Mechanistically Guided Materials Chemistry: Synthesis of Ternary Nitrides, CaZrN₂ and CaHfN₂

Recent computational studies have predicted many new ternary nitrides, revealing synthetic opportunities in this underexplored phase space. However, synthesizing new ternary nitrides is difficult, in part because intermediate and product phases often have high cohesive energies that inhibit diffusion. Here, we report the synthesis of two new phases, calcium zirconium nitride (CaZrN₂) and calcium hafnium nitride (CaHfN₂), by solid state metathesis reactions between Ca₃N₂ and MCl₄ (M = Zr, Hf). Although the reaction nominally proceeds to the target phases in a 1:1 ratio of the precursors via Ca₃N₂ + MCl₄ → CaMN₂ + 2 CaCl₂, reactions prepared this way result in Ca-poor materials (CaM_2-N₂, x < 1). A small excess of Ca₃N₂ (ca. 20 mol %) is needed to yield stoichiometric CaMN₂, as confirmed by high-resolution synchrotron powder X-ray diffraction. In situ synchrotron X-ray diffraction studies reveal that nominally stoichiometric reactions produce Zr³⁺ intermediates early in the reaction pathway, and the excess Ca₃N₂ is needed to reoxidize Zr³⁺ intermediates back to the Zr⁴⁺ oxidation state of CaZrN₂. Analysis of computationally derived chemical potential diagrams rationalizes this synthetic approach and its contrast from the synthesis of MgZrN₂. These findings additionally highlight the utility of in situ diffraction studies and computational thermochemistry to provide mechanistic guidance for synthesis.