Transition-metal oxides often exhibit complex magnetic behavior due to the strong interplay between atomic-structure, electronic and magnetic degrees of freedom. Cobaltates, especially, exhibit complex behavior because of cobalt’s ability to adopt various valence and spin state configurations. The case of the oxygen-deficient perovskite Sr3YCo4O10+x (SYCO) has attracted considerable attention because of persisting uncertainties about its structure and the origin of the observed room temperature ferromagnetism. Here we report a combined investigation of SYCO using aberration-corrected scanning transmission electron microscopy and density functional theory calculations. Guided by theoretical results on Co-O distances projected on different planes, the atomic-scale images of several different orientations, especially of the fully oxygenated planes, allow the unambiguous extraction of the underlying structure. The calculated magnetic properties of the new structure are in excellent agreement with the experimental data.

The aluminate perovskites YAlO 3 have been identified as potential scintillator materials due to their high light output and short decay time. However, the performance of YAlO 3 is still low. Despite the promising optical properties of YAlO 3 , it has been proposed that their potential as scintillators has not been fully realized [1]. Recently, the focus has been on carrier traps as the limiting factor in the performance of YAlO 3 , but the nature of these traps has not been identified. An investigation using firstprinciples calculations suggested that anti-site defects in which B-site Aluminum and A-site Yttrium are interchanged have a lower energy than Shottky or Frenkel defects [2,3]. Despite numerous studies, there is still no direct evidence to show the existence of anti-site defects.We have used a fifth-order aberration-corrected Nion UltraSTEM200 scanning transmission electron microscope to investigate the local atomic structure of YAlO 3 doped with 0.1% Ce. YAlO 3 :Ce 3+ are expected to have both types of cation anti-site defects (Al on the Y site and Y on the Al site). Highangle annular dark-field (HAADF) imaging was used to detect the presence of anti-site defects in YAlO 3 :Ce 3+ .A typical HAADF image from YAlO 3 :Ce 3+ is seen in Figure1b along with an image simulation based on the multi-slice method. The image was obtained at 200 kV using a probe convergence angle of 30 mrad. We can see some B-sites Al+O columns with brighter intensity as indicated by white circles and a blue arrow in Figure1b. And we also can see an A-site column with reduced intensity as indicated by a red arrow in Figure1b and as shown line profile in Figure 2. To interpret these intensity variations, HAADF image simulations using a structure with between 0-10% Y on the Al B-site and 0-10% Al on the A-site Y were performed. As shown in Figure 2, the experimental and simulated intensities at A-sites and Bsites with and without 10% anti-site defects are in good agreement. Additional details on these YAlO 3 :Ce crystals provided by DFT calculations and optical property measurements will be discussed.

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Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.