Solvothermal Metal Azide Decomposition Routes to Nanocrystalline Metastable Nickel, Iron, and Manganese Nitrides

This paper describes the use of solvothermally moderated metal azide decomposition as a route to nanocrystalline mid to late transition metal nitrides. This method utilizes exothermic solid-state metathesis reaction precursor pairs, namely, metal halides (NiBr2, FeCl3, MnCl2) and sodium azide, but conducts the metathesis reaction and azide decomposition in superheated toluene. The reaction temperatures are relatively low (<300 °C) and yield thermally metastable nanocrystalline hexagonal Ni3N and Fe2N, and tetragonal MnN. These solvothermally moderated metal nitride metathesis reactions require several days to produce high yields of the intended nitrides. The products are aggregated nanoparticulates with room temperature magnetic properties consistent with their known bulk structures, for example, Fe2N and Ni3N are known ferromagnets. The stirred reactions with dispersed fine reagent powders benefit from solvothermal moderation more effectively than submerged pressed reagent pellets. Pellet reactions produced manganese nitrides with lower nitrogen content and higher aggregation than loose powder reactions, consistent with the occurrence of significant local exothermic heating in the pellet metathesis reactions.