Mapping cation‐vacancy ordering and oxygen octahedral distortions in A‐site deficient perovskites by monochromated core‐loss EELS

Abstract

A–site defficient perovskites are of particular interest for a range of engineering applications, such as ionic conductors, batteries and thermoelectric materials. The attraction of these systems lies in particular in their chemically stability over a wide range of compositions and operating conditions. Moreover, they can exhibit different degrees structural ordering, such as cation‐vacancy or oxygen‐octahedral‐tilt‐domain ordering [1,2] which can significantly affect their macroscopic physical properties. In this work we combine state‐of‐the‐art monochromated core loss electron energy loss spectroscopy (EELS) measurements with advanced image analysis to investigate the interplay between A‐site cation‐vacancy ordering and oxygen octahedral tilting domains in a series of Nd 2/3x TiO 3 double perovskites, exhibiting different degrees of structural ordering as a function of their heat treatment. [3,4] As a result they exhibit vastly different thermoelectric properties related to the presence of domain boundaries which can suppress the thermal conductivity due to increased phonon scattering. [5,6] High‐angle annular‐dark‐field (HAADF) imaging and large scale atomically‐resolved EELS maps were used to investigate the chemical ordering of cations on the so‐called A site of the perovskite structure (Figure 1), while pm‐precision annular bright field (ABF) imaging of these compounds reveals the presence of TiO 6 tilting domains, which we show can be correlated with variations in the A‐site occupancy (Figure 2). Furthermore, advanced image analysis of the electron micrographs was used to measure local distortions in the TiO 6 lattice These local distortions are further investigated using atomically‐resolved monochromated EELS measurements with an energy resolution better than 0.1eV, in a Nion UltraSTEM 100MC. In particular, measurements of the of the Ti L 2,3 edge (Figure 3), reveal local changes of the near‐edge fine structure, usually not discernible with conventional EELS measurements. Changes in the Ti L 2,3 pre‐peak intensity, as well as subtle local variations in the Ti L 3 e g /t g intensity ratios, clearly observable thanks the relatively high beam current still available at this energy resolution, are indicative of local distortions in the oxygen octahedral sublattice.