<P> When the pressure is below the triple point, solid nitrogen directly sublimes to gas . The triple point is at 63.14 ± 0.06 K and 0.1255 ± 0.0005 bar . The vapour pressure has been measured from 20 K up to the triple point . For α - nitrogen (below 35 K) the logarithm of the pressure is given by 12.40 − 807.4 × T − 3926 T + 6.297 × 10 T − 4.633 × 10 T 1.325 × 10 T. For β - nitrogen it is given by 8.514 − 458.4 T − 19870 T 4.800 × 10 T − 4.524 × 10 T. Where the solid is not pure nitrogen, the vapour pressure can be estimated using Raoult's law in which the pressure is reduced by the molar concentration . This calculation is relevant for the atmosphere of outer solar system bodies, where there could be a 1% contamination with carbon monoxide and methane . </P> <P> There are several known solid forms of molecular dinitrogen . At ambient pressures there are two solid forms . β - N is a hexagonal close packed structure which exists from 35.6 K up to 63.1 5K at which point it melts . At 45 K the unit cell has a = 4.050 Å and c = 6.604 Å . At 4125 atmospheres pressure, and 49K the unit cell sizes have shrunk to a = 3.861 Å c = 6.265 Å . If the pressure is increased the c / a ratio stays the same . </P> <P> In the β phase, the molecule centres are hexagonal close packed . This means that the c / a ratio is ≈ 1.633 = √ 8 / 3 . The nitrogen molecules are randomly tipped at an angle of 55 ° from the c - axis . There is a strong quadrupole - quadrupole interaction between the molecules . </P> <P> Another phase termed α - N exists below 35.6 K at low pressure and has a cubic structure . The space group is Pa3 . At 21 K the unit cell dimension is 5.667 Å . Under 3785 bars this reduces to 5.433 Å . At low temperatures the α - phase can be compressed to 3500 atmospheres before it changes (to γ), and as the temperature rises above 20 K, this pressure rises to about 4500 atmospheres . </P>

At what temperature does nitrogen become a solid