Electronic structures and mechanical properties of boron and boron-rich crystals (Part I)

Shirai, Katsuhiko

doi:10.3103/s1063457610030068

Cited by 44 publications

(45 citation statements)

References 161 publications

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“…This weak bonding of the Si atoms with the icosahedra (bond length = 0.1764 nm) could further stabilize the icosahedra by donating electrons owing to the less electro-negativity of silicon. Shirai [26] noted that in boron carbide, (1) the intraicosahedral bonds are weaker than the intericosahedral bonds, and (2) the restoring force against the displacement perpendicular to the chain bond axis is much lower than the bond stretching force, which would result in lower energetic barrier for chain bending vs. chain stretching. Ab initio simulations by several groups implied that the amorphization in boron carbide must be triggered by the bending (not breaking) of the 3-atom chain that leads to formation of a new bond between the displaced chain center atom and an atom in the neighboring icosahedron, pulling this atom out of the icosahedron and subsequently resulting in the destruction of the crystalline phase [2,6,[27][28][29].…”

Section: Resultsmentioning

confidence: 99%

Locating Si atoms in Si-doped boron carbide: A route to understand amorphization mitigation mechanism

et al. 2018

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The well-documented formation of amorphous bands in boron carbide (B 4 C) under contact loading has been identified in the literature as one of the possible mechanisms for its catastrophic failure. To mitigate amorphization, Si-doping was suggested by an earlier computational work, which was further substantiated by an experimental study. However, there have been discrepancies between theoretical and experimental studies, about Si replacing atom/s in B 12 icosahedra or the C-B-C chain. Dense single phase Si-doped boron carbide is produced through a conventional scalable route. A powder mixture of SiB 6 , B 4 C, and amorphous boron is reactively sintered, yielding a dense Si-doped boron carbide material.A combined analysis of Rietveld refinement on XRD pattern coupled with electron density difference Fourier maps and DFT simulations were performed in order to investigate the location of Si atoms in boron carbide lattice. Si atoms occupy an interstitial position, between the icosahedra and the chain. These Si atoms are bonded to the chain end C atoms and result in a kinked chain. Additionally, these Si atoms are also bonded to the neighboring equatorial M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT2 B atom of the icosahedra, which is already bonded to the C atom of the chain, forming a bridge like geometry. Si atoms are found to reside around the chain, resulting in a kinked chain. These Si atoms lie close to boron atom of the neighboring icosahedra. Owing to this bonding, distance suggests weak bonding and Si is anticipated to stabilize the icosahedra through electron donation, which is expected to help in mitigating stress-induced amorphization. Possible supercell structures are suggested along with the most plausible structure for Si-doped boron carbide.

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Section: Resultsmentioning

confidence: 99%

Locating Si atoms in Si-doped boron carbide: A route to understand amorphization mitigation mechanism

et al. 2018

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“…Such explanation is in line with the results, obtained from the theoretical calculations of electronic properties, demonstrating that B 12 (CBC) is metallic [53,59,72]. Even though B 4 C is predicted to be a semiconductor, in qualitative agreement with the experimental findings, the calculated electronic band gap of the most favorable B 11 C p (CBC) is overestimated, ranging from 2.6 up to 3.0 eV [9,73,74]. It is worth noting that the electronic band gaps, almost without exception, are typically underestimated in standard density functional theory calculations due to the approximation of the exchange-correlation functionals (see Chapter 2.4).…”

supporting

confidence: 88%

“…Although the crystal symmetry and space group of β-rhombohedral boron are identical to those of α-rhombohedral boron, its crystal structure is far more complicated in which, apart from regular B 12 icosahedral clusters, it can consist of larger clusters of boron atoms (B 84 ), deltahedra (B 10 ), and individual boron atoms [9,10]. Due to its highly complex atomic structure, several structural models of β-boron have been proposed in the literature [11][12][13].…”

Section: Introductionmentioning

confidence: 99%

Disordered Icosahedral Boron-Rich Solids: A Theoretical Study of Thermodynamic Stability and Properties

Ektarawong¹

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The cover page illustrates the crystal structure of disordered boron carbide (B 4 C), projected on the (100) r crystal plane in rhombohedral symmetry.The cover image is visualized by the VESTA package. Red and white spheres represent boron and carbon atoms, respectively.During the course of research underlying this thesis, Annop Ektarawong was enrolled in Agora Materiae, a multidisciplinary doctoral program at Linköping University, Sweden.c Annop Ektarawong Printed by LiU-Tryck 2017 AbstractThis thesis is a theoretical study of configurational disorder in icosahedral boronrich solids, in particular boron carbide, including also the development of a methodological framework for treating configurational disorder in such materials, namely superatom-special quasirandom structure (SA-SQS). In terms of its practical implementations, the SA-SQS method is demonstrated to be capable of efficiently modeling configurational disorder in icosahedral boron-rich solids, whiles the thermodynamic stability as well as the properties of the configurationally disordered icosahedral boron-rich solids, modeled from the SA-SQS method, can be directly investigated, using the density functional theory (DFT).In case of boron carbide, especially B 4 C and B 13 C 2 compositions, the SA-SQS method is used for modeling configurational disorder, arising from a high concentration of low-energy B/C substitutional defects. The results, obtained from the DFT-based calculations, demonstrate that configurational disorder of B and C atoms in boron carbide is not only thermodynamically favored at high temperature, but it also plays an important role in altering the properties of boron carbide − for example, restoration of higher rhombohedral symmetry of B 4 C, a metal-tononmetal transition and a drastic increase in the elastic moduli of B 13 C 2 . The configurational disorder can also explain large discrepancies, regarding the properties of boron carbide, between experiments and previous theoretical calculations, having been a long standing controversial issue in the field of icosahedral boronrich solids, as the calculated properties of the disordered boron carbides are found to be in qualitatively good agreement with those, observed in experiments. In order to investigate the configurational evolution of B 4 C as a function of temperature, beyond the SA-SQS level, a brute-force cluster-expansion method in combination with Monte Carlo simulations is implemented. The results demonstrate that configurational disorder in B 4 C indeed essentially takes place within the icosahedra in a way that justifies the focus on low-energy defect patterns of the superatom picture.The investigation of the thermodynamic stability of icosahedral carbon-rich iii iv boron carbides beyond the believed solubility limit of carbon (20 at.% C) demonstrates that, apart from B 4 C generally addressed in the literature, B 2.5 C represented by B 10 C p 2 (CC) is predicted to be thermodynamically stable with respect to B 4 C as well as pure boron and carbon under high pressure...

