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3.3.1 Crystal Structures

Solid nitrogen has three known crystal structures - labelled $\alpha$-, $\beta$- and $\gamma$-N2 - in the P-T plane. [20] The $\gamma$ phase exists only under high pressures - greater than about 3500 atm, well beyond pressures attainable in thin-walled cells and is therefore not presently accessible with low energy surface muon beams.

The $\alpha$ phase exists under equilibrium vapour pressure and at temperatures below $T_{\alpha\beta}$=36.61 K, in which the N2 molecules are centred on the sites of an fcc lattice of space group Pa3. In this structure the molecules are oriented (on average) lengthwise along body diagonals of the cubic cell. The x-ray structure yields a lattice constant a=5.660(2) Å at 20 K. [23]

Above $T_{\alpha\beta}$, also at low pressures, the $\beta$ phase has the molecular centres on a hcp lattice of space group P$6_3/{\rm mmc}$. The unit cell has dimensions a=4.036Å and c=6.630Å. [21] The molecules' orientations on this lattice are not fixed.