Local Observations of Defects and Disorder in Ferroelectrics and Their Impact on Phase Transition Behavior

Local defects often play a decisive role in functional behavior of materials, affecting both baseline properties and dynamic response. Quantitative scanning transmission electron microscopy (STEM) has been an indispensable tool in studies of such defects, providing information on local distortions and chemical composition down to atomic scale. Advanced data analysis methods have further expanded the capabilities of local studies with STEM to analyses of spatial correlations and other statistical properties. Coupled with in-depth functional characterization with scanning probe microscopy (SPM), we can use this approach to understand the impact of the defects on global material behavior.<br/>In perovskite BaTiO₃thin films, we were able to connect enhanced electromechanical response and unusual dynamic behavior to the presence of oxygen vacancies. Fully relaxed 80 nm BaTiO₃ thin film was grown on SrRuO₃ // SrTiO₃ (001) using pulsed laser deposition. STEM imaging has identified areas of modulated lattice spacings within the film, as well as characteristic changes in O K EELS edge indicative of the presence of oxygen vacancies. Dynamic variable-temperature SPM studies combined with an array of theoretical methods point to a created internal electric field that further enhances the intrinsic electrostriction as the mechanism for enhanced electromechanical response [1].<br/>Recently, layered ferroelectrics of the thiophosphate family, such as Copper Indium Thiophosphate (CIPS), have garnered attention of researchers due to high potential for tunability and therefore a wealth of possible phase transitions. Our recent work has identified that layer stacking in these materials exhibits a wide variety of defects that can be expected to affect polarization and switching behavior. Defects of different polarity and with different number of nodes are observed, interspersed by relatively unstrained areas of 5-15 nm. We have also investigated spatial variability and statistical distribution of the interlayer spacings over the entire dataset of cross-sectional STEM images. Using k-means clustering algorithm, we were able to identify three types of clusters with distinct average spacing. Clusters appear to form nm-scale domains; their boundaries are not associated with defects [2]. Interestingly, theoretical studies of this system have predicted three possible ferroelectric states with different interlayer spacings. [3]. We are also exploring 3D ferroelectrics from the thiophosphate family such as Sn₂P₂S_6.This material has a complex monoclinic structure and its defect behavior is not well understood.<br/>The work was supported by the U.S. Department of Energy, Office of Science, Materials Sciences and Engineering Division. The work was conducted in part at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.<br/> <br/>[1] K.P. Kelley, A.N. Morozovska, E. A. Eliseev, V. Sharma, D.E. Yilmaz, A.C.T. van Duin, P. Ganesh, A. Borisevich, S. Jesse, P. Maksymovych, N. Balke, S.V. Kalinin, R.K. Vasudevan, Adv. Mater., DOI 10.1002/adma.202106426 (2021)<br/>[2] R.K. Vasudevan, S.M. Neumayer, M.A. Susner, M.A. McGuire, S.T. Pantelides, P. Maksymovych, D.N. Leonard, N.Balke, A.Y. Borisevich ACS Appl. Nano Mater., 8 8161 (2020)<br/>[3] J.A. Brehm, S.M. Neumayer, L. Tao, A. O’Hara, M. Chyasnavichus, M.A. Susner, M.A. McGuire, S.V. Kalinin, S. Jesse, P. Ganesh, S. T. Pantelides, P. Maksymovych & N. Balke, Nat. Mater., 19, 43 (2020).