Long-term relaxation of orientational disorder and structural modifications in molecular nitrogen at high pressure

Up to 17 GPa, the crystalline phases of N₂ are characterized by pronounced orientational disorder, whereas the higher-pressure phases of molecular N₂ are ordered. This raises the question about long-term relaxation of orientational disorder within the low- to intermediate-pressure regime. Here, this question is addressed by comparing synthetic with natural, chemically pure, solid N₂ that resides as inclusions in diamonds at 300 K for about 10⁸ years at pressures up to 11 GPa. It is shown that disorder prevails at 8.7 GPa, 300 K, where both synthetic and natural N₂ assume the same structure. However, at 10.8 GPa, natural solid N₂ exhibits monoclinic distortion and partial orientational ordering of the molecules, both of which are not observed in synthetic material. This difference is interpreted as the result of long-term structural relaxation. The ordering mechanism is examined and placed into the context of the δ- to ε-N₂ transition. We present explanations for the absence of complete ordering of δ-N₂.