Conversion of La₂Ti₂O₇ to LaTiO₂N via ammonolysis: An ab-initio investigation

Perovskite oxynitrides are, due to their reduced band gap compared to oxides, promising materials for photocatalytic applications. They are most commonly synthesized from {110} layered Carpy-Galy (A₂B₂O₇) perovskites via thermal ammonolysis, i.e. the exposure to a flow of ammonia at elevated temperature. The conversion of the layered oxide to the non-layered oxynitride must involve a complex combination of nitrogen incorporation, oxygen removal and ultimately structural transition by elimination of the interlayer shear plane. Despite the process being commonly used, little is known about the microscopic mechanisms and hence factors that could ease the conversion. Here we aim to derive such insights via density functional theory calculations of the defect chemistry of the oxide and the oxynitride as well as the oxide's surface chemistry. Our results point to the crucial role of surface oxygen vacancies in forming clusters of NH₃ decomposition products and in incorporating N, most favorably substitutionally at the anion site. N then spontaneously diffuses away from the surface, more easily parallel to the surface and in interlayer regions, while diffusion perpendicular to the interlayer plane is somewhat slower. Once incorporation and diffusion lead to a local N concentration of about 70 % of the stoichiometric oxynitride composition, the nitridated oxide spontaneously transforms to a nitrogen-deficient oxynitride.

氧氮化物钙钛矿（Perovskite oxynitrides）相较于传统氧化物具有更窄的带隙，是极具应用前景的光催化材料。这类材料通常以{110}晶面层状的卡皮-加利（Carpy-Galy，A₂B₂O₇）型钙钛矿为前驱体，通过热氨解法（thermal ammonolysis）合成，即于高温环境下暴露于氨气气流中进行处理。将层状氧化物转化为非层状氧氮化物的过程，必然涉及氮掺入、氧脱除以及最终通过消除层间剪切面实现结构转变的一系列复杂组合过程。尽管该工艺已被广泛应用，但学界对其微观机制以及可促进转化的相关因素仍知之甚少。本研究旨在通过对氧化物与氧氮化物的缺陷化学（defect chemistry）以及氧化物表面化学开展密度泛函理论（Density Functional Theory）计算，以此获取此类认知。研究结果表明，表面氧空位在形成氨气（NH₃）分解产物团簇以及氮元素掺入的过程中发挥关键作用，其中氮最优选的掺杂位点为阴离子位点的取代位。随后氮元素会自发地从表面向外扩散，沿平行于表面的方向以及层间区域的扩散更为容易，而垂直于层间平面的扩散速度则相对较慢。当掺入与扩散过程使局部氮浓度达到化学计量比氧氮化物组分的约70%时，氮化后的氧化物会自发转变为氮缺陷型氧氮化物。