Rhenium diboride

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Rhenium diboride
Magnesium-diboride-3D-balls.png
Identifiers
CAS number 12355-99-6 YesY
Properties
Molecular formula ReB2
Molar mass 207.83 g/mol
Appearance black powder
Density 12.7 g/cm3
Melting point 2,400 °C (4,350 °F; 2,670 K)[1]
Solubility in water none
Structure
Crystal structure Hexagonal, Space group P63/mmc.
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Rhenium diboride (ReB2) is a synthetic superhard material. It was first synthesized in 1962[2] and re-emerged recently due to hopes of achieving high hardness comparable to that of diamond.[3] The reported ultrahigh hardness has been questioned,[4] although this is a matter of definition as in the initial test rhenium diboride was able to scratch diamond.[3]

The production method of this material does not involve high pressures as with other hard synthetic materials, such as cubic boron nitride, which makes production cheap. However, rhenium itself is an expensive metal.

The compound is formed from a mixture of rhenium, noted for its resistance to high pressure, and boron, which forms short, strong covalent bonds with rhenium.

Synthesis[edit]

ReB2 can be synthesized by at least three different methods at atmospheric pressure: solid-state metathesis, melting in an electric arc, and direct heating of the elements.[3]

In the metathesis reaction, rhenium trichloride and magnesium diboride are mixed and heated in an inert atmosphere and the magnesium chloride byproduct is washed away. Excess boron is needed to prevent formation of other phases such as Re7B3 and Re3B.

In the arc-melting method, rhenium and boron powders are mixed and a large electric current is passed through the mixture, also in an inert atmosphere.

In the direct reaction method, the rhenium-boron mixture is sealed in a vacuum and held at a high temperature over a longer period (1000 °C for five days).

At least the last two methods are capable of producing pure ReB2 without any other phases, as confirmed by X-ray crystallography.

Properties[edit]

The hardness of ReB2 exhibits considerable anisotropy because of its hexagonal layered structure (see structure model). Its value (HV ~ 22 GPa)[4] is much lower than that of diamond and is comparable to that of tungsten carbide, silicon carbide, titanium diboride or zirconium diboride.[4]

ReB2 slowly reacts with water, converting into a hydroxide.

Two factors contribute to ReB2's high hardness: a high density of valence electrons, and an abundance of short covalent bonds.[3][5] Rhenium has one of the highest valence electron densities of any transition metal (476 electrons/nm3, compare to 572 electrons/nm3 for osmium and 705 electrons/nm3 for diamond[6]). The addition of boron requires only a 5% expansion of the rhenium lattice, because the small boron atoms fill the existing spaces between the rhenium atoms. Furthermore, the electronegativities of rhenium and boron are close enough (1.9 and 2.04 on the Pauling scale) that they form covalent bonds in which the electrons are shared almost equally.

See also[edit]

References[edit]

  1. ^ Gaidar', L. M.; Zhilkin, V. Z. (1968). "Forward slip in the rolling of strip from metal powders". Soviet Powder Metallurgy and Metal Ceramics 7 (4): 258. doi:10.1007/BF00775787. 
  2. ^ La Placa, S. J.; Post, B. (1962). "The crystal structure of rhenium diboride". Acta Crystallographica 15 (2): 97. doi:10.1107/S0365110X62000298. 
  3. ^ a b c d Chung, Hsiu-Ying et al. (April 20, 2007). "Synthesis of Ultra-Incompressible Superhard Rhenium Diboride at Ambient Pressure". Science 316 (5823): 436–9. doi:10.1126/science.1139322. PMID 17446399. 
  4. ^ a b c Qin, Jiaqian; He, Duanwei; Wang, Jianghua; Fang, Leiming; Lei, Li; Li, Yongjun; Hu, Juan; Kou, Zili; Bi, Yan (2008). "Is Rhenium Diboride a Superhard Material?". Advanced Materials 20 (24): 4780. doi:10.1002/adma.200801471. 
  5. ^ W. Zhou, H. Wu, and T. Yildirim (2007). "Electronic, dynamical, and thermal properties of ultra-incompressible superhard rhenium diboride: A combined first-principles and neutron scattering study". Phys. Rev. B 76 (18): 184113–184119. doi:10.1103/PhysRevB.76.184113. 
  6. ^ Cumberland, Robert W. et al. (April 27, 2005). "Osmium Diboride, An Ultra-Incompressible, Hard Material". Journal of the American Chemical Society 127 (20): 7264–5. doi:10.1021/ja043806y. PMID 15898746.