Investigating the Effect of Atmosphere on Domains in BaTiO₃ Using In Situ TEM

Ferroelectrics possess a spontaneous polarisation that can be switched with an external field, a property that has led to their incorporation in many modern device technologies. The characteristic marker of a ferroelectric material is the formation of regions of uniform polarisation, called domains, below the material’s Curie temperature (T_C). These domains are physically separated by domain walls, which are mobile entities that respond to external stimuli such as heat, electric field, and strain [1]. In recent years, the chemistry of ferroelectric surfaces has become a topic of great interest, because the domain’s dipole orientations can be manipulated to tailor the surface reactivity for specific applications [2]. For example, the polarity of ferroelectric domains has been shown to affect the physisorption of molecules on the ferroelectric surface [3], the catalytic activity [4] and the equilibrium stoichiometry [5]. Conversely, the chemical environment plays a dominant role in ferroelectric behaviour [6], by determining ionic compensation at the surface in terms of vacancies, surface dipoles and electron density [7]. Theoretical studies show that the domain structures of thin films can be altered with an oxidising atmosphere [8], which has been verified experimentally mainly using <i>in situ</i> synchrotron X-ray scattering to study the effects of p(O₂) in PbTiO₃films [9-11]. However, the literature is still lacking an experimental study that directly investigates the dynamical response of domains to different chemical environments.<br/><br/><i>In situ</i> transmission electron microscopy (TEM) is a powerful tool for studying the dynamic response of ferroelectric materials. With recent advancements to dedicated <i>in situ </i>holders, the immediate response of a ferroelectric to external stimuli can be explored in real-time, with high spatial and temporal resolution [12]. In this work, we use a dedicated <i>in situ</i> heating gas holder to map the thermodynamic response of ferroelectric-ferroelastic domains in free-standing single crystal &lt;100&gt;_pc BaTiO₃ lamellae, using scanning transmission electron microscopy (STEM) techniques. We investigate the effect that chemical environments have on the domain behaviour, as the sample cools down from T_C, providing direct evidence that a vacuum, inert, or oxygen-rich environment will affect the domain microstructure and subsequent domain behaviours. This work aims to further our understanding of the interaction between chemical environment and ferroelectric polarity and appeals to those interested in creating new devices which exploit ferroelectric surfaces, for example, chemical sensors or catalysts.<br/><br/>[1] G. Catalan et al., Rev. Mod. Phys., 84, 2012.<br/>[2] D. Li et al., Nat. Mater., 7, 2008.<br/>[3] Y. Yun et al., J. Am. Chem. Soc., 219, 2007.<br/>[4] A. Kolpak., Phys. Rev. Lett., 98, 2007.<br/>[5] S. Levchenko et al., Phys. Rev Lett., 100, 2008.<br/>[6] K. Garrity et al., Adv. Mater., 22, 2010.<br/>[7] A. Kakekhani et al., Surf. Sci., 650, 2016.<br/>[8] D. Ma et al., Appl. Phys. Lett., 99, 2011.<br/>[9] R. Wang et al., Phys. Rev. Lett., 102, 2009.<br/>[10] M. Highland et al., Phys. Rev. Lett., 105, 2010.<br/>[11] M. Highland et al., Phys. Rev. Lett., 107, 2011.<br/>[12] L. Li et al., Rep. Prog. Phys., 82, 2019.