Perovskite structures based on the formulation Ca_1-xLa_2x/3TiO₃ 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-xLa_2x/3TiO₃, 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 La³⁺ / Ca²⁺ 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 La³⁺ 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 / La³⁺, 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.

Acknowledgements

Funding is acknowledged from the UK’s Engineering and Physical Sciences Research Council (EPSRC) under grants EP/K029770/1 and EP/L005581/1. SuperSTEM is the UK National Facility for Aberration-Corrected STEM, supported by EPSRC.

References

[1] V Vashook et al., J. Alloys Compd. 2003, 354 (1-2), 13–23.

[2] I-S Kim et al., Mater. Res. Bull. 1995, 30 (3), 307–316.

[3] EKH Salje et al., ChemPhysChem 2010, 11 (5), 940–950.

[4] E Salje et al., Phase Transit. 2009, 82 (6), 452–469.

[5] Z Zhang et al., J. Solid State Chem. 2007, 180 (3), 1083–1092.

[6] AE Ringwood et al., Nature 1979, 278 (5701), 219–223.

Figures:

Figure 1- CLTO x = 0.2 specimen viewed along [010]p zone axis: (a) HAADF image, (b) HAADF during SI acquisition image along with elemental intensity maps.

Figure 2- CLTO x = 0.9 specimen along [100]p zone axis, (a) HAADF image from an area with long-range ordering double-perovskite structure, (b) HAADF image acquired during SI and elemental maps.

Figure 3- CLTO x = 0.9 specimen (same particle and zone axis as Figure 2), (a) HAADF image showing a twin boundary. The inset in (a) shows the region with twins at low magnification. (b) HAADF image within the same particle showing another type of boundary between two double perovskite domains.

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EMC Abstracts - https://emc-proceedings.com/abstract/characterisation-of-ordering-in-the-a-site-deficient-perovskite-ca1-xla2x3tio3-using-stem-eels/