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Date: 23 July 2017
Amazing: boron alone form a ionic crystal!
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Amazing: boron alone form a ionic crystal! :: 01 January, 2002


   
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Long defied boron chemists and physicists. An international group of researchers has shown that high pressure, it behaves like an ionic crystal, similar to table salt, which appears to contradict the laws of crystal chemistry as such requires two different atoms. But it is not!

The saga of boron begins in 1808 when, in France, Gay-Lussac and Thénard announce the discovery of a new, independently and at the same time that Sir Humphrey Davy in London. No luck, it is in fact a compound containing only 60 to 70% of boron. Later, the Nobel Prize in Chemistry Henri Moissan also believe obtaining boron in pure form but it is still a 90% compound.

Boron chemists laughs but also crystallographers. F. Wöhler, although not a specialist in organic chemistry, said in 1858 that boron can exist in two crystalline forms, one resembling the other graphite and diamond. Still missed! This is still composed of two distinct forms AlB12 and B48C2Al respectively.

Physicists are also at pains to define it. With its three valence electrons, it is in the periodic table of elements, between metals and insulators, only closer to the former than the latter. Tiny changes in temperature or pressure, even the mere presence of impurities, enough to make moving from one state to another.

It was not until 1909 that we obtain from pure boron to 99%. The fact remains that 16 forms of boron are known, although there still is suspected impurities. Remember qu'i is a molecule PuB100, therefore consists of one atom of plutonium surrounded by 100 atoms of boron. This construction is extremely rare in itself illustrates the difficulty of obtaining pure boron or almost pure as well as to study their properties.

An atom chameleon

In 2004, Chen and Vladimir Jiuhu Solozhenko got a strange material based on boron at a pressure of 100,000 atmospheres but could not specify either the structure or composition. They turned to the great theorist of crystallography Artem Oganov, which was recently heard after his work on the germane.

Oganov used a computer program using the equations of quantum mechanics to simulate what had happened during the experiments at high pressures. He was able to make a series of predictions subsequently confirmed by analysis of Carlo Gatti, a chemist at the University of Milan.

He had to face the facts. We were in the presence of an almost pure ionic crystal boron! Such a result seems absurd at first because a crystal ion requires two atoms with different electronegativity. But nature is clever.

Pressure experiments (at least up to 900,000 atmospheres according to the calculations), two types of clusters of atoms are formed, with very different electronic properties. Appear associations of B12 icosahedron and B2 pairs, organized as the atoms of sodium and chlorine in table salt, the archetypal ionic crystal.

Details of the discovery are published in Nature.

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