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

Abstract

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 R3̅m) 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 ($A+ZrN+\frac{1}{6}{N}_2$). 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 R3̅m 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.