skip to main content
DOE PAGES title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Mysterious SiB 3 : Identifying the Relation between α- and β-SiB 3

Abstract

Binary silicon boride SiB 3 has been reported to occur in two forms, as disordered and nonstoichiometric α-SiB 3–x, which relates to the α-rhombohedral phase of boron, and as strictly ordered and stoichiometric β-SiB 3. Similar to other boron-rich icosahedral solids, these SiB 3 phases represent potentially interesting refractory materials. However, their thermal stability, formation conditions, and thermodynamic relation are poorly understood. Here, we map the formation conditions of α-SiB 3–x and β-SiB 3 and analyze their relative thermodynamic stabilities. α-SiB 3–x is metastable (with respect to β-SiB 3 and Si), and its formation is kinetically driven. Pure polycrystalline bulk samples may be obtained within hours when heating stoichiometric mixtures of elemental silicon and boron at temperatures 1200–1300 °C. At the same time, α-SiB 3–x decomposes into SiB6 and Si, and optimum time-temperature synthesis conditions represent a trade-off between rates of formation and decomposition. The formation of stable β-SiB 3 was observed after prolonged treatment (days to weeks) of elemental mixtures with ratios Si/B = 1:1–1:4 at temperatures 1175–1200 °C. The application of high pressures greatly improves the kinetics of SiB 3 formation and allows decoupling of SiB 3 formation from decomposition. Quantitative formation of β-SiB 3 was seen atmore » 1100 °C for samples pressurized to 5.5–8 GPa. β-SiB 3 decomposes peritectoidally at temperatures between 1250 and 1300 °C. The highly ordered nature of β-SiB 3 is reflected in its Raman spectrum, which features narrow and distinct lines. In contrast, the Raman spectrum of α-SiB 3–x is characterized by broad bands, which show a clear relation to the vibrational modes of isostructural, ordered B 6P. The detailed composition and structural properties of disordered α-SiB 3–x were ascertained by a combination of single-crystal X-ray diffraction and 29Si magic angle spinning NMR experiments. Notably, the compositions of polycrystalline bulk samples (obtained at T ≤ 1200 °C) and single crystal samples (obtained from Si-rich molten Si–B mixtures at T > 1400 °C) are different, SiB 2.93(7) and SiB 2.64(2), respectively. The incorporation of Si in the polar position of B 12 icosahedra results in highly strained cluster units. This disorder feature was accounted for in the refined crystal structure model by splitting the polar position into three sites. The electron-precise composition of α-SiB 3–x is SiB 2.5 and corresponds to the incorporation of, on average, two Si atoms in each B 12 icosahedron. Accordingly, α-SiB 3–x constitutes a mixture of B 10Si 2 and B 11Si clusters. The structural and phase stability of α-SiB 3–x were explored using a first-principles cluster expansion. The most stable composition at 0 K is SiB 2.5, which however is unstable with respect to the decomposition β-SiB 3 + Si. Modeling of the configurational and vibrational entropies suggests that α-SiB 3–x only becomes more stable than β-SiB 3 at temperatures above its decomposition into SiB 6 and Si. Hence, we conclude that α-SiB 3–x is metastable at all temperatures. Density functional theory electronic structure calculations yield band gaps of similar size for electron-precise α-SiB 2.5 and β-SiB 3, whereas α-SiB 3 represents a p-type conductor.« less

Authors:
 [1];  [2];  [3]; ORCiD logo [1]; ORCiD logo [1];  [1];  [4];  [4];  [5]; ORCiD logo [6]; ORCiD logo [2]; ORCiD logo [1]
  1. Stockholm Univ. (Sweden)
  2. Augsburg Univ. (Germany)
  3. Chulalongkorn Univ., Bangkok (Thailand); Thailand Center of Excellence in Physics, Bangkok (Thailand)
  4. Linköping Univ. (Sweden)
  5. Tsinghua Univ., Beijing (China)
  6. Carnegie Mellon Univ., Pittsburgh, PA (United States)
Publication Date:
Research Org.:
Carnegie Mellon Univ., Pittsburgh, PA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22); German Research Foundation (DFG); Swedish Research Council (SRC); National Science Foundation (NSF)
OSTI Identifier:
1574460
Grant/Contract Number:  
SC0014506
Resource Type:
Accepted Manuscript
Journal Name:
ACS Omega
Additional Journal Information:
Journal Volume: 4; Journal Issue: 20; Journal ID: ISSN 2470-1343
Publisher:
American Chemical Society (ACS)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY

