Solid nitrogen is a number of solid forms of the elementnitrogen, 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 theouter Solar System and high-temperature, high-pressure solid nitrogen is a powerful explosive, with higher energy density than any other non-nuclear material.^[1]

Karol Olszewski first observed solid nitrogen in 1884, by firstliquefying hydrogen with evaporatingliquid nitrogen, and then allowing the liquid hydrogen to freeze the nitrogen.^[2] By evaporating vapour from the solid nitrogen, Olszewski also generated the extremely low temperature of48 K, at the time a world record.^[3]

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.^[4]

Solid nitrogen forms a large part of the surface ofPluto (where it mixes with solidcarbon monoxide andmethane) and the Neptunian moonTriton. On Pluto it was directly observed for the first time in July 2015 by theNew Horizons space probe and on Triton it was directly observed by theVoyager 2 space probe in August 1989.^[5][6][7]

Even at the low temperatures of solid nitrogen it is fairly volatile and cansublime to form an atmosphere, or condense back into nitrogen frost. Compared to other materials, solid nitrogen losescohesion at low pressures and flows in the form ofglaciers when amassed. Yet its density is higher than that of water ice, so the forces ofbuoyancy 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.^[5]

On Triton, solid nitrogen takes the form of frost crystals and a transparent sheet layer ofannealed nitrogen ice, often referred to as a "glaze".^[7] Eruptions of nitrogen gas were observed byVoyager 2 to spew from the subpolar regions around Triton's southern polar ice cap.^[8] 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.

At standard atmospheric pressure, the melting point of N₂ is63.23 K.^[9]

Like most substances, nitrogen melts at a higher temperature with increasing ambient pressure until50 GPa, when liquid nitrogen is predicted topolymerize.^[10][11] Within that region, melting point increases at a rate of approximately190 K/GPa.^[10] Above50 GPa, the melting point drops.^[11]

Nitrogen has atriple point at63.14±0.06 K and0.1255±0.0005 bar; below this pressure, solid nitrogensublimes directly to gas.^[12] At these low pressures, nitrogen exists in only two known allotropes: α-nitrogen (below35 K) and β-nitrogen (35–63 K). Measurements of the vapour pressure from20–63 K suggest the followingempirical formulae:^[12]

${\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}(\alpha )}$${\displaystyle \ln \left(\frac{P_{\text{subl}}}{1\text{ bar}}\right)=8.514-\frac{458.4\text{ K}}{T}-\frac{19870{\text{ K}}^2}{T^2}+\frac{4.800\cdot {10}^5{\text{ K}}^3}{T^3}-\frac{4.524\cdot {10}^6{\text{ K}}^4}{T^4}(\beta )}$

Solid nitrogen is slightly soluble inliquid hydrogen. Based on solubility in60–75 Kgaseous hydrogen,^[13] Seidal et al. estimated that liquid hydrogen at15 K can dissolve(1–10)×10¹⁰ (moleculeN₂)/cm³.^[14] At the boiling point of hydrogen with excess solid nitrogen, the dissolved molar fraction is 10⁻⁸. At32.5 K (just below the boiling point ofH₂) and15 atm, the maximum molar concentration of dissolved N₂ is7.0×10⁻⁶.^[15]Nitrogen and oxygen are miscible in liquid phase but separate in solid phase. 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 formsN₂ molecules; at low temperatureLondon dispersion forces suffice to solidify these molecules.^[17]

Solid nitrogen admits two phases at ambient pressure: α- and β-nitrogen.

Below35.6 K, nitrogen adopts acubic structure withspace groupPa3; theN₂ molecules are located on the body diagonals of the unit cell cube. At low temperatures the α-phase can be compressed to3500 atm before it changes (to γ), and as the temperature rises above20 K, this pressure rises to about4500 atm. At21 K, the unit cell dimension is5.667 Å, decreasing to5.433 Å under3785 bar.^[10][18]

Above35.6 K (until it melts), nitrogen adopts ahexagonal close packed structure, with unit cell ratioc⁄a ≈ 1.633 =√8⁄3. The nitrogen molecules are randomly tipped at an angle of55°, due to strongquadrupole-quadrupole interaction. At45 K the unit cell hasa =4.050 Å andc =6.604 Å, but these shrink at4125 atm and49 K toa =3.861 Å andc =6.265 Å. At higher pressures, thec⁄a displays practically no variation.^[10][18]

The tetragonal γ form exists at low temperatures below44.5 K and pressures around0.3–3 GPa. The α/β/γ₂ triple point occurs at0.47 GPa and44.5 K. Formation of γ-dinitrogen exhibits a substantialisotope effect: at20 K, the isotope¹⁵N converts to the γ form at a pressure 400 atm (0.041 GPa) lower than natural nitrogen.

