Nitrogen has two solid phases, a and 1, being stable below and above phase transition point, T== 35. 61 K, respectively. However, a number of other phases have been observed at high pressures.
The structure of the low phase of solid nitrogen was investigated by neutron powder diffraction technique. Nitrogen of high purity with natural isotopic abundance was used. A method was used to form high quality powder specimens with random1y oriented fine grains directly from the gas phase. A special sample tube set-up was used which did not produce any extra peaks in the neutron powder diffraction patterns.
The neutron powder diffraction patterns were recorded with two wavelengths (l.3702? and 2.1223 ?)using two different diffract meters each of them having five counters. The diffraction patterns recorded with different counters were treated separately but the results obtained from them are consistent with each other. No departure from the cubic structure was observed even with the long wavelength. The edge length of the primitive unit cell at 4. 2 K was found to be a=5. 650±0. 001 ?
The systematic absences led to the conclusion that the space group is pa3 which is Centro symmetric. The unit cell contains 4 molecules each of them is aligned along one of the body diagonals of the cube with the molecular centres located on the face-centred lattice points. The atoms are in special positions with Wyckoff notation 8c.
The refinement of the positional parameters with isotropic temperature factor using Rietveld profile program gave X=0.0542±0.0003 which corresponds to a molecular bond length of 1. 06 ± 0. 01 ?.
The possibility of other space groups such as P4232 and p213 was considered. On the basis of the neutron powder diffraction patterns obtained at 4.2 K, both space groups were rejected. The molar volume and the density of solid nitrogen at 4.2 K were found to be 27.15 cm3 and 1.029 gr/cm3 respectively which are in agreement with other measurements.