Crystal Structures of the Three Modifications of Nitrogen 14 and Nitrogen 15 at High Pressure

We have determined from x‐ray diffraction photographs that high‐pressure $\gamma$ N₂ is tetragonal with two molecules per unit cell in special position $f$ of space group ${\text{P4}}_2\mathrm{\hspace{0.17em}/\hspace{0.17em}mnm}$ . At an average pressure and temperature of 4015 atm and 20.5°K, respectively, the unit cell dimensions are $\text{a\hspace{0.17em}=\hspace{0.17em}3.957 Å}$ and $\text{c\hspace{0.17em}=\hspace{0.17em}5.109 Å}$ , giving a molar volume in good agreement with that from $\mathrm{p–V–T}$ measurements. The $\beta$ N₂ solid modification, which is contiguous with the melting curve, remains hexagonal up to at least 4125 atm and 49°K where the unit cell constants are $\text{a\hspace{0.17em}=\hspace{0.17em}3.861 Å}$ and $\text{c\hspace{0.17em}=\hspace{0.17em}6.265 Å}$ . Over the pressure range investigated, the $\mathrm{c\hspace{0.17em}/\hspace{0.17em}a}$ ratio is very close to the ideal value for closest packing of hard spheres. The atomic positions in hexagonal $\mathrm{(\beta )}$ N₂, which are known to be highly disordered at low pressure, show no evidence of ordering at the highest pressures studied. The third allotrope, $\alpha$ N₂, is cubic at 3785 atm and 19.6°K with four molecules in a unit cell 5.433 Å on a side. Both the $\text{Pa3}$ and ${\text{P2}}_{1}3$ space groups, which have been reported at zero pressure, appear to be spatially possible at limiting high pressures for $\alpha$ N₂. However, from our diffraction measurements on the cubic solid at all pressures, we were unable to prove the existence of the ${\text{P2}}_{1}3$ structure. The measurements on pure ³⁰N₂ show that this isotope also exists in the same three solid modifications as ²⁸N₂ with cell dimensions that are similar for both isotopes. However, the transition to $\gamma$ N₂ at 20°K for the mass‐15 isotope occurs 400 atm lower than that for the mass‐14 isotope.