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 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+16N2). 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.三元氮化物材料的合成极具挑战性，这在很大程度上源于单质氮气（N₂）相对惰性的化学性质。尽管如此，锂可与其他金属及氮气顺利发生反应，因此锂-金属-氮（Li-M-N）体系是三元氮化物中数量最多的子类。本研究以合成工艺简便的三元氮化物Li₂ZrN₂作为前驱体，开展针对AZrN₂（A=Mg、Fe、Cu、Zn）的离子交换反应。原位同步辐射粉末X射线衍射（in situ synchrotron powder X-ray diffraction）研究表明，Li⁺与Mg²⁺以拓扑化学方式发生离子交换，完整保留了八面体[ZrN₆]的层状结构。该反应生成的是亚稳层状MgZrN₂多晶型（空间群（space group）Roverline{3}m），而非理论计算得到的基态结构（I41/amd）。漫反射光谱测量结果显示，该多晶型的光吸收起始点约为2.0 eV，与该多晶型的计算带隙（bandgap）值相符。我们尝试将该离子交换方法拓展至FeZrN₂、CuZrN₂及ZnZrN₂的合成时，得到了分解产物（A+ZrN+16N₂）。我们的计算结果表明，相较于MgZrN₂，这些物相的亚稳性（metastability）更高，由此解释了该实验现象。我们通过Li₂HfN₂成功合成了MgHfN₂，将该离子交换方法推广至其他Li-M-N前驱体体系。除实验合成MgZrN₂与MgHfN₂的亚稳Roverline{3}m多晶型外，本研究还证实了63种已知Li-M-N相作为前驱体，用于合成其他多种三元氮化物材料的应用潜力。