronic structure and defect properties of B6O from hybrid functional and many-body perturbation theory calculations: A possible ambipolar transparent conductor

${\mathrm{B}}_{6}\mathrm{O}$ is a member of icosahedral boron-rich solids known for their physical hardness and stability under irradiation bombardment, but it has also recently emerged as a promising high mobility $p\ensuremath{-}$type transparent conducting oxide. Using a combination of hybrid functional and many-body perturbation theory calculations, we report on the electronic structure and defect properties of this material. Our calculations identify ${\mathrm{B}}_{6}\mathrm{O}$ has a direct band gap in excess of 3.0 eV and possesses largely isotropic and low effective masses for both holes and electrons. Of the native defects, we identify no intrinsic origin to the reported $p\ensuremath{-}$type conductivity and confirm that $p\ensuremath{-}$type doping is not prevented by intrinsic defects such as oxygen vacancies, which we find act exclusively as neutral defects rather than hole-killing donors. We also investigate a number of common impurities and plausible dopants, finding that isolated acceptor candidates tend to yield deep states within the band gap or act instead as donors, and cannot account for $p\ensuremath{-}$type conductivity. Our calculations identify the only shallow acceptor candidate to be a complex consisting of interstitial H bonded to C substituting on the O site ${(\mathrm{CH})}_{\mathrm{O}}$. We therefore attribute the origins of $p\ensuremath{-}$type conductivity to these complexes formed during growth or more likely via isolated ${\mathrm{C}}_{\mathrm{O}}$ which later binds with H within the crystal. Lastly, we identify Si as a plausible $n\ensuremath{-}$type dopant, as it favorably acts as a shallow donor and does not suffer from self-compensation as may the C-related defects. Thus, in addition to the observed $p\ensuremath{-}$type conductivity, ${\mathrm{B}}_{6}\mathrm{O}$ exhibits promise of $n\ensuremath{-}$type dopability if the stoichiometry and both native and extrinsic sources of compensation can be sufficiently controlled.

An α-rhombohedral boron-related compound with sulfur: Synthesis, structure and thermoelectric properties

The structural and thermoelectric properties of a series of icosahedral boron-rich solids, B6S1 − x obtained from the elements in a Ga melt were studied to estimate their potential as thermoelectric materials. Rietveld analysis of synchrotron and conventional X-ray powder data displayed the rhombohedral structure with the α-rhombohedral boron framework of B12 icosahedra and a narrow filling fraction range for sulfur atoms in the interstitial voids (0.37 < x ⩽ 0.40). The samples obtained exhibited p-type semiconducting behavior with large Seebeck coefficients, reaching a maximum of ∼220 μV K−1.

ChemInform Abstract: High‐Pressure Route to Superhard Boron‐Rich Solids

AbstractReview: 43 refs.

High-pressure route to superhard boron-rich solids

Novel superhard phases are expected to be found among various high-pressure polymorphs of light element compounds. Besides diamond-like phases, the icosahedral boron-rich solids are of particular interest because they could combine high hardness with advanced electronic and phonon transport properties, lightness, high thermal and chemical stability. Here we review some recent results on high-pressure synthesis of novel boron-rich solids.

Present knowledge of electronic properties and charge transport of icosahedral boron-rich solids

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.

Are there bipolarons in icosahedral boron-rich solids?

The charge transport of boron carbide, often incorrectly denoted asB4C, has been controversially discussed. It is shown that the bipolaron hypothesis is notcompatible with numerous experimental results. In particular, the determined realmicrostructure of boron carbide and its related electronic properties disprove severalassumptions, which are fundamental to the bipolaron hypothesis. In contrast, the actualenergy band scheme derived mainly from optical investigations is confirmed by carefulevaluation of the high-temperature electrical conductivity, and allows a consistentdescription at most of the experimental results.

Unusual properties of icosahedral boron-rich solids

Icosahedral boron-rich solids are materials containing boron-rich units in which atoms reside at an icosahedron's 12 vertices. These materials are known for their exceptional bonding and the unusual structures that result. This article describes how the unusual bonding generates other distinctive and useful effects. In particular, radiation-induced atomic vacancies and interstitials spontaneously recombine to produce the “self-healing” that underlies these materials’ extraordinary radiation tolerance. Furthermore, boron carbides, a group of icosahedral boron-rich solids, possess unusual electronic, magnetic and thermal properties. For example, the charge carriers, holes, localize as singlet pairs on icosahedra. The unusual origin of this localization is indicated by the absence of a concomitant photo-ionization. The thermally assisted hopping of singlet pairs between icosahedra produces Seebeck coefficients that are unexpectedly large and only weakly dependent on carrier concentration. These properties are exploited in devices: (1) long-lived high-power high-capacity beta-voltaic cells, (2) very high temperature thermoelectrics and (3) solid-state neutron detectors.

