Scientists Just Solved a 50-Year Mystery Hiding in Solid Nitrogen

For half a century, solid nitrogen has been holding out on us. Not the nitrogen in the air — the weird, high-pressure, super-cold kind that forms strange structures. Specifically, a form called γ-N2, which has been the bane of many a scientist's existence, refusing to give up its secrets.

Now, a team led by Professor Xiaodi Liu from the Hefei Institute of Solid State Physics, along with international collaborators, has finally cracked the code. They've confirmed that γ-N2 has a monoclinic P21/c structure. Which, if you're not a crystallographer, means each unit cell contains two nitrogen molecules. Let that satisfyingly precise number sink in.

This isn't just a win for long-suffering theories; it turns out γ-N2 is far more robust than we thought, existing across a much wider range of pressures and temperatures. Because apparently, even solid nitrogen likes to surprise us.

The Case of the Stubborn Crystal

The reason this mystery persisted for 50 years? γ-N2 is incredibly camera-shy. It doesn't form neat, single crystals that are easy to analyze. Instead, it prefers to show up as a low-quality powder, making traditional methods about as useful as a chocolate teapot.

To outsmart it, the scientists pulled out all the stops, combining a veritable arsenal of techniques: synchrotron X-ray diffraction, Raman spectroscopy, infrared spectroscopy, and some serious computer calculations based on density functional theory. The scientific equivalent of throwing everything but the kitchen sink at the problem, and then some. Crucially, all these high-tech methods pointed to the same P21/c structure, finally confirming it over other contenders.

But wait, there's more. Previous Raman measurements had shown an extra vibration signal that seemed to defy explanation. Was γ-N2 even weirder than imagined? Nope. The new study revealed this signal came from a tiny fraction of nitrogen molecules containing the rare nitrogen-15 isotope. As pressure cranked up, this weaker signal got cozy with a stronger one from regular nitrogen, creating what scientists charmingly call a Fermi-like resonance. Because even isotopes like to party.

They also discovered that γ-N2 has a surprisingly close cousin in θ-N2, another solid phase. Despite forming under wildly different conditions, they share similar molecular arrangements and Raman signals. It seems even in the world of high-pressure nitrogen, family resemblances persist. And now, after half a century, we finally know who γ-N2 really is.