The space group of the γ phase isP4₂/mnm. At20 K and4000 bar, the unit cell has lattice constantsa =3.957 Å andc =5.109 Å.

The nitrogen molecules themselves are arranged inP4₂/mnm patternf^{[Note 1]} and take the shape of aprolate spheroid with long dimension4.34 Å and diameter3.39 Å.^{[Note 2]} The molecules can vibrate up to10° on theab plane, and up to15° in the direction of thec axis.^[10][18]

At high pressure (but ambient temperature), dinitrogen adopts the cubic δ form, with space grouppm3n and eight molecules per unit cell. This phase admits alattice constant of6.164 Å (at300 K and4.9 GPa).^[19] δ-N₂ admits two triple points. The (δ-N₂, β-N₂, liquid) triple point occurs somewhere around8–10 GPa and555–578 K.^[10] The (δ-N₂, β-N₂, γ-N₂) triple point occurs at2.3 GPa and150 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, δ_loc, in which the molecular orientations progressively order, a distinction that is only visible viaRaman spectroscopy.^[21] At high pressure (roughly2–13 GPa) and low temperature,^{[Note 3]} the dinitrogen molecule orientations fully order into therhombohedral ε phase, which follows space groupR3c.^[10][20][23] Cell dimensions area =8.02 Å,b =8.02 Å,c =11.104 Å,α =β =90°,γ =120°, volume618.5 Å³,Z = 24.^[24]

DissolvedHe can stabilize ε-N₂ at higher temperatures or lower pressures from transforming into δ-N₂ (see§ Related substances).^[20]

Above60 GPa, ε-N₂ transforms to an orthorhombic phase designated by ζ-N₂. There is no measurable discontinuity in the volume per molecule between ε-N₂ and ζ-N₂. The structure of ζ-N₂ is very similar to that of ε-N₂, with only small differences in the orientation of the molecules. ζ-N₂ adopts the monoclinic space groupC2/c, and has lattice constants ofa =7.580 Å,b =6.635 Å,c =5.018 Å andβ =97.64° with sixteen molecules per unit cell.^[25]

Further compression and heating produces two crystalline phases of nitrogen with surprising metastability.^[26]

A ζ-N₂ phase compressed to95 GPa and then heated to over600 K produces a uniformly translucent structure called θ-nitrogen.^[26]

The ι phase can be accessed by isobarically heating ε-N₂ to750 K at65 GPa or isothermal decompression of θ-N₂ to69 GPa at850 K.^[27] The ι-N₂ crystal structure is characterised by primitive monoclinic lattice with unit-cell dimensions of:a =9.899(2) Å,b =8.863(2) Å,c =8.726(2) Å andβ =91.64(3)° at56 GPa and ambient temperature. The space group isP2₁/c and the unit cell contains 48N₂ molecules arranged into a layered structure.^[28]

Upon pressure release, θ-N₂ does not return to ε-N₂ until around30 GPa; ι-N₂ transforms to ε-N₂ until around23 GPa.^[26]

When compressing nitrogen to pressures120–180 GPa and temperatures above4000 °C,^[29][30] nitrogen adopts a crystal structure ("bp-N") identical to that ofblack phosphorus (orthorhombic,Cmce space group).^[31] Like black phosphorus, bp-N is an electrical conductor.^[32] The existence of bp-N structure matches the behavior of heavierpnictogens, and reaffirms the trend that elements at high pressure adopt the same structures asheavier congeners at lower pressures.^[33]

Hexagonal layered polymeric nitrogen (HLP-N) was experimentally synthesized at244 GPa and3300 K. It adopts a tetragonal unit cell (P4₂bc) in which the single-bonded nitrogen atoms form two layers of interconnectedN₆ hexagons. HPL-N is metastable to at least 66 GPa.^[34]

The decomposition ofhydrazinium 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). Simulations suggest thatN₈ is stable at low temperatures and pressures (< 20 GPa); in practice, the reportedN₈ decomposes to the ε allotrope below 25 GPa but a residue remains at pressure as low as 3 GPa.^[35][36]

