Solid nitrogen

Solid nitrogen is a number of solid forms of the element nitrogen, first observed in 1884. Solid nitrogen is mainly the subject of academic research, but low-temperature, low-pressure solid nitrogen is a substantial component of bodies in the outer Solar System and high-temperature, high-pressure solid nitrogen is a powerful explosive, with higher energy density than any other non-nuclear material.

Generation Karol Olszewski first observed solid nitrogen in 1884, by first liquefying hydrogen with evaporating liquid nitrogen, and then allowing the liquid hydrogen to freeze the nitrogen. By evaporating vapour from the solid nitrogen, Olszewski also generated the extremely low temperature of 48 K, at the time a world record. Modern techniques usually take a similar approach: solid nitrogen is normally made in a laboratory by evaporating liquid nitrogen in a vacuum. The solid produced is porous.

Occurrence in nature Solid nitrogen forms a large part of the surface of Pluto (where it mixes with solid carbon monoxide and methane) and the Neptunian moon Triton. On Pluto it was directly observed for the first time in July 2015 by the New Horizons space probe and on Triton it was directly observed by the Voyager 2 space probe in August 1989.

Even at the low temperatures of solid nitrogen it is fairly volatile and can sublime to form an atmosphere, or condense back into nitrogen frost. Compared to other materials, solid nitrogen loses cohesion at low pressures and flows in the form of glaciers when amassed. Yet its density is higher than that of water ice, so the forces of buoyancy will naturally transport blocks of water ice towards the surface. Indeed, New Horizons observed "floating" water ice atop nitrogen ice on the surface of Pluto. On Triton, solid nitrogen takes the form of frost crystals and a transparent sheet layer of annealed nitrogen ice, often referred to as a "glaze". Eruptions of nitrogen gas were observed by Voyager 2 to spew from the subpolar regions around Triton's southern polar ice cap. A possible explanation of this observed phenomenon is that the Sun shines through the transparent layer of nitrogen ice, heating the layers beneath. Nitrogen sublimes and eventually erupts through holes in the upper layer, carrying dust along with it and creating dark streaks.

Transitions to fluid allotropes

Melting At standard atmospheric pressure, the melting point of N2 is 63.23 K. Like most substances, nitrogen melts at a higher temperature with increasing ambient pressure until 50 GPa, when liquid nitrogen is predicted to polymerize. Within that region, melting point increases at a rate of approximately 190 K/GPa. Above 50 GPa, the melting point drops.

Sublimation Nitrogen has a triple point at 63.14±0.06 K and 0.1255±0.0005 bar; below this pressure, solid nitrogen sublimes directly to gas. At these low pressures, nitrogen exists in only two known allotropes: α-nitrogen (below 35 K) and β-nitrogen (35–63 K). Measurements of the vapour pressure from 20–63 K suggest the following empirical formulae: ln ⁡ ( P subl 1 bar ) = 12.40 − 807.4 K T − 3926 K 2 T 2 + 6.297 ⋅ 10 4 K 3 T 3 − 4.633 ⋅ 10 5 K 4 T 4 + 1.325 ⋅ 10 6 K 5 T 5 ( α ) {\displaystyle \ln {\left({\frac {P_{\text{subl}}}{1{\text{ bar}}}}\right)}=12.40-{\frac {807.4{\text{ K}}}{T}}-{\frac {3926{\text{ K}}^{2}}{T^{2}}}+{\frac {6.297\cdot 10^{4}{\text{ K}}^{3}}{T^{3}}}-{\frac {4.633\cdot 10^{5}{\text{ K}}^{4}}{T^{4}}}+{\frac {1.325\cdot 10^{6}{\text{ K}}^{5}}{T^{5}}}\quad \quad \quad (\alpha )}

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