Kinetically Controlled Low-Temperature Solid-State Metathesis of Manganese Nitride Mn₃N₂

The synthesis of inorganic metal nitrides poses a challenge due to the low reactivity of N2 gas at low temperatures, yet entropy driven formation of N2 gas at high temperatures. In contrast, synthetic approaches using more activated forms of nitrogen can be used to overcome the inertness of N2, but increased exothermicity can also result in diminished stoichiometric control and the activation of deleterious competing pathways. Here, kinetically controlled solid-state metathesis reactions are used to prepare Mn3N2 without the use of experimental conditions that increase the chemical potential of nitrogen and are known to produce phase impurity (e.g., NH3, N2-based plasma, azides, or high pressure). The solid-state metathesis reaction between MnCl2 and Mg2NCl or Mg3N2 is shown to generate Mn3N2, a phase on the border of stability. Highly exothermic control reactions performed with Li3N, Ca3N2, and Ca2NCl yield poorly crystalline, nitrogen-deficient Mn–N phases and N2 gas. The reactions with Mg2NCl and Mg3N2 do not self-propagate and have the lowest predicted free energies of reaction. A series of reactions performed at different times and temperatures, as well as in situ synchrotron X-ray diffraction, illustrate the importance of kinetic competence, and the results implicate the mechanism for this competence: the formation of a solid-solution, MgxMn1–xCl2, between the halide precursor (MnCl2) and the halide product (MgCl2) coupled to a mildly exothermic reaction. Kinetically controlled solid-state metathesis continues to provide an avenue toward the synthesis of materials that cannot be prepared under traditional, high-temperature ceramic methods.