High-pressure chemistry researchers disproved a golden rule of fundamental elements, creating a crystalline structure out of nitrogen with two-dimensional atomic layers — showing great potential for high-tech electronics in ways that could resemble graphene. Dubbed "black nitrogen," the new material was explained in a study published in the journal Physical Review Letters.
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Breaking the golden rule with 'black nitrogen'
Under high pressures, carbon, oxygen, and other light elements follow a golden rule: they have similar structures to heavier elements in the same group of elements.
However, nitrogen was always the black sheep under the golden rule.
When chemical elements are arranged in ascending order according to each one's number of protons, their properties recur at large intervals (periods). The periodic table arranges these periods to be more visual.
Elements with similar properties are placed one below another in the same column, forming a group of elements. At the top of each column is an element with the fewest protons and the lowest weight in its group. Nitrogen plays this role for group 15, but the reason it was the "black sheep" came from the way it behaved under high pressure.
In previous high-pressure experiments, nitrogen didn't show a similar structure to the ones exhibited by other heavier elements in its group under normal conditions. Namely, phosphorus, antimony, and arsenic.
Instead, under high pressure, nitrogen's structure looked more like those seen in neighboring groups' lightest elements: carbon and oxygen.
Nitrogen not the exception to periodic table rule
So it seems nitrogen isn't the exception to the rule, after all. Researchers working at the Laboratory for Crystallography at the University of Bayreuth and the Bavarian Research Institute of Experimental Geochemistry & Geophysics (BGI) have proven this using a newly-developed measuring method.
Dominique Laniel led the study that made an unexpected discovery. At high pressures and temperatures, nitrogen atoms coalesce into a crystalline structure resembling black phosphorus — a specific variant of phosphorus. This structure also happens in arsenic and antimony. Composed of two-dimensional layers of cross-linked, zig-zagging nitrogen atoms, these 2D layers resemble graphene — a recently-discovered material with great potential for high-tech applications.
This is why black phosphorus is now subject to intensive studies for its potential use as a highly-efficient transistor, semiconductor, or other future electronic components.
Bayreuth researchers have thus proposed an analogous name for the allotrope of nitrogen they came across: black nitrogen.
The bad news is that black nitrogen can only exist at the extreme temperatures and pressures produced in a laboratory. In normal conditions, it immediately dissolves. "Because of this instability, industrial applications are currently not feasible. Nevertheless, nitrogen remains a highly interesting element in materials research. Our study shows by way of example that high pressures and temperatures can produce material structures and properties that researchers previously did not know existed," said Laniel.
'Black nitrogen' structure revealed via particle accelerators
To make black nitrogen, scientists had to create — and we can't stress this enough — extreme conditions. Compression pressure reached 1.4 million times one Earth atmosphere (what we feel at sea level), and the temperature shot beyond 4,000 degrees Celsius (7,232 degrees Fahrenheit). The Bayreuth scientists worked with the German Electron Synchrotron (DESY) in Hamburg and the U.S.-based Advanced Photon (APS) at the Argonne National Laboratory to explore how atoms behave in these wild conditions.
"We were surprised and intrigued by the measurement data suddenly providing us with a structure characteristic of black phosphorus. 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," said Laniel.