A new form of boron

Boron is a simple atom: five protons,five or six neutrons,five electrons. It is not as ubiquitous as hydrogen.

Published:February 8, 2009 3:31 pm

Boron is a simple atom: five protons,five or six neutrons,five electrons. It is not as ubiquitous as hydrogen. It does not,as helium does,make your voice sound like Donald Duck. It is not as famous as carbon,its neighbour to the right on the periodic table.
Perhaps it is held back by its name–sounds like boring. Yet it remains an element of mystery.

For more than two centuries,boron has confounded scientists,resulting in what Artem R. Oganov,a professor of geosciences at Stony Brook University,calls “a stream of discoveries and misdiscoveries”.
Now researchers led by Oganov have added to the actual discoveries. They have found a form of boron that is nearly as hard as diamond.
This discovery even illustrates the power of the idea of evolution,using a so-called genetic algorithm to decipher the structure of the new boron crystal.

“This work is a beautiful example of cooperation between theory and experiment,” said Aitor Bergara,a physicist at the University of the Basque Country in Spain. Bergara was not involved with the research,which appears in the current issue of the journal Nature.

Boron has a long history. Mentions of boron compounds like borax date back millennia. In 1808,within a week and a half of each other,two research efforts,led by the great chemists Sir Humphrey Davy in London and Joseph Louis Gay-Lussac and Louis-Jacques Thenard in Paris,announced that they had isolated boron. They had not. Another great chemist,Henri Moissan,later showed that the two earlier groups had made a compound consisting of 60 per cent boron.

Moissan also claimed to have isolated boron. He too was wrong,although he did do better: a compound with 90 per cent boron. Not until 1909 was a sample of 99 per cent pure boron produced.
Just as pure carbon can come as diamond or graphite,boron comes in multiple forms–as many as 16 have been reported. But even tiny amounts of impurities can alter the structure,and it seems that the element has only three pure forms,Oganov said.

One,known as alpha boron,is a dark but transparent red. Beta boron is black and looks like coal. Even today,scientists do not definitively know which of these two forms is the stable form. (It is probably beta boron.) The third form is the newly discovered one.
Boron’s unusual properties come from the three electrons in its outer electronic layer. For similar elements like aluminum,one row down on the periodic table,the three outer electrons are easily torn away,and the element behaves as a metal. But boron is smaller,and so its nucleus holds on to the electrons tighter,more like an insulator.

“Boron is a truly schizophrenic element,” Oganov said. “It’s an element of complete frustration. It doesn’t know what it wants to do. The outcome is something horribly complicated.”
Two of the collaborators on the Nature paper,Jiuhua Chen of Florida International University and Vladimir L. Solozhenko of the National Council for Scientific Research in France,independently produced the new boron phase in high-temperature,high-pressure experiments in 2004. But they were not able to deduce what exactly they had produced.

For that,they turned to Oganov,who employed a computational technique that encodes parameters of the crystal structure in a string of data. Starting with a number of trial crystal structures,the program calculates the energy needed to hold each together,and discards the versions that do not pack together comfortably. Then,as occurs in biological evolution,the crystal parameters are tweaked (the equivalent of mutation) and portions of the structure swapped (the equivalent of recombination). After generations of calculation,the answer converges on the stable form.

This algorithm had previously revealed new phases of iron sulfide,calcium carbonate,sulphur and even a superconducting form of oxygen,subsequently confirmed by experiment.
The newly discovered form of boron is stable at super-high pressures–more than 100,000 times the normal atmospheric pressure of 14.7 pounds per square inch–and consists of two substructures. One is a spherical shape of 12 boron atoms. The other is a dumbbell shape of a pair of boron atoms. These two sub-substructures stack together in the same way that table salt (sodium chloride) does.

When the high pressures were eased,the boron remained in the new configuration. Subsequent experiments confirmed that the material had the properties predicted by the algorithm. Although gamma boron is not quite as hard as diamond,it is more heat-resistant,which could make it attractive for certain industrial uses.
_ KENNETH CHANG,NYT

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