Sodium nitride

Sodium nitride can be synthesized in two different ways: by the thermal decomposition of NaNH₂ or by the direct reaction of the elements.[2] The most common way to successfully synthesize sodium nitride has been done by Dieter Fischer & Martin Jansen and Grigori Vajenine using the latter method. The first way is to introduce desired ratios of Na and N₂ in gas phase separately and depositing them in a vacuum chamber on a cooled substrate, which is then heated to room temperate (298 K) to crystallize.[1] The second method is to react elemental sodium with plasma activated nitrogen on a metal surface. This synthesis can be further facilitated by introducing liquid Na-K alloy to the compound with the excess liquid removed and washed with fresh alloy. The solid is then separated from the liquid using a centrifuge. However Vajenine’s method is very air-sensitive and can decompose and combust rapidly, unless exposed to a pure oxygen (O₂) environment.[3]

Sodium nitride can be of reddish brown or dark blue color depending on the synthesis of the compound due to intrinsic properties.[1][3] It shows no signs of decomposition after several weeks when at room temperature.[3] The compound does not have a melting point as it decomposes back into its elemental forms as demonstrated using mass spectrometry around 360 K.[1][2] The estimated enthalpy of formation for the compound is +64 kJ/mol.[3]

Sodium nitride seems to be about 90% ionic at room temperature, but has the band gap typical for a semiconductor.[2][3] It adopts the anti-ReO₃ structure with a simple lattice made up of NNa₆ octahedra.[1][2][3][4] The compound has N­−Na bond lengths of 236.6 pm.[1][3] This structure has been confirmed through X-ray diffraction and more recently neutron diffraction on powder and single crystals.[1][2][3][4]