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

Abstract

We have determined from x-ray diffraction photographs that high-pressure γ N2 is tetragonal with two molecules per unit cell in special position f of space group P42 / mnm. At an average pressure and temperature of 4015 atm and 20.5°K, respectively, the unit cell dimensions are a = 3.957 Å and c = 5.109 Å, giving a molar volume in good agreement with that from p–V–T measurements. The β 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 a = 3.861 Å and c = 6.265 Å. Over the pressure range investigated, the c / a ratio is very close to the ideal value for closest packing of hard spheres. The atomic positions in hexagonal (β) N2, which are known to be highly disordered at low pressure, show no evidence of ordering at the highest pressures studied. The third allotrope, α 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 Pa3 and P213 space groups, which have been reported at zero pressure, appear to be spatially possible at limiting high pressures for α N2. However, from our diffraction measurements on the cubic solid at all pressures, we were unable to prove the existence of the P213 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 γ N2 at 20°K for the mass-15 isotope occurs 400 atm lower than that for the mass-14 isotope.