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“…Note that the band gaps, almost without exception, are underestimated in standard density functional theory calculations due to the approximation of the exchangecorrelation functionals. The calculated band gaps of the presumed B 11 C p (CBC), within the range from 2.6 up to 3.0 eV, are reported [5,60,61]. Note also that a smaller band gap of about 1.6 eV can be achieved from the high-energy B 12 (CCC) unit due to the appearance of the mid gap states [62].…”

Section: Boron Carbidementioning

confidence: 86%

“…However, a limited intermixing of B 6 O and B 13 C 2 to form solid solutions at high temperature is predicted, e.g. a solid solution of ∼5% B 13 C 2 in B 6 O and ∼20% B 6 O in B 13 C 2 at 2000 K. Introduction processes the same rhombohedral symmetry as does α-boron, the situation is far more complicated in which, apart from regular icosahedra, it can consists of larger clusters of boron atoms (B 84 ), deltahedra (B 10 ), and individual boron atoms [5,6]. Due to its highly complex atomic structure, several structural models of β-boron have been proposed in the literature [7][8][9].…”

Section: Introductionmentioning

confidence: 99%

First-principles study of configurational disorder in icosahedral boron-rich solids

Ektarawong¹

2015

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This thesis is a theoretical study of configurationally disordered icosahedral boronrich solids, in particular boron carbides, using density functional theory and alloy theory. The goal is to resolve discrepancies, regarding the properties of boron carbides, between experiments and previous theoretical calculations which have been a controversial issue in the field of icosahedral boron-rich solids. For instance, B 13 C 2 is observed experimentally to be a semiconductor, meanwhile electronic band structure calculations reveal a metallic character of B 13 C 2 due to its electron deficiency. In B 4 C, on the other hand, the experimentally observed band gap is unexpectedly smaller, not the usual larger, than that of standard DFT calculations. Another example is given by the existence of a small structural distortion in B 4 C, as predicted in theoretical calculations, which reduces the crystal symmetry from the experimentally observed rhombohedral (R3m) to the based-centered monoclinic (Cm). Since boron carbide is stable as a single-phase over a broad composition range (∼8-20 at.% C), substitution of boron and carbon atoms for one another is conceivable. For this reason, the discrepancies have been speculated in the literature, without a proof, to originate from configurational disorder induced by substitutional defects. However, owing to its complex atomic structure, represented by 12-atom icosahedra and 3-atom intericosahedral chains, a practical alloy theory method for direct calculations of the properties of the relevant configurations of disordered boron carbides, as well as for a thermodynamic assessment of their stability has been missing.In this thesis, a new approach, the superatom-special quasirandom structure (SA-SQS), has been developed. The approach allows one to model configurational disorder in boron carbide, induced by high concentrations of low-energy B/C substitutional defects. B 13 C 2 and B 4 C are the two stoichiometries, mainly considered in this study, as they are of particular importance and have been in focus in the literature. The results demonstrate that, from thermodynamic considerations, both B 13 C 2 and B 4 C configurationally disorder at high temperature. In the case of B 13 C 2 , the configurational disorder splits off some valence states into the band gap that in turn compensates the electron deficiency in ordered B 13 C 2 , thus resulting in a semiconducting character. As for B 4 C, the configurational disorder eliminates the monoclinic distortion, thus resulting in the restoration of the higher rhombohedral symmetry. Configurational disorder can also account for an exceliii iv lent agreement on elastic moduli of boron carbide between theory and experiment. Thus, several of the previous discrepancies between theory and experiments are resolved.Inspired by attempts to enhance the mechanical properties of boron suboxide by fabricating boron suboxide-boron carbide composites, as recently suggested in the literature, the SA-SQS approach is used for modeling mixtures of boron sub...

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Electronic structures and mechanical properties of boron and boron-rich crystals (Part I)

Cited by 44 publications

References 161 publications

Locating Si atoms in Si-doped boron carbide: A route to understand amorphization mitigation mechanism

Locating Si atoms in Si-doped boron carbide: A route to understand amorphization mitigation mechanism

Disordered Icosahedral Boron-Rich Solids: A Theoretical Study of Thermodynamic Stability and Properties

First-principles study of configurational disorder in icosahedral boron-rich solids

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