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Disordered Icosahedral Boron-Rich Solids: A Theoretical Study of Thermodynamic Stability and Properties
Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is a theoretical study of configurational disorder in icosahedral boron-rich 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 B4C and B13C2 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 B4C, a metal-to-nonmetal transition and a drastic increase in the elastic moduli of B13C2. The configurational disorder can also explain large discrepancies, regarding the proper- ties of boron carbide, between experiments and previous theoretical calculations, having been a long standing controversial issue in the field of icosahedral boron- rich 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 B4C 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 B4C indeed essentially takes place within the icosahedra in a way that justifies the focus on lowenergy defect patterns of the superatom picture.

The investigation of the thermodynamic stability of icosahedral carbon-rich boron carbides beyond the believed solubility limit of carbon (20 at.% C) demonstrates that, apart from B4C generally addressed in the literature, B2.5C represented by B10Cp2(CC) is predicted to be thermodynamically stable with respect to B4C as well as pure boron and carbon under high pressure, ranging between 40 and 67 GPa, and also at elevated temperature. B2.5C is expected to be metastable at ambient pressure, as indicated by its dynamical and mechanical stabilities at 0 GPa. A possible synthesis route of B2.5C and a fingerprint for its characterization from the simulations of x-ray powder diffraction pattern are suggested.

Besides modeling configurational disorder in boron carbide, the SA-SQS method also opens up for theoretical studies of new alloys between different icosahedral boron-rich solids − for example, (B6O)1−x(B13C2)x and B12(As1−xPx)2. As for the pseudo-binary (B6O)1−x(B13C2)x alloy, it is predicted to display a miscibility gap resulting in B6O-rich and either ordered or disordered B13C2-rich domains for intermediate global compositions at all temperatures up to melting points of the materials. However, some intermixing of B6O and B13C2 to form solid solutions is also predicted at high temperature. A noticeable mutual solubility of icosahedral B12As2 and B12P2 in each other to form B12(As1−xPx)2 disordered alloy is predicted even at room temperature, and a complete closure of a pseudo-binary miscibility gap is achieved at around 900 K.

Apart from B12(As1−xPx)2, the thermodynamic stability of other compounds and alloys in the ternary B-As-P system is also investigated. For the binary B-As system, zincblende BAs is found to be thermodynamically unstable with respect to icosahedral B12As2 and gray arsenic at 0 K and increasingly so at higher temperature, indicating that BAs may merely exist as a metastable phase. This is in contrast to the binary B-P system, in which zinc-blende BP and icosahedral B12P2 are both predicted to be stable. Owing to the instability of BAs with respect to B12As2 and gray arsenic, only a tiny amount of BAs is predicted to be able to dissolve in BP to form BAs1−xPx disordered alloy at elevated temperature. For example, less than 5% BAs can dissolve in BP at 1000 K. As for the binary As-P system, As1−xPx disordered alloys are predicted at elevated temperature − for example, a disordered solid solution of up to ∼75% As in black phosphorus as well as a small solubility of ∼1% P in gray arsenic at 750 K, together with the presence of miscibility gaps.

The thermodynamic stability of three different compositions of α-rhombohedral boron-like boron subnitride, having been proposed so far in the literature, is investigated. Those are, B6N, B13N2, and B38N6, represented respectively by B12(N-N), B12(NBN), and [B12(N-N)]0.33[B12(NBN)]0.67. It is found that, out of these sub- nitrides, only B38N6 is thermodynamically stable from 0 GPa up to ∼7.5 GPa, depending on the temperature, and is thus concluded as a stable composition of α-rhombohedral boron-like boron subnitride.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2017.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1843
Keyword [en]
Icosahedral boron-rich solids, Boron carbide, Configurational disorder, First-principles calculations, Thermodynamic stability, Superatom-special quasirandom structure
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:liu:diva-136813DOI: 10.3384/diss.diva-136813ISBN: 978-91-7685-544-7 (print)OAI: oai:DiVA.org:liu-136813DiVA: diva2:1091559
Public defence
2017-06-16, Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2017-04-27 Created: 2017-04-27 Last updated: 2017-05-16Bibliographically approved
List of papers
1. First-principles study of configurational disorder in B4C using a superatom-special quasirandom structure method
Open this publication in new window or tab >>First-principles study of configurational disorder in B4C using a superatom-special quasirandom structure method
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2014 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 2, 024204Article in journal (Refereed) Published
Abstract [en]

