Crystal Structures of the Three Modifications of Nitrogen 14 and Nitrogen 15 at High Pressure
Abstract
We have determined from xray diffraction photographs that highpressure γ N_{2} 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 pVT measurements. The β N_{2} 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) N_{2}, which are known to be highly disordered at low pressure, show no evidence of ordering at the highest pressures studied. The third allotrope, α N_{2}, 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 N_{2}. 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 ^{30}N_{2} show that this isotope also exists in the same three solid modifications as ^{28}N_{2} with cell dimensions that are similar for both isotopes. However, the transition to γ N_{2} at 20°K for the mass15 isotope occurs 400 atm lower than that for the mass14 isotope.
 Publication:

Journal of Chemical Physics
 Pub Date:
 June 1970
 DOI:
 10.1063/1.1672899
 Bibcode:
 1970JChPh..52.6000S