Scientists have succeeded in making what they call “black nitrogen” an allotrope (form) expected to exist based on observations of comparable elements, but which had stubbornly eluded them for decades.
The periodic table was essential to chemistry becoming a true science, based on grouping elements that behave in a similar manner into columns so that their behavior could be predicted. It was a long time before we discovered the reason for these similarities – that electrons orbit in “shells” and each element in a column has an outermost shell with the same number of electrons, with each row representing an additional shell.
As technology advanced, chemists observed the similarities are even greater than they first appear. For example, when placed under great pressure elements near the top of the table take on similar structures to those further down their column under more normal conditions. The sole exception to this has been nitrogen, which stubbornly refused to mimic phosphorus no matter how much pressure was applied. A paper in Physical Review Letters changes that, describing a crystalline state of nitrogen that resembles black phosphorus.
Phosphorus is a versatile element most commonly found in two major forms, red and highly volatile white allotropes. When heated under pressure, however, white phosphorus converts to the most stable structure, known as black phosphorus, whose sheets of zigzagging atoms bears some resemblance to graphite. Further down the table arsenic and antimony can take similar forms.
Dr Dominique Laniel of the University of Bayreuth applied 1.4 million atmospheres of pressure to nitrogen while heating it to 4,000ºC (7,200ºF) – vastly greater temperatures and pressures than are needed to transform phosphorus. Such conditions don't lend themselves to the easy study of a sample, but Laniel and co-authors bombarded the product with X-rays to learn its structure.
"We were surprised and intrigued by the measurement data suddenly providing us with a structure characteristic of black phosphorus,” Laniel said in a statement. “Further experiments and calculations have since confirmed this finding. This means there is no doubt about it: Nitrogen is, in fact, not an exceptional element, but follows the same golden rule of the periodic table as carbon and oxygen do."
Although nitrogen is in plentiful supply, the conditions required to turn it to black are so extreme the new allotrope is unlikely to be of any direct use. However, the study of new elemental forms can often improve our understanding of those they most resemble. In this light, the similarity of black nitrogen’s two-dimensional layers to wonder-material graphene could yield some value.