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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 intermediateand product phases often have high cohesive energies that inhibit diffusion.Here, we report the synthesis of two new phases, calcium zirconium nitride(CaZrN$_2$) and calcium hafnium nitride (CaHfN$_2$), by solid state metathesisreactions between Ca$_3$N$_2$ and $M$Cl$_4$ ($M$ = Zr, Hf). Although thereaction nominally proceeds to the target phases in a 1:1 ratio of theprecursors via Ca$_3$N$_2$ + $M$Cl$_4$ $\rightarrow$ Ca$M$N$_2$ + 2 CaCl$_2$,reactions prepared this way result in Ca-poor materials (Ca$_xM_{2-x}$N$_2$,$x<1$). A small excess of Ca$_3$N$_2$ (ca. 20 mol\%) is needed to yieldstoichiometric Ca$M$N$_2$, as confirmed by high-resolution synchrotron powderX-ray diffraction. In situ synchrotron X-ray diffraction studies reveal thatnominally stoichiometric reactions produce Zr$^{3+}$ intermediates early in thereaction pathway, and the excess Ca$_3$N$_2$ is needed to reoxidize Zr$^{3+}$intermediates back to the Zr$^{4+}$ oxidation state of CaZrN$_2$. Analysis ofcomputationally-derived chemical potential diagrams rationalizes this syntheticapproach and its contrast from the synthesis of MgZrN$_2$. These findingsadditionally highlight the utility of in situ diffraction studies andcomputational thermochemistry to provide mechanistic guidance for synthesis.

Mechanistically-guided materials chemistry: synthesis of new ternary nitrides, CaZrN$_2$ and CaHfN$_2$