Lithium nitride
Lithium nitride is an inorganic compound with the chemical formulaLi3N. It is the only stable alkali metal nitride. It is a reddish-pink solid with a high melting point.
Preparation and handling
Lithium nitride is prepared by direct reaction of elemental lithium with nitrogen gas:
- 6 Li + N2 → 2 Li3N
Instead of burning lithium metal in an atmosphere of nitrogen, a solution of lithium in liquid sodium metal can be treated withN2.
Lithium nitride is an extremely strong base, so it must be protected from moisture as it reacts violently with water to produce ammonia:
- Li3N + 3 H2O → 3 LiOH + NH3
Structure and properties
- alpha-Li3N (stable at room temperature and pressure) has an unusual crystal structure that consists of two types of layers: one layer has the compositionLi2N− contains 6-coordinate N centers and the other layer consists only of lithium cations.
Two other forms are known:
- beta-Li3N, formed from the alpha phase at 0.42 GPa has the sodium arsenide (Na3As) structure;
- gamma-Li3N (same structure as lithium bismuthideLi3Bi) forms from the beta form at 35 to 45 GPa.
Lithium nitride shows ionic conductivity forLi+, with a value of c. 2×10−4 Ω−1cm−1, and an (intracrystal) activation energy of c. 0.26 eV (c. 24 kJ/mol). Hydrogen doping increases conductivity, whilst doping with metal ions (Al, Cu, Mg) reduces it. The activation energy for lithium transfer across lithium nitride crystals (intercrystalline) has been determined to be higher, at c. 68.5 kJ/mol. The alpha form is a semiconductor with band gap of c. 2.1 eV.
Reactions
Reacting lithium nitride with carbon dioxide results in amorphous carbon nitride (C3N4), a semiconductor, and lithium cyanamide (Li2CN2), a precursor to fertilizers, in an exothermic reaction.
Under hydrogen at around 200°C, Li3N will react to form lithium amide.
- Li3N + 2 H2 → 2LiH + LiNH2
At higher temperatures it will react further to form ammonia and lithium hydride.
- LiNH2 + H2 → LiH + NH3
Lithium imide can also be formed under certain conditions. Some research has explored this as a possible industrial process to produce ammonia since lithium hydride can be thermally decomposed back to lithium metal.
Lithium nitride has been investigated as a storage medium for hydrogen gas, as the reaction is reversible at 270 °C. Up to 11.5% by weight absorption of hydrogen has been achieved.