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silicon boride is a group of light ceramic compounds formed between silicon and boron. In a number of different forms, these materials can be used for many high-temperature thermoelectric applications such as solar cells and fuel cell components.
The first silicon boride was discovered in 1891 by American scientist Edward G. Acheson. During experiments to produce artificial diamonds, Acheson heated a mixture of clay and coke in an iron bowl and he was surprised to find bright green crystals attached to the carbon electrode.
He then drained the material and poured it into a glass beaker. He found the resulting silicate slurry had an extremely hard texture similar to that of diamond. The mineral name alumina was later adopted to describe the new compound.
In 1903, French scientist Henri Moissan synthesized a similar compound from quartz and carbon. He also described its resemblance to diamond, but he ascribed the discovery of this new compound to Acheson.
This compound is one of the strongest synthetic materials known and has a Mohs hardness rating of 9 which approximates diamond. It is used in abrasives, refractories and ceramics, and is an excellent electrical conductor.
Among the various silicon borides, b-SiB6 has received most attention because of its interesting structural features. It is a cubic phase, denoted as g-SiB6-type, which crystallizes in space group Pm-3m (no. 221), with unit cell parameters of a = 4.130 A (GGA) and c = 6.407 A (LDA).
Within the b-modification, four boron atoms form distinct B12 icosahedra with atom-atom distances of 1.73 A to 2.87 A, as visualized in Figure 2a. Additionally, the silicon atoms in this structure are in four-fold coordination with interatomic distances of 2 x 3.638 A and 2 x 3.977 A, respectively. These properties, coupled with its wide band-gap and thermal conductivity, make boron-rich silicon borides highly attractive for high-temperature thermoelectric devices.