Magnesium nitride, which possesses the chemical formula Mg3N2, is an inorganic compound of magnesium and nitrogen. At room temperature and pressure it is a greenish yellow powder.
History When measuring the boiling point of magnesium, Henri Étienne Sainte-Claire Deville and Henri Caron identified that molten magnesium they distilled covered itself by "small colorless and transparent needles which are destroyed fairly quickly by transforming into ammonia and magnesia". In their 1857 publication the chemists interpreted it as a likely nitride similar to those discovered by Friedrich Wöhler and Heinrich Rose. It was indeed confirmed in 1862 when Friedrich Briegleb and Johann Georg Anton Geuther synthesized the compound on purpose and first studied it.
Preparation Magnesium nitride may be prepared by passing dry, very pure nitrogen over heated magnesium at 800–850 °C (1,470–1,560 °F) for 90 minutes:
3 Mg + N2 → Mg3N2 Or by passing dry ammonia over heated magnesium at 800–850 °C (1,470–1,560 °F) for 4 hours:
3 Mg + 2 NH3 → Mg3N2 + 3 H2 This second method is preferred despite the additional time required due to the difficulty of fully purifying nitrogen gas to prevent the formation of unwanted oxides.
Chemistry Magnesium nitride reacts with water to produce magnesium hydroxide and ammonia gas, as do many metal nitrides.
Mg3N2(s) + 6 H2O(l) → 3 Mg(OH)2(aq) + 2 NH3(g) In fact, when magnesium is burned in air, some magnesium nitride is formed in addition to the principal product, magnesium oxide. Thermal decomposition of magnesium nitride at 700–1,500 °C (1,292–2,732 °F) gives magnesium and nitrogen gas. At high pressures, the stability and formation of new nitrogen-rich nitrides (N/Mg ratio equal or greater to one) were suggested and later discovered. These include the Mg2N4 and MgN4 solids which both become thermodynamically stable near 50 GPa (7.3 million psi). The Mg2N4 is composed of exotic cis-tetranitrogen N4−4 species with N-N bond orders close to one. This Mg2N4 compound was recovered to ambient conditions, along with the N4−4 units, marking only the fourth polynitrogen entity bulk stabilized at ambient conditions.
Uses and history When isolating argon, William Ramsay passed dry air over copper to remove oxygen and over magnesium to remove the nitrogen, forming magnesium nitride. Magnesium nitride was the catalyst in the first practical synthesis of borazon (cubic boron nitride). Robert H. Wentorf, Jr. was trying to convert the hexagonal form of boron nitride into the cubic form by a combination of heat, pressure, and a catalyst. He had already tried all the logical catalysts (for instance, those that catalyze the synthesis of diamond), but with no success. Out of desperation and curiosity (he called it the "make the maximum number of mistakes" approach), he added some magnesium wire to the hexagonal boron nitride and gave it the same pressure and heat treatment. When he examined the wire under a microscope, he found tiny dark lumps clinging to it. These lumps could scratch a polished block of boron carbide, something only diamond was known to do. From the smell of ammonia, caused by the reaction of magnesium nitride with the moisture in the air, he deduced that the magnesium metal had reacted with the boron nitride to form magnesium nitride, which was the true catalyst. Magnesium nitride has also been applied to synthesize aluminum nitride nanocrystals, cubic boron nitride and nitrides of aluminum and group 3 elements. It has also been proposed as an intermediate in a fossil-fuel-free nitrogen fixation process.
References
Further reading Wu, P.; Tiedje, T. (2018). "Molecular beam epitaxy growth and optical properties of Mg3N2 films". Applied Physics Letters. 113 (8). AIP: 082101. Bibcode:2018ApPhL.113h2101W. doi:10.1063/1.5035560. S2CID 125356057.
