Decompression transition of ionic NH7

Determine whether the synthesized P4₁2₁2-NH₇ phase undergoes a structural transition to the R̅3m phase upon further decompression to pressures below approximately 120 GPa.

Background

The experiments identify the laser-heated ammonia product as the ionic P4₁2₁2-NH₇ phase. First-principles calculations predict a structural transition from P4₁2₁2-NH₇ to an R̅3m-NH₇ structure at approximately 60 GPa.

During decompression, the diamond anvils failed before the pressure could be reduced below approximately 120 GPa. Consequently, the experiment could not test whether the synthesized P4₁2₁2-NH₇ phase transforms into the predicted R̅3m structure at lower pressure.

References

It is noteworthy that the theoretically predicted transition between the $R\bar{3}m$ and $P4_12_12$ structures occurs at approximately 60 GPa. In our experiments, however, the diamond anvils failed during decompression before the pressure could be reduced below $\sim$120 GPa. By comparison, substantially lower pressures can typically be reached during decompression in polymeric-nitrogen experiments. We speculate that the reduced diamond stability may be related to hydrogen generated during laser heating and its subsequent diffusion into the diamond anvils. Consequently, we were unable to determine whether the synthesized $P4_12_12$-NH$_7$ phase undergoes a structural transition in to $R\bar{3}m$ phase upon further decompression to lower pressures.

— Hydrogen Reorganization in Hot Dense Ammonia  (2609.19605 - Tao et al., 17 Sep 2026) in Experimental Results, paragraph discussing the calculated R̅3m–P4₁2₁2 transition