Boron suboxide
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| Names | |
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IUPAC name
Boron suboxide
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| Other names
Hexaboron monoxide
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| Identifiers | |
| Properties | |
| B6O | |
| Molar mass | 80.865 g/mol |
| Appearance | Reddish icosahedral twinned crystals |
| Density | 2.56 g/cm3 |
| Melting point | 2,000 °C (3,630 °F; 2,270 K) |
| Structure | |
| Rhombohedral, hR42 | |
| R3, No. 166 | |
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a = 0.53824 nm, b = 0.53824 nm, c = 1.2322 nm
α = 90°, β = 90°, γ = 120°
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Formula units (Z)
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6 |
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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| Infobox references | |
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.
B6O can be synthesized by reducing B2O3 with boron or by oxidation of boron with zinc oxide or other oxidants. 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.
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.
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.
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