Stølen Svein, Bakken Egil, Mohn Chris E
Department of Chemistry and Centre for Materials Science and Nanotechnology, University of Oslo, Postbox 1033 Blindern, N0315 Oslo, Norway.
Phys Chem Chem Phys. 2006 Jan 28;8(4):429-47. doi: 10.1039/b512271f. Epub 2005 Nov 11.
The present review focuses on links between structure, energetics and ion transport in oxygen-deficient perovskite oxides, ABO(3-delta). The perfect long-range order, convenient for interpretations of the structure and properties of ordered materials, is evidently not present in disordered materials and highly defective perovskite oxides are spatially inhomogeneous on an intermediate length scale. Although this makes a fundamental description of these and other disordered materials very difficult, it is becoming increasingly clear that this complexity is often essential for the functional properties. In the present review we advocate a potential energy barrier description of the disordered state in which the possible local (or inherent) structures are seen to correspond to separate local minima on the potential energy surface. We interpret the average structure observed experimentally at any temperature as a time and spatial average of the different local structures which are energetically accessible. The local structure is largely affected by preferences for certain polyhedron coordinations and the oxidation state stability of the transition metals, and the strong long-range electrostatic interactions present in non-stoichiometric oxides imply that only a small fraction of the local energy minima on the potential energy surface are accessible at most temperatures. We will show that models neglecting the spatial inhomogeneity and thus the local structure serve as useful empirical tools for particular purposes, e.g. for understanding the main features of the complex redox properties that are so crucial for many applications of these oxides. The short-range order is on the other hand central for understanding ionic transport. Oxide ion transport involves the transformation of one energetically accessible local structure into another. Thus, strongly correlated transport mechanisms are expected; in addition to the movement of the oxygen ions giving rise to the transport, other ions are involved and even the A and B atoms move appreciably in a cooperative fashion along the transition path. Such strongly correlated or collective ionic migration mechanisms should be considered for fast oxide ion conductors in general and in particular for systems forming superstructures at low temperatures. Structural criteria for fast ion conduction are discussed. A high density of low-lying local energy minima is certainly a prerequisite and for perovskite-related A(2)B(2)O(5) oxides, those containing B atoms that have energetic preference for tetrahedral coordination geometry are especially promising.
本综述聚焦于缺氧钙钛矿氧化物ABO(3-δ)的结构、能量学与离子传输之间的联系。对于有序材料的结构和性质解释而言十分便利的完美长程有序，在无序材料中显然并不存在，且高度缺陷的钙钛矿氧化物在中等长度尺度上在空间上是不均匀的。尽管这使得对这些及其他无序材料进行基本描述极为困难，但越来越清楚的是，这种复杂性对于功能特性往往至关重要。在本综述中，我们提倡对无序状态进行势能垒描述，其中可能的局部（或固有）结构被视为对应于势能面上不同的局部极小值。我们将在任何温度下实验观测到的平均结构解释为不同能量可及的局部结构在时间和空间上的平均值。局部结构在很大程度上受特定多面体配位偏好以及过渡金属氧化态稳定性的影响，并且非化学计量氧化物中存在的强长程静电相互作用意味着在大多数温度下，势能面上只有一小部分局部能量极小值是可及的。我们将表明，忽略空间不均匀性进而忽略局部结构的模型可作为用于特定目的的有用经验工具，例如用于理解对这些氧化物的许多应用至关重要的复杂氧化还原性质的主要特征。另一方面，短程有序对于理解离子传输至关重要。氧离子传输涉及一种能量可及的局部结构向另一种结构的转变。因此，预计会有强相关的传输机制；除了产生传输的氧离子移动外，其他离子也会参与，甚至A和B原子会沿着转变路径以协同方式显著移动。一般而言，对于快速氧离子导体，尤其是对于在低温下形成超结构的体系，应考虑这种强相关或集体离子迁移机制。文中讨论了快速离子传导的结构标准。低能量局部极小值的高密度肯定是一个先决条件，对于与钙钛矿相关的A(2)B(2)O(5)氧化物，那些含有对四面体配位几何结构有能量偏好的B原子的氧化物尤其有前景。