Configurationally disordered crystalline boron carbide, with the composition B4C, is studied using first-principles calculations. We investigate both dilute and high concentrations of carbon-boron substitutional defects. For the latter purpose, we suggest a superatoms picture of the complex structure and combine it with a special quasirandom structure approach for disorder. In this way, we model a random distribution of high concentrations of the identified low-energy defects: (1) bipolar defects and (2) rotation of icosahedral carbon among the three polar-up sites. Additionally, the substitutional disorder of the icosahedral carbon at all six polar sites, as previously discussed in the literature, is also considered. Two configurational phase transitions from the ordered to the disordered configurations are predicted to take place upon an increase in temperature using a mean-field approximation for the entropy. The first transition, at 870 K, induces substitutional disorder of the icosahedral carbon atoms among the three polar-up sites; meanwhile the second transition, at 2325 K, reveals the random substitution of the icosahedral carbon atoms at all six polar sites coexisting with bipolar defects. Already the first transition removes the monoclinic distortion existing in the ordered ground-state configuration and restore the rhombohedral system (R3m). The restoration of inversion symmetry yielding the full rhombohedral symmetry (R (3) over barm) on average, corresponding to what is reported in the literature, is achieved after the second transition. Investigating the effects of high pressure on the configurational stability of the disordered B4C phases reveals a tendency to stabilize the ordered ground-state configuration as the configurationally ordering/disordering transition temperature increases with pressure exerted on B4C. The electronic density of states, obtained from the disordered phases, indicates a sensitivity of the band gap to the degree of configurational disorder in B4C.

Place, publisher, year, edition, pages
American Physical Society, 2014
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-109591 (URN)10.1103/PhysRevB.90.024204 (DOI)000339481700003 ()
Conference
Conference name
Available from: 2014-08-21 Created: 2014-08-21 Last updated: 2017-05-16
2. Configurational order-disorder induced metal-nonmetal transition in B13C2 studied with first-principles superatom-special quasirandom structure method
Open this publication in new window or tab >>Configurational order-disorder induced metal-nonmetal transition in B13C2 studied with first-principles superatom-special quasirandom structure method
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2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 1, 014202Article in journal (Refereed) Published
Abstract [en]

Due to a large discrepancy between theory and experiment, the electronic character of crystalline boron carbide B13C2 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 the ordered ground state B13C2 is metallic, the configurationally disordered B13C2, modeled with a superatom-special quasirandom structure method, goes through a metal to nonmetal 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 B11Ce(BBC) unit. The atomic configuration of the substitutionally disordered B13C2 thus tends to be dominated by a mixture between B-12(CBC) and B11Ce(BBC). Due to splitting of valence states in B11Ce(BBC), the electron deficiency in B-12(CBC) is gradually compensated.

Place, publisher, year, edition, pages
American Physical Society, 2015
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-120344 (URN)10.1103/PhysRevB.92.014202 (DOI)000357484100001 ()
Note

Funding Agencies|Swedish Research Council (VR) [621-2011-4417, 330-2014-6336, 2014-4750]; CeNano at Linkoping University; LiLi-NFM; Swedish Government Strategic Research Area Grant in Materials Science

Available from: 2015-07-31 Created: 2015-07-31 Last updated: 2017-05-16
3. Carbon-rich icosahedral boron carbides beyond B4C and their thermodynamic stabilities at high temperature and pressure from first principles
Open this publication in new window or tab >>Carbon-rich icosahedral boron carbides beyond B4C and their thermodynamic stabilities at high temperature and pressure from first principles
2016 (English)In: PHYSICAL REVIEW B, ISSN 2469-9950 (print); 2469-9969 (online), Vol. 94, no 5, 054104Article in journal (Refereed) Published
Abstract [en]

We investigate the thermodynamic stability of carbon-rich icosahedral boron carbide at different compositions, ranging from B4C to B2C, using first-principles calculations. Apart fromB4C, generally addressed in the literature, B2.5C, represented by B10C2p (C-C), where C-p and (C-C) denote a carbon atom occupying the polar site of the icosahedral cluster and a diatomic carbon chain, respectively, is predicted to be thermodynamically stable under high pressures with respect to B4C as well as pure boron and carbon phases. The thermodynamic stability of B2.5C is determined by the Gibbs free energy G as a function of pressure p and temperature T, in which the contributions from the lattice vibrations and the configurational disorder are obtained within the quasiharmonic and the mean-field approximations, respectively. The stability range of B2.5C is then illustrated through the p-T phase diagrams. Depending on the temperatures, the stability range of B2.5C is predicted to be within the range between 40 and 67 GPa. At T greater than or similar to 500 K, the icosahedral C-p atoms in B2.5C configurationally disorder at the polar sites. By investigating the properties of B2.5C, e.g., elastic constants and phonon and electronic density of states, we demonstrate that B2.5C is both mechanically and dynamically stable at zero pressure, and is an electrical semiconductor. Furthermore, based on the sketched phase diagrams, a possible route for experimental synthesis of B2.5C as well as a fingerprint for its characterization from the simulations of x-ray powder diffraction pattern are suggested.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2016
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-131508 (URN)10.1103/PhysRevB.94.054104 (DOI)000381304300001 ()
Note

Funding Agencies|Swedish Research Council (VR) [621-2011-4417, 330-2014-6336, 2014-4750]; Marie Sklodowska Curie Actions [INCA 600398]; CeNano at Linkoping University; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]

