Considering boron’s prominent place near the top of the periodic table, along with the fact we have known about its existence for more than 200 years, it is remarkable how much we continue to learn about this element. It can form unique three-centre, two-electron bonds, creating new forms of boron that were still being identified as recently as last year.
Boron has regularly frustrated and fascinated researchers by demonstrating unusual features. For example, some boron compounds are second only to diamond in hardness, while demonstrating better high-temperature stability. Adding to its list of talents is the tantalizing observation that boron is disordered as it approaches absolute zero. This information adds to the understanding of the third law of thermodynamics, which states that all ordered crystalline materials have an entropy of zero at absolute zero. “Every other element, as far as we know, has a completely ordered structure if the temperature were going to absolute zero — but not boron,” says Dalhousie University chemistry professor Mary Anne White, who has spent the past 30 years exploring the field of fundamental thermodynamics.
Her interest in the subject was sparked in 2009 when she read a Nature paper describing a new elemental form of boron. The article noted in passing that it was still not known which of the various permutations of boron would be stable at room temperature and pressure. “That’s like saying for carbon that we don’t know which is more stable at room conditions, graphite or diamond,” she says.
White and her co-workers addressed this question with an experimental thermodynamic analysis of two key allotropes of boron and published their findings in Angewandte Chemie at the beginning of the year. They found that under typical ambient pressure β-boron — which is disordered with a random occupation of some sites in its crystal lattice — is the most stable form at temperatures as high as 2000 K. This disordered form demonstrated high entropy at low temperatures, with extrapolations showing that residual entropy would be retained at absolute zero.
For White, this news confirmed her appreciation for basic scientific studies along with her opinion that boron is one thoroughly weird element. “Most people would have thought that when it comes to the early elements, all of the fundamental things are already known,” she says. “But it’s not so. Boron is not boring.”