Oxygen-deficient perovskites: linking structure, energetics and ion transport

Abstract

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.

Structural phases driven by oxygen vacancies at the La0.7Sr0.3MnO3/SrTiO3 hetero-interface

“…This amount of oxygen vacancies is also consistent with the oxygen fine structure, which is similar to the one reported by Yao et al 23 This large number of oxygen vacancies breaks the MnO 6 octahedron, and hence, destroys the perovskite structure. 24 A known cation non-stoichiometric phase is the Ruddlesden-Popper structure. 25 However, our high-resolution STEM-HAADF data are not consistent with the clear signature of such a phase.…”

Redox chemistry of the novel fast oxide ion conductor CeNbO4+d determined through an in-situ spectroscopic technique

“…More recently, materials that contain oxygen interstitials have shown promise as both electrolytes [4][5][6] and electrodes [6][7][8][9], suggesting that the structural complexity of these materials does not preclude fast ion transport and hence the need for three dimensional isotropic conductors could be reduced. Building on this concept, CeNbO 4 + d materials have been investigated and identified as fast oxide ion conductors at relatively modest temperatures [10][11][12].…”

Photoluminescence properties of Ba3B′B″2O9 (B′=Mg, Co or Zn and B″=Nb or Ta) ceramics with perovskite structure

“…According to their data the tolerance factor for the perovskites studied here is larger than 1 and it is on the verge of stability for the perovskite structure. In addition to a broad range of cation substitutions, the perovskite structure is tolerant to both cation and anion vacancies [25,26]. Deviation from stoichiometry on either A or B atomic sites would introduce atomic vacancies, thereby relaxing the structure, and can result in the formation of secondary phases.…”