Ion Exchange Synthesizes a Metastable Layered Polymorph of MgZrN2 and MgHfN2 Semiconductors

The synthesis of ternary nitride materials is uniquely difficult, in large part because elemental N2 is relatively inert. However, lithium reacts readily with other metals and N2, making Li-M-N the most numerous subset of ternary nitrides. Here, we use Li2ZrN2, a ternary nitride compound with a simple synthesis recipe, as a precursor for ion exchange reactions toward AZrN2 (A = Mg, Fe, Cu, Zn). In situ synchrotron powder X-ray diffraction studies show that Li+ and Mg2+ undergo ion exchange topochemically, preserving the layers of octahedral [ZrN6]. This reaction yields a metastable layered polymorph of MgZrN2 (space group R3m) rather than the calculated ground state structure (I41/amd). Diffuse reflectance measurements show an optical absorption onset near 2.0 eV, consistent with the calculated bandgap for this polymorph. Our experimental attempts to extend this ion exchange method toward FeZrN2, CuZrN2, and ZnZrN2 resulted in decomposition products (A+ZrN+1/6N2). This experimental outcome is explained by our computational results via the higher metastability of these phases compared to MgZrN2. We successfully extended this ion exchange method to other Li-M-N precursors by synthesizing MgHfN2 from Li2HfN2. In addition to the experimental synthesis of metastable R3m polymorphs of MgZrN2 and MgHfN2, this work highlights the potential of the 63 known Li-M-N phases as precursors to synthesize many other ternary nitride materials.