Theoretical analyses in 2016 predicted that an analogous allotrope with six nitrogens should exist at ambient conditions.^[37] The synthesis ofC_2h-N₆ was published in 2025. It is stable at liquid nitrogen temperature and atmospheric pressure.^[38]

Non-molecular forms of solid nitrogen exhibit the highest known non-nuclear energy density.^[1]

When the ζ-N₂ phase is compressed at room temperature over150 GPa an amorphous form is produced.^[1] This is a narrow gap semiconductor, and designated the μ-phase. The μ-phase has been brought to atmospheric pressure by first cooling it to100 K.^[39]

η-N is a semiconducting amorphous form of nitrogen. It forms at pressures around80–270 GPa and temperatures10–510 K. In reflected light it appears black, but does transmit some red or yellow light. In the infrared there is an absorption band around1700 cm⁻¹. Under even higher pressure of approximately280 GPa, theband gap closes and η-nitrogen metallizes.^[40]

At pressures higher than110 GPa and temperatures around2000 K, nitrogen forms anetwork solid, bound bycovalent bonds in acubic-gauche structure, abbreviated as cg-N. The cubic-gauche form has space groupI2₁3. Each unit cell has edge length3.805 Å, and contains eight nitrogen atoms.^[23] As a network, cg-N consists of fused rings of nitrogen atoms; at each atom, the bond angles are very close to tetrahedral. The position of the lone pairs of electrons is ranged so that their overlap is minimised.^[39]

The cubic-gauche structure for nitrogen is predicted to have bond lengths of 1.40 Å, bond angles of 114.0° anddihedral angles of −106.8°. The termgauche refers to the odd dihedral angles, if it were 0° it would be calledcis, and if 180° it would be calledtrans. The dihedral angle Φ is related to the bond angle θ by sec(Φ) = sec(θ) − 1. 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).^[41]

All bonds in cg-N have the same length:1.346 Å at115 GPa.^[1][42] This suggests that all bonds have the sameorder: asingle bond carrying4.94 eV/atom. In contrast, thetriple bond ingaseous nitrogen carries only0.83 eV/atom, so that relaxation to the gaseous form involves tremendous energy release: more than any other non-nuclear reaction.^[1][43] For this reason, cubic-gauche nitrogen is being investigated for use in explosives and rocket fuel.^[1] Estimates of its energy density vary: simulations predict10–33 kJ/g is predicted, which is160%–300% the energy density ofHMX.^[44][45]

cg-N is also very stiff with abulk modulus around298 GPa, similar to diamond.^[42]

Another network solid nitrogen called poly-N and abbreviatedpN was predicted in 2006.^[23]pN has space groupC2/c and cell dimensions a = 5.49 Å, β = 87.68°. Other higher pressure polymeric forms are predicted in theory, and a metallic form is expected if the pressure is high enough.^[46]

Yet other phases of solid dinitrogen are termed ζ'-N₂ and κ-N₂.^[39]

At58 K theultimate compressive strength is 0.24 MPa. Strength increases as temperature lowers becoming 0.54 MPa at 40.6 K.Elastic modulus varies from 161 to 225 MPa over the same range.^[47] This property is relevant in understanding the behaviour of nitrogen ice sheets and glaciers.

Thethermal conductivity of solid nitrogen is 0.7 W m⁻¹ K⁻¹.^[48] Thermal conductivity varies with temperature and the relation is given byk = 0.1802×T^0.1041  W m⁻¹ K⁻¹.^[49]Specific heat is given by 926.91×e^0.0093T joules per kilogram per kelvin.^[49]Its appearance at 50 K is transparent, while at 20 K it is white.

Nitrogen frost has a density of 0.85 g cm⁻³.^[50] As a bulk material the crystals are pressed together and density is near that of water. It is temperature dependent and given by ρ = 0.0134T² − 0.6981T + 1038.1 kg/m³.^[49] The volume coefficient of expansion is given by 2×10⁻⁶T² − 0.0002T + 0.006 K⁻¹.^[49]

Theindex of refraction at 6328 Å is 1.25 and hardly varies with temperature.^[50]

Thespeed 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. The transverse velocity is much lower ranging from 900 to 800 m/s over the same temperature range.^[17]

