Solid nitrogen
[2] By evaporating vapour from the solid nitrogen, Olszewski also generated the extremely low temperature of 48 K, at the time a world record.
Compared to other materials, solid nitrogen loses cohesion at low pressures and flows in the form of glaciers when amassed.
[7] Eruptions of nitrogen gas were observed by Voyager 2 to spew from the subpolar regions around Triton's southern polar ice cap.
Nitrogen sublimes and eventually erupts through holes in the upper layer, carrying dust along with it and creating dark streaks.
[14] At the boiling point of hydrogen with excess solid nitrogen, the dissolved molar fraction is 10−8.
Thus excess nitrogen (melting at 63 K) or oxygen (melting at 55 K) freeze out first, and the eutectic liquid air freezes at 50 K.[16] At ambient and moderate pressures, nitrogen forms N2 molecules; at low temperature London dispersion forces suffice to solidify these molecules.
Below 35.6 K, nitrogen adopts a cubic structure with space group Pa3; the N2 molecules are located on the body diagonals of the unit cell cube.
[10][18] Above 35.6 K (until it melts), nitrogen adopts a hexagonal close packed structure, with unit cell ratio c⁄a ≈ 1.633 = √8⁄3.
The nitrogen molecules are randomly tipped at an angle of 55°, due to strong quadrupole-quadrupole interaction.
[10][18] At high pressure (but ambient temperature), dinitrogen adopts the cubic δ form, with space group pm3n and eight molecules per unit cell.
The (δ-N2, β-N2, liquid) triple point occurs somewhere around 8–10 GPa and 555–578 K.[10] The (δ-N2, β-N2, γ-N2) triple point occurs at 2.3 GPa and 150 K.[19] Within the lattice cells, the molecules themselves have disordered orientation,[20] but increases in pressure causes a phase transition to a slightly different phase, δloc, in which the molecular orientations progressively order, a distinction that is only visible via Raman spectroscopy.
[21] At high pressure (roughly 2–13 GPa) and low temperature,[Note 3] the dinitrogen molecule orientations fully order into the rhombohedral ε phase, which follows space group R3c.
[26] A ζ-N2 phase compressed to 95 GPa and then heated to over 600 K produces a uniformly translucent structure called θ-nitrogen.
The decomposition of hydrazinium azide at high pressure and low temperature produces a molecular solid made of linear chains of 8 nitrogen atoms (N≡N+−N−−N=N−N−−N+≡N).
[35][36] Grechner et al. predicted in 2016 that an analogous allotrope with six nitrogens should exist at ambient conditions.
[37] Non-molecular forms of solid nitrogen exhibit the highest known non-nuclear energy density.
The μ-phase has been brought to atmospheric pressure by first cooling it to 100 K.[38] η-N is a semiconducting amorphous form of nitrogen.
[39] At pressures higher than 110 GPa and temperatures around 2000 K, nitrogen forms a network solid, bound by covalent bonds in a cubic-gauche structure, abbreviated as cg-N.
The coordinate of one atom in the unit cell at x,x,x also determines the bond angle by cos(θ) = x(x-1/4)/(x2+(x-1/4)2).
[49] The speed of sound[clarification needed] in solid nitrogen is 1452 m/s at 20 K and 1222 m/s at 44 K. The longitudinal velocity ranges from 1850 m/s at 5 K to 1700 m/s at 35 K. With temperature rise the nitrogen changes phase and the longitudinal velocity drops rapidly over a small temperature range to below 1600 m/s and then it slowly drops to 1400 m/s near the melting point.
[17] Under pressure nitrogen can form crystalline van der Waals compounds with other molecules.
[51] Solid nitrogen can dissolve 2 mole % helium under pressure in its disordered phases such as the γ-phase.
Under higher pressure 9 mol% helium, He can react with ε-nitrogen to form a hexagonal birefringent crystalline van der Waals compound.
When the pressure on He(N2)11 exceeds 135 GPa the substance changes from clear to black, and takes on an amorphous form similar to η-N2.
These mixtures are prevalent in outer Solar System objects such as Pluto that have both nitrogen and methane on their surfaces.
[54] At room temperature there is a clathrate of methane and nitrogen in 1:1 ratio formed at pressures over 5.6 GPa.
[55] The carbon monoxide molecule (CO) is very similar to dinitrogen in size, and it can mix in all proportions with solid nitrogen without changing crystal structure.
[56] Neon or xenon atoms can also be included in solid nitrogen in the β and δ phases.
[57] A van der Waals compound of neon and nitrogen was shown using Raman spectroscopy.
[64] Solid nitrogen is used as a matrix on which to store and study reactive chemical species, such as free radicals or isolated atoms.
