Maria Varela
Facultad de CC. Fisicas & Instituto Pluridisciplinar, Universidad Complutense de Madrid 28040 Madrid (Spain).
The properties of complex oxides are extremely sensitive to minor changes in doping. For example, in manganites it is still not clear whether chemical order (or disorder) is in any way connected to phenomena such as double exchange, electronic phase separation or charge ordering. By means of aberration corrected scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) combined with simulations it is possible to carry out atomic resolution studies of these systems, such as the colossal magnetoresistant manganite La2-2xSr1+2xMn2O7 [1-3]. For this material we find a significant degree of long-range chemical ordering for a number of values of doping (x), which increases in the antiferromagnet- charge ordered range. However, the degree of ordering is never complete. Our results show that chemical ordering over distinct crystallographic sites is not needed for electronic ordering phenomena to appear in manganites, and other explanations, including electronic degrees of freedom, play a determining role when trying to explain the complex electronic behavior of manganite oxides. In fact, STEM-EELS studies of this type provide the key to harness other oxide systems where minor changes in local composition may have a direct effect on macroscopic properties. An example can be found in ionic conductors such as those in polycrystalline solid electrolytes. They are the main component in solid-state electrochemical devices, such as fuel cells and batteries but their performance is strongly limited by ion blocking at grain boundaries in the electrolytes. Understanding the chemical, structural and electronic properties of these materials at the atomic scale is the key to improve functional properties and the door to designing low power energy generation and storage devices [4,5]. Here, we will discuss a STEM-EELS study of a single grain boundary in a bicrystal of yttria (9% mol) stabilized zirconia, an emblematic oxide ionic conductor. From strain analysis and atomic resolution EEL spectrum images, we find a strong tendency to yttrium segregation at the expansive site of the grain boundary cores, doubling the bulk relative Y concentration. Contrary to previous reports [6], we detect a depletion of oxygen within a region of nanometric dimensions around the boundary, which may be explained by the presence of O vacancies [7]. We will discuss these findings in the light of density-functional theory calculations.
Acknowledgements: Work done in collaboration with Gabriel Sanchez-Santolino, J. Salafranca, M. A. Frechero, M. Rocci, Rainer Schmidt, M. R. Díaz-Guillén, O. J. Durá, A. Rivera-Calzada, R. Mishra, M. A. Roldan, M. P. Oxley, Q. A. Li, H. Zheng, K. E. Gray, J. F. Mitchell, Carlos Leon, S. T. Pantelides, Jacobo Santamaría and Stephen J. Pennycook. Research at UCM sponsored by Fundación BBVA and Spanish MINECO MAT2015-66888-C3-3-R. Research at Oak Ridge National Laboratory supported by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division, and through a user project supported by ORNL’s Shared Research Equipment (ShaRE) User Program, which is also sponsored by DOE-BES.
[1] Q. Li et al., Phys. Rev. Lett. 98, 167201 (2007).
[2] J.F. Mitchell t al., J. Phys. Chem. B 105, 10731-11052 (2001).
[3] M. A. Roldan et al., Microscopy & Microanalysis 20, 1791-1797 (2014).
[4] B. C. H. Steele & A. Heinzel, Nature 414, 345-352 (2001).
[5] A. S. Arico. et al., Nature Materials 4, 366-377 (2005).
[6] X. Guo and R. Waser, Prog. Mat. Sci, 51, 151-210 (2006).
[7] M. A. Frechero et al., Scientific Reports 5, 17229 (2015).
