8层孪生型和位移型六方钙钛矿的构型选择机制及结构与微波介电性能关系

Dielectric ceramics are used in mobile phone network and satellite communication as microwave dielectric resonators and filters, which are short of available materials with high dielectric permittivity, low dielectric loss and temperatrue-stable resonant frequency. The fundmental chemistry and physics of microwave dielectric properties are not well understood and the tunning the micrwoave dielectric properties and discovery of new microwave dielectric materials are basically empirical. As one class of dominate materials in the currently commercial microwave dielectric ceramics, cubic perovskite dielectrics have already acttracted considerable attentions because of their excellet microwave dielectric properties, which require controlling the processing for the cation order-disorder to acheive the high qaulity factors. Hexagonal perovskites such as the 8-layer twinned and shifted series, containing the same structural units as those in the cubic perovskites, possess relatively simpler processings and high qaulity factors, demonstrating potential application in microwave circuits as dielectrics. In this proposal, we are going to investigate the stablization mechanisms of twinned and shifted structure of 8-layer hexagonal perovskites as well as their structrual details, microwave dielectric properties and the relationship between the structure and microwave dielectric properties. This project is expected to provide guidance on the exploitation of new high performance microwave dielectrics and controlling the microwave dielectric properties and it is believed to have significant contributions to understanding the underlying formalism on chemistry of microwave dielectric properties for the hexagonal perovskites as well as the cubic perovskites.

移动通讯和卫星通讯的发展需要高性能的微波介质陶瓷谐振器和滤波器。目前符合应用要求的谐振频率对温度稳定的高介电常数，低介电损耗的介质陶瓷材料面临短缺。人们对微波介电性能的化学和物理机制的认识有限，目前对微波介质陶瓷材料的开发主要还是基于经验。立方钙钛矿介质材料由于其良好的微波介电性能已受到了广泛的关注和研究，但它需要严格控制合成工艺来调整离子排列的有序-无序才能获得高品质因子。与立方钙钛矿具有相同结构单元的六方钙钛矿，如8层孪生型和位移型六方钙钛矿，其化学合成工艺过程相对简单并且具有高的品质因子，在微波电子元器件中呈现了潜在的应用价值。本项目将研究8层六方钙钛矿在孪生型和位移型两种构型之间的选择机制，并开展结构和微波介电性能及其之间关系的研究。这项研究对探索新的高性能微波介质材料和调控材料的微波介电性能具有重要指导意义，并有助于理解六方钙钛矿甚至立方钙钛矿材料家族的微波介电性能的化学机制。

符合微波器件要求的高介电常数，低介电损耗、且谐振频率对温度稳定的介质陶瓷材料面临短缺。立方钙钛矿因其良好的微波介电性能已受到了广泛的关注和研究，但它需要严格控制合成工艺来调整离子有序才能获得高品质因子。8 层六方钙钛矿其合成工艺相对简单且具有高品质因子，在微波器件中呈现了潜在的应用价值。本项目主要围绕新的8层六方钙钛矿的探索和孪生-位移构型选择机制及微波介电性质与结构之间的关系开展研究。取得的主要研究成果有：(1)在8层六方钙钛矿铌酸盐和钽酸盐体系中提出了Nb5+和Ta5+之间在二阶Jahn-Teller效应上的微小差异主导的孪生-位移构型选择机制以及8层位移型结构中B位空位有序及NbO6八面体的二阶Jahn-Teller畸变主导的纳米级离子有序和位移型结构稳定机制；(2)获得了第一例长周期的14层孪生型六方钙钛矿Ba8MnTa6O24并对该结构及其离子有序进行了原子级的成像，对粉末衍射结构解析和离子有序研究提供了具有重要参考意义的互补性手段；(3)制备了多个新的8层六方钙钛矿铌酸盐和钽酸盐介质材料，在Ba8M2+Nb6O24铌酸盐组成中提出了M2+二价离子和Nb5+离子间的尺寸差异是影响位移-孪生构型选择的又一重要因素，进一步发展了8层六方钙钛矿孪生-位移构型选择机制。(4)发现了少量的大尺寸Zr对Ti的等价取代对Ba8Ti3Ta4O24造成的8H-10H孪生物相竞争和相互转化现象，提出B位离子尺寸和加热温度是影响孪生型六方钙钛矿的堆积的周期长度的重要因素。(5) 进一步确认了共面八面体离子间排斥及空位浓度、离子有序、磁性离子等因素是六方钙钛矿微波介电损耗的重要因素；通过熔盐法调控8层六方孪生型钙钛矿晶粒生长各向异性和谐振频率温度系，对六方钙钛矿温度系数调控提供了有意义的织构工程思路。(6)获得了介电常数在22-40，Qxf大于或接近30000GHz且Tf在+/-10 ppm/oC之内正负可调的复合立方和六方钙钛矿和金红石微波介质陶瓷材料。已在Chem. Mater.、J. Mater. Chem. C、Dalton Trans.等发表8篇论文，待发表论文2篇；获1项中国发明专利授权。项目培养了5名硕士研究生，负责人获得了国家优青，项目实施促进了西部地区高层次人才的培养。