Thermally Driven Domains in BaTiO₃—An In Situ Study

Ferroelectric materials are widely used in sensors and actuators, due to their unique properties that can be controlled with external fields [1]. The characteristic marker of a ferroelectric is the formation of domains below a critical temperature (T_C) - a spontaneous response to the change in the directional symmetry of the unit cell. In BaTiO₃(BTO), both ferroelectric and ferroelastic domains form below T_C (∼130°C) into energetically favourable patterns, based on a complex interplay between the system’s spontaneous strain and polarisation. Domains are physically separated by domain walls (DWs), which are moveable entities that respond to external stimuli such as heat, bias and mechanical loading. The overall structure and mobility of domains heavily influences the macroscopic properties of the material [2], therefore the study and control of domain behaviour under external fields is of great technological importance. While the observation of ferroelectric domain nucleation and switching as a function of external biasing has been well documented [3-5], the domain formation (and associated dynamics) as a function of temperature has been studied to a much lesser extent [6-10], with still many aspects of this behaviour not yet fully understood.<br/>New frontiers of <i>in situ</i> electron microscopy have seen the development of a variety of in situ holders and fast acquisition cameras, which have become an ideal tool for probing the dynamics of a material. The work here presented utilises an in-situ heating holder to map the local dynamic response of 90° domains in single crystal BTO lamella, by STEM techniques. The results here obtained, revealed an interesting and unique insight into the physical phenomena of thermally activated domain dynamics near and below T_C. Heat cycles (RT to 200°C) revealed that, as expected, the material accepts a thermal expansion, and loss of polarisation, as it moves closer to the cubic paraelectric state. However, we observe (i) T_C is pushed to higher temperatures to that of the bulk (~ 150°), in agreement with previous reports in thin films [11], (ii) the domain widths were found to be larger than expected according to Roytburd’s treatment of Kittel’s law [12] and (iii) the domains showed significant mobility at lower temperature. In addition, the domain structure did not fully reorganise after cooling from T_C. This has been rationalised as the presence of pinning sites in the parent crystal. Given that the domains here observed are 90° ferroelastic domains, strain mapping techniques were employed to map the local strain fields of the domain variants as a function of temperature, in order to gain insight into the local elastic competition between domain variants. Furthermore, with the rationale of removing potential local defects in the crystal, the sample was ‘hard reset’ by heating to 800°C. This resulted in the loss of the initial memory, allowing the domains to fully reorganise and span across the volume of the lamella, and the domain’s width to decrease. In subsequent heat cycles, the mobility was significantly enhanced between 20°C – 50°C, showing in-between metastable domain configurations and greater competition between the 90° domains. Possible mechanisms for this behaviour are explored, including defect diffusion, oxygen vacancies, gallium expulsion and recrystallisation.<br/>[1] M. Acosta et al., Appl. Phys. Rev., 4, 2017.<br/>[2] Y. Ivry et al., Phys. Rev. B., 86, 2012.<br/>[3] L. Li et al., Rep. Prog. Phys., 82. 2019.<br/>[4] C. Nelson et al., Science, 334, 2011.<br/>[5] P. Gao et al., Nat. Commun., 2, 2011.<br/>[6] A. Everhardt et al., Phys. Rev. Lett., 123, 2019.<br/>[7] M. Varlioglu et al., J. Appl. Phys., 107, 2010.<br/>[8] L. McGilly et al., Phys. Rev. B., 85, 2012.<br/>[9] M. Barzilay et al., ACS Appl. Electron. Mater., 11, 2019.<br/>[10] N. Orlovskaya et al., Act. Mater., 51, 2003.<br/>[11] K. Choi et al., Science, 306, 2004.<br/>[12] A. Schilling et al., Phys. Rev. B., 74, 2006.