Low-temperature magnetism of the compound GdB18Si5

Low-temperature magnetic properties of single crystals of the B12 cluster compound GdB18Si5(R3̄m)were investigated. A sharp drop in the a– bin-plane magnetic susceptibility is observed at TN = 3.2 Kindicating an antiferromagnetic transition. Anisotropy between the in-plane andc-axissusceptibilities is observed, consistent with the spins ordering in the a– bplane. The logarithm of the resistivity follows the hoppingT−0.25-behaviourwhich has been typically observed for B12 icosahedral boron-rich solids,and this indicates that the typical Ruderman–Kittel–Kasuya–Yosidainteraction is not the mediating interaction. A λ-likepeak is observed in the magnetic specific heat at 3.2 K and supports the notion ofa long-range-order antiferromagnetic transition occurring in this system.

Interband Transitions and Optical Phonons of B48Al3C2

The optical absorption edge of B48Al3C2 has been measured. It is decomposed into several interband transitions between 0.97 and 2.5 eV. Additionally, several gap-state-related transitions with energies between 0.45 and 0.96 eV were determined. They are attributed to defects in the structure, whose character is not yet known. The spectra of IR- and Raman-active phonons and their resonance frequencies are presented. According to other icosahedral boron-rich solids, the phonons can roughly be separated into vibrations of icosahedra and those of single atoms. The vibrations of the tetrahedrally bonded Al1 atom are discussed in analogy to the tetrahedral XY4 molecule.

Structural Defects of Some Icosahedral Boron-Rich Solids and Their Correlation with the Electronic Properties

For β-rhombohedral boron and boron carbide, the hitherto best-investigated icoasahedral boron-rich solids, the concentrations of structural defects and electronic gap states are quantitatively correlated. In this way the theoretically determined valence electron deficiencies are exactly compensated, and the metallic character of these solids resulting in the theoretical calculations on hypothetical idealized structures is changed to the experimentally proved semiconducting behavior. Obviously, the structural defects in these crystals are the necessary consequence of the valence electron deficiency. It is suggested that this correlation holds for icosahedral boron-rich solids in general.

Atomic structure and vibrational properties of icosahedral α-boron and B4C boron carbide

The Raman and infrared spectra of α-rhombohedral boron B12 and of B4C boron carbide have been determined by accurate first-principles calculations based on density-functional perturbation theory. Our results account for all the features observed experimentally, including the characteristic Raman-active mode around 530 cm−1, which is attributed to the libration of the icosahedra. A comparison of the calculated vibrational spectra with experimental data allows the first unambiguous determination of the atomic structure of B4C. Analysis of our data shows that the high bulk moduli of α-rhombohedral boron and of B4C boron carbide – 220 and 240 GPa, respectively – are mainly determined by the stiff intramolecular bonding within each icosahedron. This finding is at variance with the current interpretation of recent neutron diffraction data on B4C in terms of a postulated larger stiffness of the intermolecular bonds in icosahedral solids (inverted molecular compressibility). Our results show that icosahedral boron-rich solids should be considered as members of a new class of covalently bonded materials.

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.

The Complete Optical Spectrum ofβ-Rhombohedral Boron

To give a quantitative overview of the interaction between electromagnetic waves and icosahedral boron-rich solids, the reflectivity and absorption index spectra ofβ-rhombohedral boron between dc and about 250 eV are presented. They are put together from reliable results obtained from literature and new measurements partly with very high resolution. From a suitable combination of the experimental data the real and the imaginary parts of the dielectric function were calculated The interband transitions prove to be the strongest interaction in the whole spectrum.

The Bonding Nature of Icosahedral Clusters of the Group III Elements

The bonding nature of icosahedral clusters of the group III elements was investigated to obtain the relation between properties of the clusters and their aggregates, i.e., icosahedral boron-rich solids and icosahedral aluminum-based quasicrystals. By means of a molecular orbital method of the bonding nature was evaluated whether it is metallic or covalent type. It is found that the 12-atom icosahedra (B12and Al12) and the 13-atom icosahedra (B13and Al13) show covalent and metallic-type bonding, respectively. Solid state properties of a cluster solid of the group III elements are considered to be profoundly related to the question whether the central icosahedral position is occupied or not.