Available from: 2016-09-26 Created: 2016-09-23 Last updated: 2017-05-16
4. Effects of configurational disorder on the elastic properties of icosahedral boron-rich alloys based on B6O, B13C2, and B4C, and their mixing thermodynamics
Open this publication in new window or tab >>Effects of configurational disorder on the elastic properties of icosahedral boron-rich alloys based on B6O, B13C2, and B4C, and their mixing thermodynamics
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2016 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 144, no 13, 134503Article in journal (Refereed) Published
Abstract [en]

The elastic properties of alloys between boron suboxide (B6O) and boron carbide (B13C2), denoted by (B6O)1−x(B13C2)x, as well as boron carbide with variable carbon content, ranging from B13C2 to B4C are calculated from first-principles. Furthermore, the mixing thermodynamics of (B6O)1−x(B13C2)x is studied. A superatom-special quasirandom structure approach is used for modeling different atomic configurations, in which effects of configurational disorder between the carbide and suboxide structural units, as well as between boron and carbon atoms within the units, are taken into account. Elastic properties calculations demonstrate that configurational  disorder in B13C2, where a part of the C atoms in the CBC chains substitute for B atoms in the B12 icosahedra, drastically increase the Young’s and shear modulus, as compared to an atomically ordered state, B12(CBC). These calculated elastic moduli of the disordered state are in excellent agreement with experiments. Configurational disorder between boron and carbon can also explain the experimentally observed almost constant elastic moduli of boron carbide as the carbon content is changed from B4C to B13C2. The elastic moduli of the (B6O)1−x(B13C2)x system are also practically unchanged with composition if boron-carbon disorder is taken into account. By investigating the mixing thermodynamics of the alloys, in which the Gibbs free energy is determined within the mean-field approximation for the configurational entropy, we outline the pseudo-binary phase diagram of (B6O)1−x(B13C2)x. The phase diagram reveals the existence of a miscibility gap at all temperatures up to the melting point. Also, the coexistence of B6O-rich as well as ordered or disordered B13C2-rich domains in the material prepared through equilibrium routes is predicted.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2016
National Category
Physical Sciences
Identifiers
urn:nbn:se:liu:diva-122425 (URN)10.1063/1.4944982 (DOI)000374527900023 ()27059576 (PubMedID)
Note

Funding agencies:Swedish Research Council (VR) [621-2011-4417, 330-2014-6336, 2011-42-59]; CeNano at Linkoping University; Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST "MISiS" [K3-2014-049]; LiLi-

At the time for thesis presentation publication was in status: Manuscript

Available from: 2015-11-02 Created: 2015-11-02 Last updated: 2017-05-16Bibliographically approved
5. Thermodynamic stability and properties of boron subnitrides from first principles
Open this publication in new window or tab >>Thermodynamic stability and properties of boron subnitrides from first principles
2017 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 6, 064206Article, review/survey (Refereed) Published
Abstract [en]

We use the first-principles approach to clarify the thermodynamic stability as a function of pressure and temperature of three different alpha-rhombohedral-boron-like boron subnitrides, with the compositions of B6N, B13N2, and B38N6, proposed in the literature. We find that, out of these subnitrides with the structural units of B-12(N-N), B-12(NBN), and [B-12(N-N)](0.33)[B-12(NBN)](0.67), respectively, only B38N6, represented by [B-12(N-N)](0.33)[B-12(NBN)](0.67), is thermodynamically stable. Beyond a pressure of about 7.5 GPa depending on the temperature, also B38N6 becomes unstable, and decomposes into cubic boron nitride and a-tetragonalboron- like boron subnitride B50N2. The thermodynamic stability of boron subnitrides and relevant competing phases is determined by the Gibbs free energy, in which the contributions from the lattice vibrations and the configurational disorder are obtained within the quasiharmonic and the mean-field approximations, respectively. We calculate lattice parameters, elastic constants, phonon and electronic density of states, and demonstrate that [B-12(N-N)](0.33)[B-12(NBN)](0.67) is bothmechanically and dynamically stable, and is an electrical semiconductor. The simulated x-ray powder-diffraction pattern as well as the calculated lattice parameters of [B-12(N-N)](0.33)[B-12(NBN)](0.67) are found to be in good agreement with those of the experimentally synthesized boron subnitrides reported in the literature, verifying that B38N6 is the stable composition of a-rhombohedral-boron-like boron subnitride.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2017
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-136168 (URN)10.1103/PhysRevB.95.064206 (DOI)000394658500001 ()
Note

Funding Agencies|Swedish Research Council (VR) [621-2011- 4417, 330-2014-6336, 2014-4750]; Marie Sklodowska Curie Actions [INCA 600398]; CeNano at Linkoping University; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]

Available from: 2017-04-03 Created: 2017-04-03 Last updated: 2017-05-16

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