KTN及其表面界面中氧空位与掺杂离子的第一性原理研究

In Nature,Y.Saito et al.report that most of the piezoelectric properties of a KNbO3-based lead-free piezoelectric ceramic are comparable to those of the PZT. Therefore there has been a flurry of studies on the topic of lead-free piezoceramics. Potassium tantalate niobate (KTN) is KNbO3-based ferroelectric, which has drawn much attention recently, and doping KTNs have considerable technological applications in electro-optics, electroholography, transducers, microwave tunable phase shifter, upconversion laser, etc. For instance, in Nature Photonics, A.J.Agranat et al. indicate that copper-doped KTN:Li leads to a completely new paradigm for ultraresolved imaging and microscopy. Point defects have traditionally been considered to play an essential role in device design, and first-principles have been particularly helpful and highly efficient in elucidating the issues. In consequence, we intend to present a first-principles study of defects in KTN by means of hybrid density functional. The KTN supercells are optimized to obtain stable geometry structures, and then the electronic structure, optical properties and piezoelectricity are calculated using the optimized structure. We will pay particular attention to formation energy and clustering of oxygen vacancy. The piezoelectric properties of KTN are often controlled by doping ions, which will be explained through our calculations. We also investigate the complexes formed by doping ions and oxygen vacancy. The formation of two dimension electron gases (2DEGs) at KTN surfaces and interfaces will be explored, and we will illustrate the microscopic mechanisms of interfacial conductivity in KTN ultrathin films. The above calculations will cause the vital findings of new phenomena and effects, which will expand new directions for KTN piezoceramics and serve as the theoretical predications for lead-free piezoelectric devices. As a result, our project has the important scientific meaning and practical significance.

Y.Saito在Nature 上报道 KNbO3 基压电陶瓷所有性能几乎都可以与PZT媲美，由此掀起无铅压电研究热潮。钽铌酸钾(KTN)是研究最多的 KNbO3 基材料，而且掺杂KTN在电光、电控全息、传感器、移相器、上转换激光等器件应用方面潜力巨大。众所周知点缺陷对器件设计至关重要，且第一性原理预测高效省时。因此本项目拟利用杂化密度泛函进行KTN 点缺陷第一性原理研究。探寻缺陷晶格构型并计算电子结构、光学性质和压电特性。给出氧空位形成能和能级位置，重点研究氧空位团簇。理论解释掺杂离子对KTN 压电性能的调控作用，并研究掺杂离子与氧空位联合作用。探索 KTN 表面界面二维电子气形成过程，给出超薄薄膜界面导电的微观机制。本项目发现的新现象与新效应，将进一步扩展无铅 KTN 的研究领域，因此具有重要科学意义，为KTN无铅压电器件应用提供理论基础，具有重大实用价值。

钽铌酸钾(KTN)是广泛研究的 KNbO3 基晶体， 因为它在技术应用方面潜力巨大，被认为是最有前途的无铅压电材料。而氧空位、离子掺杂以及表面界面效应对KTN的物理性能有着重要影响，这就为器件开发提供了便利。基于此，本项目对KTN展开如下研究。（1）晶体：系统地研究了含中性与带电氧空位KTN晶体的几何结构和电子性质。利用局域密度近似(LDA)、广义梯度近似(GGA)方法(PBE)、修正GGA方法(WC) 进行几何结构优化。相对于广泛应用的PBE和LDA，WC非常完美地改进了材料的几何结构预测。带电氧空位是最稳定的状态，而中性氧空位只是过渡态；然后进行KTN的Li、Na、H、Cu和Zn离子的掺杂研究，发现离子掺杂对KTN的几何结构影响很小。但是H、Cu和Zn离子明显改变了KTN晶体的能带带隙。KTN:Zn的导电、透光性能很好，在透明电极方面有很大应用前景；KTN固溶体的几何结构和光学性能对高压比较敏感。（2）表面界面：近年来表面与小分子吸附物的相互作用成为表面研究的前沿热点。选取常见吸附物水和CO2作为对象，研究它们对掺钠的KTN（KNTN）表面形态、电子结构和光学性质的影响。发现水分子和CO2分子吸附确实对KNTN的物理特性产生重要影响；掺La的KNbO3薄膜和掺Y的KNbO3薄膜界面均可以产生二维电子气。与掺Y的SrTiO3超晶格是绝缘体的结论截然不同，说明KNbO3的铁电极化导致二维电子气出现；此外，还通过结构搜索软件USPEX与第一性原理软件VASP的联合使用，对Li-N二元体系的高压性能进行了深入探索，发现一系列新材料和新结构。该工作为以后Li-N体系实验研究以及储能材料的探索提供理论基础。