Fig. 1.

The crystal structure of the MgN₄ and β-Mg₂N₄ salts at 58.5 GPa. a The unit cell of MgN₄ (the light blue, dark blue and orange spheres represent the N1, N2 and Mg atoms, respectively); b a projection of the MgN₄ structure along the c-axis, emphasising 1D chains of nitrogen atoms aligned along the a-axis; c a repeating N₄²⁻ subunit of a chain with the N-N distances and angles indicated; d the unit cell of β-Mg₂N₄ (the light green and dark green spheres represent the four distinct nitrogen atoms forming the a-N₄⁴⁻ and b-N₄⁴⁻ units, respectively, the orange spheres represent Mg atoms); e a projection of the β-Mg₂N₄ structure along the b-axis allowing to see the alternating layers of isolated a-N₄⁴⁻ and b-N₄⁴⁻ units, intercalated with Mg²⁺ ions. f The a-N₄⁴⁻ (left) and b-N₄⁴⁻ (right) entities with bond lengths and angles indicated