Thebulk modulus of s-N₂ is 2.16 GPa at 20 K, and 1.47 GPa at 44 K.^[17] At temperatures below 30 K solid nitrogen will undergobrittle failure, particularly if strain is applied quickly. Above this temperature the failure mode isductile failure. Dropping 10 K makes the solid nitrogen 10 times as stiff.^[17]

Under pressure nitrogen can formcrystallinevan der Waals compounds with other molecules. It can form an orthorhombic phase withmethane above 5 GPa.^[51] With helium, He(N₂)₁₁ is formed.^[20] N₂ crystallizes with water innitrogen clathrate and in a mixture with oxygen O₂ and water inair clathrate.^[52]

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 hexagonalbirefringent crystalline van der Waals compound. The unit cell contains 22 nitrogen atoms and 2 helium atoms. It has a volume of 580 Å³ for a pressure of 11 GPa decreasing to 515 Å³ at 14 GPa.^[20] It resembles the ε-phase.^[53] At 14.5 GPa and 295 K the unit cell has space groupP6₃/m and a=7.936 Å c=9.360 Å. At 28 GPa a transition happens in which the orientation of N₂ molecules becomes more ordered. 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.^[54]

Solid nitrogen can crystallise with some solid methane included. At 55 K the molar percentage can range up to 16.35% CH₄, and at 40 K only 5%. In the complementary situation, solid methane can include some nitrogen in its crystals, up to 17.31% nitrogen. As the temperature drops, less methane can dissolve in solid nitrogen, and in α-N₂ there is a major drop in methane solubility. These mixtures are prevalent in outer Solar System objects such asPluto that have both nitrogen andmethane on their surfaces.^[55] At room temperature there is aclathrate of methane and nitrogen in 1:1 ratio formed at pressures over 5.6 GPa.^[56]

Thecarbon monoxide molecule (CO) is very similar to dinitrogen in size, and it can mix in all proportions with solid nitrogen without changing crystal structure. Carbon monoxide is also found on the surfaces of Pluto andTriton at levels below 1%. Variations in the infrared linewidth of carbon monoxide absorption can reveal the concentration.^[57]

Neon orxenon atoms can also be included in solid nitrogen in the β and δ phases. Inclusion of neon pushes the β−δ phase boundary to higher pressures.^[58]Argon is also very miscible in solid nitrogen.^[58] For compositions of argon and nitrogen with 60% to 70% nitrogen, the hexagonal form remains stable to 0 K.^[59] Avan der Waals compound of xenon and nitrogen exists above 5.3 GPa.^[58] A van der Waals compound of neon and nitrogen was shown usingRaman spectroscopy.^[58] The compound has formula (N₂)₆Ne₇. It has a hexagonal structure, with a=14.400 c=8.0940 at a pressure of 8 GPa. A van der Waals compound with argon is not known.^[60]

Withdideuterium, a clathrate (N₂)₁₂D₂ exits around 70 GPa.^[61]

Solid nitrogen can take up to a one fifth substitution byoxygen O₂ and still keep the same crystal structure.^[62] δ-N₂ can be substituted by up to 95% O₂ and retain the same structure. Solid O₂ can only have a solid solution of 5% or less of N₂.^[62]

Solid nitrogen is used in a slush mixture withliquid nitrogen in order to cool faster than with liquid nitrogen alone, useful for applications such assperm cryopreservation.^[63] The semi-solid mixture can also be calledslush nitrogen^[64] or SN2.^[65]

Solid nitrogen is used as a matrix on which to store and study reactive chemical species, such asfree radicals or isolated atoms.^[66] One use is to studydinitrogen complexes of metals in isolation from other molecules.^[67]

When solid nitrogen is irradiated by high speed protons or electrons, several reactive radicals are formed, including atomic nitrogen (N), nitrogen cations (N⁺),dinitrogen cation (N₂⁺),trinitrogen radicals (N₃ and N₃⁺), andazide (N₃⁻).^[68]

• Media related toSolid nitrogen at Wikimedia Commons
• Jessica Orwig:Freezing Liquid Nitrogen Creates Something Amazing. On: BusinessInsider. Jan 28, 2015 - Videos of nitrogen boiling, freezing, and spontaneously changing crystal form.
• Xiaoli Wang, J. Li, N. Xuet al. (2015):Layered polymeric nitrogen in RbN₃ at high pressures. In: Scientific Reports volume 5, Article number: 16677.doi:10.1038/srep16677.

• Nitrogen
• Allotropes of nitrogen
• Pnictogens
• Diatomic nonmetals
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