School of Electronic and Information Engineering and State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China; Peter Grünberg Institute and Ernst Ruska-Center for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich, Jülich 52425, Germany.
Peter Grünberg Institute and Ernst Ruska-Center for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich, Jülich 52425, Germany.
Ultramicroscopy. 2018 Sep;192:57-68. doi: 10.1016/j.ultramic.2018.05.009. Epub 2018 May 30.
Pyrochlores characterized by the chemical formula ABO form an extended class of materials with interesting physical and chemical properties. The compound BiZnNbO is prototypical. Its excellent dielectric properties make it attractive, e.g. for capacitors, tunable microwave devices and electric-energy storage equipment. BiZnNbO shows an intriguing frequency-dispersive dielectric relaxation at 50 K ≤ T ≤ 250 K, which has been studied intensively but is still not fully understood. In this first study on a pyrochlore by atomic-resolution transmission electron microscopy we observe the Bi atoms on A sites since, due to their low nuclear charge, the contribution of Zn atoms to the contrast of these sites is negligible. We find in our [1¯00]and [112] oriented images that the position of the atomic intensity maxima do not coincide with the projected Wyckoff positions of the basic pyrochlore lattice. This supplies atomic-scale evidence for displacive disorder on split A-type sites. The Bi atoms are sessile, only occasionally we observe in time sequences of images jumps between individual split-site positions. The apertaining jump rate of the order of 0.1-1 Hz is by ten orders of magnitude lower than the values derived in the literature from Arrhenius plots of the low-temperature dielectric relaxation data. It is argued that these jumps are radiation induced. Therefore our observations are ruling out a contribution of Bi-atom jumps to low-temperature dielectric A sites-related relaxation. It is suggested that this relaxation is mediated by jumps of Zn atoms.
钙钛矿结构的化学通式为 ABO，具有有趣的物理和化学性质，是一个扩展的材料家族。化合物 BiZnNbO 是典型的代表。其优异的介电性能使其具有吸引力，例如用于电容器、可调微波器件和电能存储设备。BiZnNbO 在 50 K ≤ T ≤ 250 K 的温度范围内表现出有趣的频率色散介电弛豫，已经得到了深入研究，但仍未完全理解。在通过原子分辨透射电子显微镜对钙钛矿的首次研究中，我们观察到 A 位上的 Bi 原子，这是因为由于它们的核电荷较低，Zn 原子对这些位置的对比度贡献可以忽略不计。我们在 [1¯00] 和 [112] 取向的图像中发现，原子强度最大值的位置与基本钙钛矿晶格的投影威科夫位置不重合。这为分裂 A 型位的置换无序提供了原子尺度的证据。Bi 原子是不动的，我们只是偶尔在图像时间序列中观察到它们在单个分裂位之间的跳跃。相应的跳跃率约为 0.1-1 Hz，比文献中根据低温介电弛豫数据的 Arrhenius 图得出的值低十个数量级。据认为这些跳跃是辐射诱导的。因此，我们的观察结果排除了 Bi 原子跳跃对低温介电 A 位相关弛豫的贡献。有人提出，这种弛豫是由 Zn 原子的跳跃介导的。
