Abstract
The vacancy ordering behavior of an A-site deficient perovskite system, Ca1-xLa2x/3TiO3, was studied 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), adopting Pbnm symmetry, there was no indication of long-range ordering. 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 displacement 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. 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, these highly ordered domains were separated by twin boundaries, with 90° or 180° lattice rotations across boundaries. The occurrence and characteristics of these ordered structures are discussed and compared with similar perovskite systems.
Highlights
Perovskite structures based on the formulation Ca1−xLa2x/3TiO3 have been studied extensively for use across a wide range of possible applications, such as anodes for solid oxide fuel cells (SOFCs),[1] as dielectric resonators,[2] as high-density memory storage devices,[3] as host matrices for inert matrix nuclear fuels, and as containment media for high-level nuclear waste forms.[4−6] Understanding the crystallographic ordering at the atomic scale and the nature of defects is essential in order to successfully utilize this class of perovskites across the multitude of applications
Article the La−the B-site cation (Ti)−O system, with Ti in the +4 oxidation state, was first reported by Kestigian and Ward.10 [With Ti in the purely +3 oxidation state, LaTiO3 is formed with Pbnm symmetry, with a = 5.63 Å, b = 5.61 Å, and c = 7.94 Å.11,12] Abe and Uchino were the first to synthesize La2/3TiO3−λ as a single phase, with the suggested ionic arrangement of La23/+3Ti14−+ 2λTi23λ+Ti32−−λ and demonstrating with XRD that with a small amount of O deficiency (λ = 0.007) the {002} line is split into three peaks
In the Pbnm case, identification of such boundaries was not trivial and required access to both high-angle annular dark-field (HAADF) and BF signals, whereas, for Cmmm symmetry, discussed below, the tilt boundary was readily evident in the HAADF signal, as is highlighted by change in the La3+/V2C−a ordering direction
Summary
Perovskite structures based on the formulation Ca1−xLa2x/3TiO3 have been studied extensively for use across a wide range of possible applications, such as anodes for solid oxide fuel cells (SOFCs),[1] as dielectric resonators,[2] as high-density memory storage devices,[3] as host matrices for inert matrix nuclear fuels, and as containment media for high-level nuclear waste forms.[4−6] Understanding the crystallographic ordering at the atomic scale and the nature of defects is essential in order to successfully utilize this class of perovskites across the multitude of applications. Article the La−Ti−O system, with Ti in the +4 oxidation state, was first reported by Kestigian and Ward.10 [With Ti in the purely +3 oxidation state, LaTiO3 is formed with Pbnm symmetry, with a = 5.63 Å, b = 5.61 Å, and c = 7.94 Å.11,12] Abe and Uchino were the first to synthesize La2/3TiO3−λ as a single phase, with the suggested ionic arrangement of La23/+3Ti14−+ 2λTi23λ+Ti32−−λ and demonstrating with XRD that with a small amount of O deficiency (λ = 0.007) the {002} line is split into three peaks This is interpreted as doubling of the unit cell along the c-axis, resulting from a more facile formation of the perovskite phase with vacancy ordering on the A-sites.[13] Critical in attracting attention to this system was the report of high ionic conductivity in Li0.34La0.51TiO2.94, due to a large number of available vacant sites to diffusing Li ions.[14] Other studies report that pure La2/3TiO3 ( written as La2Ti3O9) could not be stabilized,[15,16] or have indexed this structure as tetragonal (I4/mmm) with a = b = 3.856 Å, and c = 24.6 Å.17. These atomic scale features are inaccessible in volume-averaged X-ray or neutron diffraction experiments performed in the past,[19] upon which we are building up the present investigation
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