Boron suboxide

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Boron suboxide
Molecular formula B6O
Molar mass 80.865 g/mol
Appearance Reddish icosahedral twinned crystals
Density 2.56 g/cm3[1]
Melting point 2000 °C[2]
Except where noted otherwise, 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 containing six boron atoms and one oxygen atom. Its structure is built of eight icosahedra at the apexes of the rhombohedral unit cell (space group R3-m). 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.[3]

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 (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.[3]

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][4][5]

B 6O of the α-rohombohedral 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]

Preparation Method[edit]

The preparation method of B6O is classified into three categories: (1) solid state reaction between B and B2O3, (2) reduction of B2O3 and (3) oxidation of B. The high vapor pressure of B2O3 at elevated temperatures would cause the B excess composition in the process of the solid state reaction between B and B2O3. In the reduction of B2O3, reductants that can be used include, but not limited to, Si and Mg which remain in B6O as an impurity in the process. While in the oxidation process of B, oxidants such as ZnO would contaminate B6O in the process.[6]

Physical Properties[edit]

Some researchers have reported the crystal structure, which helped identify some mechanical properties of B6O. B6O has a strong covalent nature and is easy to compose at high temperature more than 1973 K.[6] Boron Suboxide has also been reported to exhibit a wide range of superior properties such as high hardness with low density, high mechanical strength, oxidation resistance up to high temperatures as well as its high chemical inertness.[7] Preliminary first-principle ab initio density functional calculations of the structural properties boron suboxide (B6O) suggest that the strength of bonding in B6O may be enhanced by the presence of a high electronegativity interstitial in the structure. The computational calculations confirm the shortening of covalent bonds which is believed to favor higher elastic constants and hardness values.[7]


The potential applications of B6O as wear-reduction coatings for high-speed cutting tools, abrasives, or other high wear applications, for example, has been an object of intense interest in recent years. However, despite the intensive research efforts, the commercial applications are yet to be realized. This is partly because of the low fracture toughness of hot pressed material and the considerable practical challenges associated with the densifying stoichiometric B6O material with good crystallinity. Furthermore, numerous mechanical properties of the material were until recently rather poorly understood.[7]

See also[edit]


  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. doi:10.1007/BF00367573. 
  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. doi:10.1006/jssc.1999.8272. 
  3. ^ 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. doi:10.1063/1.1494860.  and references therein
  4. ^ "A grain of boron suboxide (B6O) synthesized by scientists at the Arizona State". Arizona State University. Retrieved 2009-03-18. 
  5. ^ Durband, Dennis (1998). "Making the hard stuff". Arizona State University. Retrieved 2009-03-18. 
  6. ^ a b Akashi, T.; Tsuyoshi, I.; Gunjishima, I.; Hiroshi, M.; Goto, T. (2002). "Thermodynamic Properties of Hot-Pressed Boron Suboxide (B6O)". Materials Transactions 43 (7): 1719–1723. 
  7. ^ a b c Machaka, R.; Mwakikunga, B. W.; Manikandan, E.; Derry, T. E.; Sigalas, I.; Herrmann, M. (2012). "Mechanical and Structural Properties of Fluorine-Ion-Implanted Boron Suboxide". Advances in Materials Science and Engineering 2012. doi:10.1155/2012/792973. Article ID 792973.