Boron suboxide

Boron suboxide
Names
IUPAC name
Boron suboxide
Other names
Hexaboron monoxide
Identifiers
3D model (JSmol)
  • InChI=1S/6B.O
    Key: ULGXBKYAAIALFZ-UHFFFAOYSA-N
  • [B].[B].[B].[B].[B].[B].[O]
Properties
B6O
Molar mass 80.865 g/mol
Appearance Reddish icosahedral twinned crystals
Density 2.56 g/cm3[1]
Melting point 2,000 °C (3,630 °F; 2,270 K)[2]
Structure
Rhombohedral, hR42
R3, No. 166[3]
a = 0.53824 nm, b = 0.53824 nm, c = 1.2322 nm
α = 90°, β = 90°, γ = 120°
6
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Boron suboxide (chemical formula B6O) is a solid compound with a structure built of eight icosahedra at the apexes of the rhombohedral unit cell. Each icosahedron is composed of twelve boron atoms. Two oxygen atoms are located in the interstices along the [111] rhombohedral direction. Due to its short interatomic bond lengths and strongly covalent character, B6O displays a range of outstanding physical and chemical properties such as great hardness (close to that of rhenium diboride and boron nitride), low mass density, high thermal conductivity, high chemical inertness, and excellent wear resistance.[4]

B6O can be synthesized by reducing B2O3 with boron or by oxidation of boron with zinc oxide or other oxidants.[1] These boron suboxide materials formed at or near ambient pressure are generally oxygen deficient and non-stoichiometric (B6Ox, x<0.9) and have poor crystallinity and very small grain size (less than 5 μm). High pressure applied during the synthesis of B6O can significantly increase the crystallinity, oxygen stoichiometry, and crystal size of the products. Mixtures of boron and B2O3 powders were usually used as starting materials in the reported methods for B6O synthesis.[4]

Oxygen-deficient boron suboxide (B6Ox, x<0.9) might form icosahedral particles, which are neither single crystals nor quasicrystals, but twinned groups of twenty tetrahedral crystals.[2][5][6]

B6O of the α-rhombohedral boron type has been investigated because of its ceramic nature (hardness, high melting point, chemical stability, and low density) as a new structural material. In addition to this, these borides have unique bonding not easily accessible by the usual valence theory. Although an X-ray emission spectroscopic method indicated a probable parameter range for the oxygen site of B6O, the correct oxygen position remained open to question until Rietveld analysis of X-ray diffraction profiles on B6O powders were first carried out successfully, even though these were preliminary investigations.[1]

  1. ^ a b c Kobayashi, M.; Higashi, I.; Brodhag, C.; Thévenot, F. (1993). "Structure of B6O Boron-Suboxide by Rietveld Refinement". Journal of Materials Science. 28 (8): 2129–2134. Bibcode:1993JMatS..28.2129K. doi:10.1007/BF00367573. S2CID 137054305.
  2. ^ a b McMillan, P. F.; Hubert, H.; Chizmeshya, A.; Petuskey, W. T.; Garvie L. A. J.; Devouard, B. (1999). "Nucleation and Growth of Icosahedral Boron Suboxide Clusters at High Pressure". Journal of Solid State Chemistry. 147 (1): 281–290. Bibcode:1999JSSCh.147..281M. doi:10.1006/jssc.1999.8272.
  3. ^ Olofsson, Malin; Lundström, Torsten (1997). "Synthesis and structure of non-stoichiometric B6O". Journal of Alloys and Compounds. 257 (1–2): 91–95. doi:10.1016/S0925-8388(97)00008-X.
  4. ^ a b He, D.; Zhao, Y.; Daemen, L.; Qian, J.; Shen, T. D.; Zerda, T. W. (2002). "Boron suboxide: As hard as cubic boron nitride". Applied Physics Letters. 81 (4): 643–645. Bibcode:2002ApPhL..81..643H. doi:10.1063/1.1494860. and references therein
  5. ^ "A grain of boron suboxide (B6O) synthesized by scientists at the Arizona State". Arizona State University. Retrieved 2009-03-18.
  6. ^ Durband, Dennis (1998). "Making the hard stuff" (PDF). Arizona State University. Archived from the original (PDF) on 2016-03-03. Retrieved 2009-03-18.