Silicon borides (also known as boron silicides) are lightweight

ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelai ...

compounds formed between

silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ...

and boron. Several stoichiometric silicon boride compounds, SiB_''n'', have been reported: silicon triboride, SiB₃, silicon tetraboride, SiB₄, silicon hexaboride, SiB₆, as well as SiB_''n'' (''n'' = 14, 15, 40, etc.). The ''n'' = 3 and ''n'' = 6 phases were reported as being co-produced together as a mixture for the first time by

Henri Moissan Ferdinand Frédéric Henri Moissan (28 September 1852 – 20 February 1907) was a French chemist and pharmacist who won the 1906 Nobel Prize in Chemistry for his work in isolating fluorine from its compounds. Moissan was one of the original me ...

and Alfred Stock in 1900 by briefly heating silicon and boron in a clay vessel. The tetraboride was first reported as being synthesized directly from the elements in 1960 by three independent groups: Carl Cline and Donald Sands; Ervin Colton; and Cyrill Brosset and Bengt Magnusson. It has been proposed that the triboride is a silicon-rich version of the tetraboride. Hence, the stoichiometry of either compound could be expressed as SiB_{4 - ''x''} where ''x'' = 0 or 1. All the silicon borides are black, crystalline materials of similar density: 2.52 and 2.47 g cm⁻³, respectively, for the ''n'' = 3(4) and 6 compounds. On the

Mohs scale of mineral hardness The Mohs scale of mineral hardness () is a Qualitative property, qualitative ordinal scale, from 1 to 10, characterizing scratch hardness, scratch resistance of various minerals through the ability of harder material to scratch softer material. ...

, SiB_{4 - ''x''} and SiB₆ are intermediate between

diamond Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the chemically stable form of carbon at room temperature and pressure, ...

(10) and

ruby A ruby is a pinkish red to blood-red colored gemstone, a variety of the mineral corundum (aluminium oxide). Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called sapp ...

(9). The silicon borides may be grown from boron-saturated silicon in either the solid or liquid state. The SiB₆ crystal structure contains interconnected

icosahedra In geometry, an icosahedron ( or ) is a polyhedron with 20 faces. The name comes and . The plural can be either "icosahedra" () or "icosahedrons". There are infinitely many non- similar shapes of icosahedra, some of them being more symmetri ...

(polyhedra with 20 faces), icosihexahedra (polyhedra with 26 faces), as well as isolated silicon and boron atoms. Due to the size mismatch between the silicon and boron atoms, silicon can be substituted for boron in the B₁₂ icosahedra up to a limiting stoichiometry corresponding to SiB_2.89. The structure of the tetraboride SiB₄ is isomorphous to that of

boron carbide Boron carbide (chemical formula approximately B4C) is an extremely hard boron–carbon ceramic, a covalent material used in tank armor, bulletproof vests, engine sabotage powders, as well as numerous industrial applications. With a Vickers ha ...

(B₄C), B₆P, and B₆O. It is

metastable In chemistry and physics, metastability denotes an intermediate energetic state within a dynamical system other than the system's state of least energy. A ball resting in a hollow on a slope is a simple example of metastability. If the ball i ...

with respect to the hexaboride. Nevertheless, it can be prepared due to the relative ease of crystal nucleation and growth. Both SiB_{4 - ''x''} and SiB₆ become superficially oxidized when heated in air or oxygen and each is attacked by boiling sulfuric acid and by fluorine,

chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine is ...

, and

bromine Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table (halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simil ...

at high temperatures. The silicon borides are electrically conducting. The hexaboride has a low coefficient of thermal expansion and a high nuclear cross section for thermal neutrons. The tetraboride was used in the black coating of some of the space shuttle heat shield tiles.

References

{{reflist Inorganic silicon compounds Borides Ceramic materials