Characterisation of ordering in the A‐site deficient perovskite Ca1‐xLa2x/3TiO3 using STEM / EELS

Abstract

Perovskite structures based on the formulation Ca 1‐x La 2x/3 TiO 3 have been extensively studied across a wide range of possible applications, such as anodes for solid oxide fuel cells [1], dielectric resonators [2], high density memory storage devices [4], and as host matrices for inert matrix nuclear fuels and as containment media for high‐level nuclear waste forms [5–6]. Understanding the crystallographic ordering at the atomic scale and the nature of present defects is essential in order to successfully utilize this class of perovskites across the multitude of applications. We have studied the vacancy ordering behaviour of the A‐site deficient perovskite system, Ca 1‐x La 2x/3 TiO 3 , using atomic resolution scanning transmission electron microscopy (STEM) in conjunction with electron energy‐loss spectroscopy (EELS), with the aim of determining the role of A‐site composition changes. At low La content (x = 0.2), this system adopts Pbnm symmetry, with no indication of long‐range ordering. Atomic resolution high‐angle annular dark‐field (HAADF) STEM image, acquired along [010] p pseudo‐cubic zone axis, Figure 1(a), shows varying intensities indicating changes in La 3+ / Ca 2+ ratio across the field of view. Elemental intensity maps from characteristic core‐loss edges, shown in Figure 1(b), demonstrate anti‐correlated Ca versus La intensities. Domains, with clear boundaries, were observed in bright‐field (BF) imaging, but were not immediately visible in the corresponding high‐angle annular dark‐field (HAADF) image. These boundaries, with the aid of polarisation maps from A‐site cations in the HAADF signal, are shown to be tilt boundaries. At the La‐rich end of the composition (x = 0.9), adopting Cmmm symmetry, long‐range ordering of vacancies and La 3+ ions was observed, with alternating La‐rich and La‐poor layers on (001) p planes, creating a double perovskite lattice along the c axis. One such ordered region is imaged in Figure 2(a) along the [100] p zone axis, in conjunction with EELS elemental maps shown in panel (b), showing the alternating La‐rich and La‐poor atomic planes. These highly‐ordered domains can be found isolated within a random distribution of vacancies / La 3+ , or within a large population, encompassing a large volume. In regions with a high number density of double perovskite domains, e.g. the area imaged in Figure 3, these highly‐ordered domains were separated by twin boundaries, with 90° or 180° lattice rotations across boundaries, as shown in panels (a) and (b), respectively. The occurrence and characteristics of these ordered structures will be discussed and compared with similar perovskite systems.