At least 15 unique phases of solid nitrogen are known, including twelve molecular phases, two non-molecular phases and one amorphous state.
The elusive high-temperature, high-pressure phase called ι-N2 was discovered in 2002 by a Carnegie Institution-led team of researchers but its structure was unknown until now.
“Our study demonstrates that the appearance of complex structures under pressure is not limited only to metallic elements and raises the intriguing question of whether unusual phases could be expected in other molecular systems at extreme conditions,” said University of Edinburgh researcher Robin Turnbull and co-authors.
In the study, the physicists were able to synthesize crystals of ι–N2.
“We used a high-pressure diamond-tipped anvil to squeeze tiny amounts of nitrogen at pressures half a million times that of Earth’s atmosphere, while heating it to 477 degrees Celsius,” they explained.
“We then used specialist X-ray technology to capture an image of the resulting crystals.”
ι–N2 is characterized by an extraordinarily large unit cell containing 48 N2molecules, Turnbull and colleagues found.
“We were surprised to find that the nitrogen had formed a complicated arrangement made up of dozens of molecules,” the scientists said.
“We had expected to uncover a much simpler structure.”
“We hope that our results will prompt further investigations into why relatively simple elements should form such complex structures — it’s important that we keep searching for promising new lines of scientific investigation,” Turnbull said.
The findings appear in the journal Nature Communications.