Citation Formats

Eklöf, Daniel, Fischer, Andreas, Ektarawong, Annop, Jaworski, Aleksander, Pell, Andrew J., Grins, Jekabs, Simak, Sergei I., Alling, Björn, Wu, Yang, Widom, Michael, Scherer, Wolfgang, and Häussermann, Ulrich. Mysterious SiB 3 : Identifying the Relation between α- and β-SiB 3. United States: N. p., 2019. Web. doi:10.1021/acsomega.9b02727.
Eklöf, Daniel, Fischer, Andreas, Ektarawong, Annop, Jaworski, Aleksander, Pell, Andrew J., Grins, Jekabs, Simak, Sergei I., Alling, Björn, Wu, Yang, Widom, Michael, Scherer, Wolfgang, & Häussermann, Ulrich. Mysterious SiB 3 : Identifying the Relation between α- and β-SiB 3. United States. doi:10.1021/acsomega.9b02727.
Eklöf, Daniel, Fischer, Andreas, Ektarawong, Annop, Jaworski, Aleksander, Pell, Andrew J., Grins, Jekabs, Simak, Sergei I., Alling, Björn, Wu, Yang, Widom, Michael, Scherer, Wolfgang, and Häussermann, Ulrich. Fri . "Mysterious SiB 3 : Identifying the Relation between α- and β-SiB 3". United States. doi:10.1021/acsomega.9b02727. https://www.osti.gov/servlets/purl/1574460.
@article{osti_1574460,
title = {Mysterious SiB 3 : Identifying the Relation between α- and β-SiB 3},
author = {Eklöf, Daniel and Fischer, Andreas and Ektarawong, Annop and Jaworski, Aleksander and Pell, Andrew J. and Grins, Jekabs and Simak, Sergei I. and Alling, Björn and Wu, Yang and Widom, Michael and Scherer, Wolfgang and Häussermann, Ulrich},
abstractNote = {Binary silicon boride SiB3 has been reported to occur in two forms, as disordered and nonstoichiometric α-SiB3–x, which relates to the α-rhombohedral phase of boron, and as strictly ordered and stoichiometric β-SiB3. Similar to other boron-rich icosahedral solids, these SiB3 phases represent potentially interesting refractory materials. However, their thermal stability, formation conditions, and thermodynamic relation are poorly understood. Here, we map the formation conditions of α-SiB3–x and β-SiB3 and analyze their relative thermodynamic stabilities. α-SiB3–x is metastable (with respect to β-SiB3 and Si), and its formation is kinetically driven. Pure polycrystalline bulk samples may be obtained within hours when heating stoichiometric mixtures of elemental silicon and boron at temperatures 1200–1300 °C. At the same time, α-SiB3–x decomposes into SiB6 and Si, and optimum time-temperature synthesis conditions represent a trade-off between rates of formation and decomposition. The formation of stable β-SiB3 was observed after prolonged treatment (days to weeks) of elemental mixtures with ratios Si/B = 1:1–1:4 at temperatures 1175–1200 °C. The application of high pressures greatly improves the kinetics of SiB3 formation and allows decoupling of SiB3 formation from decomposition. Quantitative formation of β-SiB3 was seen at 1100 °C for samples pressurized to 5.5–8 GPa. β-SiB3 decomposes peritectoidally at temperatures between 1250 and 1300 °C. The highly ordered nature of β-SiB3 is reflected in its Raman spectrum, which features narrow and distinct lines. In contrast, the Raman spectrum of α-SiB3–x is characterized by broad bands, which show a clear relation to the vibrational modes of isostructural, ordered B6P. The detailed composition and structural properties of disordered α-SiB3–x were ascertained by a combination of single-crystal X-ray diffraction and 29Si magic angle spinning NMR experiments. Notably, the compositions of polycrystalline bulk samples (obtained at T ≤ 1200 °C) and single crystal samples (obtained from Si-rich molten Si–B mixtures at T > 1400 °C) are different, SiB2.93(7) and SiB2.64(2), respectively. The incorporation of Si in the polar position of B12 icosahedra results in highly strained cluster units. This disorder feature was accounted for in the refined crystal structure model by splitting the polar position into three sites. The electron-precise composition of α-SiB3–x is SiB2.5 and corresponds to the incorporation of, on average, two Si atoms in each B12 icosahedron. Accordingly, α-SiB3–x constitutes a mixture of B10Si2 and B11Si clusters. The structural and phase stability of α-SiB3–x were explored using a first-principles cluster expansion. The most stable composition at 0 K is SiB2.5, which however is unstable with respect to the decomposition β-SiB3 + Si. Modeling of the configurational and vibrational entropies suggests that α-SiB3–x only becomes more stable than β-SiB3 at temperatures above its decomposition into SiB6 and Si. Hence, we conclude that α-SiB3–x is metastable at all temperatures. Density functional theory electronic structure calculations yield band gaps of similar size for electron-precise α-SiB2.5 and β-SiB3, whereas α-SiB3 represents a p-type conductor.},
doi = {10.1021/acsomega.9b02727},
journal = {ACS Omega},
number = 20,
volume = 4,
place = {United States},
year = {2019},
month = {11}
}

Journal Article:
Free Publicly Available Full Text
Publisher's Version of Record

Save / Share: