具有奇数配位多面体的新KCoO2型层状氮(氧)化物的合成与高体相介电常数机理

High dielectric-permittivity materials are required by the capacitive microelectronic devices to increase the storage capacity and minimize the size. The high bulk dielectric permittivity in the traditional oxide materials originates from the ferroelectric spontaneous polarization, e.g. the perovskite materials containing the 6-coordinate octahedral units. The higher polarizability of nitrogen than oxygen makes the (oxy)nitrides huge possibility for exploiting the high bulk dielectric-permittivity materials. However, owing to difficulty on synthesis and instability of (oxy)nitrides, the investigation on dielectric properties of the metal (oxy)nitrides is relatively little and the understanding to their dielectric mechanisms remains insufficient. Recently we have discovered that a new KCoO2-type layered nitride CaTiN2, that contains unevenly coordinate polyhedra, displayed > 1000 high bulk dielectric permittivity but with the mechanism unanswered. In this project, we propose to design a series of new KCoO2-type layered (oxy)nitrides with different symmetries based on size effect and prepare them using multiple synthetic methods; the crystal structures and dielectric properties as well as the relationship between them for these (oxy)nitrides will be investigated, based on which the chemical mechanism for the high bulk dielectric permittivity of KCoO2-type layered (oxy)nitrides will be elucidated from the point of view on the the influence of unevenly coordinate polyhedra, polyhedral distortion and nitrogen-oxygen ordering to the dielectric (spontaneous) polarization. This project is believed to establish new dielectric polarization mechanism on the crystalline solid materials, and play a guiding role on exploiting new high bulk dielectric-permittivity materials form (oxy)nitrides even oxide materials on both theoretical and experimental aspects. The high dielectric permittivity materials obtained from this project could show potential application in the capacitive microelectronic devices.

基于电容的微电子器件需要高介电常数材料来提高容量和减小尺寸。传统氧化物的高体相介电常数来源于铁电自发极化，比如基于6配位八面体的钙钛矿。氮的极化率比氧大，在氮(氧)化物中探索高体相介电常数材料具有广阔空间。但氮(氧)化物合成困难且不稳定，目前氮(氧)化物介电性质研究较少，对其介电极化机制认识不足。最近我们发现具有奇数配位多面体的KCoO2型层状氮化物CaTiN2具有>1000的高体相介电常数，但其介电机制不明晰。本项目拟从尺寸效应来设计并采用多种手段合成系列新的具有不同对称性KCoO2型层状氮(氧)化物，研究其晶体结构和介电性质及其关系，从奇数配位多面体、多面体的畸变和氧氮有序等因素影响介电(自发)极化的角度阐述其高体相介电常数的结构化学机制。项目将获得新的晶态固体介电极化机制，对在氮(氧)化物甚至纯氧化物体系中探索高体相介电常数材料提供理论和实验指导，可为微电子器件提供有应用价值的材料。

氮的极化率比氧大使得氮(氧)化物是探索高体相介电常数材料的良好候选体系。但氮(氧)化物合成困难且不稳定，对氮(氧)化物介电性质研究较少，对其介电极化机制的认识有限。基于具有奇数配位多面体的KCoO2型层状氮化物CaTiN2大于1000的高体相介电常数，项目主要围绕AMN2型氮(氧)化物的制备、掺杂、结构与性质调控开展研究，从结构极性和多面体畸变等角度探索其介电极化机制。项目取得主要研究成果概括为六点。(1)提出了Ti-N层中的N原子平面的分裂引起结构畸变是正交CaTiN2高体相介电常数的来源。(2)预测了a-NaFeO2型SrZrN2和SrHfN2层状氮化物的高体相介电常数，并揭示其主要来源于结构中ab平面内的离子极化，以及调控ab平面内应力可以实现介电常数数量级上的增加。(3)采用一步固相法实现含Mg盐岩固溶体MgxTi1-xN的体相合成，阐明了Ti3+含量、N空位或氧杂质以及Mg-Ti有序无序等因素影响MgxTi1-xN传输性质的机制。(4)从离子尺寸匹配、金属离子与氮间的化学键特性、分解能和电子自旋等角度总结出AMN2型氮化物的稳定性及合成化学规律。(5)通过高温高压合成在严重偏离理想钙钛矿的La6Sr3Si6O24组成上获得富含缺陷，呈现离子导电的六方钙钛矿结构。(6)在RbNaMgP2O7的极性-极性结构相变中发现热驱动P2O7二聚体的平行排列渐进过程，介电常数的增加以及金属离子导电的增强现象。研究成果创新了氮化物的介电极化机制和电输运性质的构效关系，为氮化物制备及其他相关材料缺陷调控提供了合成化学新途径。项目目前已在Chem Mater、Inorg Chem、Chinese Chem Lett期刊正式发表论文4篇，1篇Inorg Chem论文修改稿正在评审中，申报发明专利1项；培养了6名硕士生、1名博士生、1名博士后，其中4名硕士生和1名博士生获得学位。项目负责人获2021年度广西青年科技杰出贡献奖。