Atomic Resolution Imaging of Nanoscale Chemical Expansion in Pr_xCe_1–xO_2−δ during In Situ Heating

Abstract

Thin film nonstoichiometric oxides enable many high-temperature applications including solid oxide fuel cells, actuators, and catalysis. Large concentrations of point defects (particularly, oxygen vacancies) enable fast ionic conductivity or gas exchange kinetics in these materials but also manifest as coupling between lattice volume and chemical composition. This chemical expansion may be either detrimental or useful, especially in thin film devices that may exhibit enhanced performance through strain engineering or decreased operating temperatures. However, thin film nonstoichiometric oxides can differ from bulk counterparts in terms of operando defect concentrations, transport properties, and mechanical properties. Here, we present an in situ investigation of atomic-scale chemical expansion in PrCeO (PCO), a mixed ionic-electronic conducting oxide relevant to electrochemical energy conversion and high-temperature actuation. Through a combination of electron energy loss spectroscopy and transmission electron microscopy with in situ heating, we characterized chemical strains and changes in oxidation state in cross sections of PCO films grown on yttria-stabilized zirconia (YSZ) at temperatures reaching 650 °C. We quantified, both statically and dynamically, the nanoscale chemical expansion induced by changes in PCO redox state as a function of position and direction relative to the film-substrate interface. Additionally, we observed dislocations at the film-substrate interface, as well as reduced cation localization to threading defects within PCO films. These results illustrate several key aspects of atomic-scale structure and mechanical deformation in nonstoichiometric oxide films that clarify distinctions between films and bulk counterparts and that hold several implications for operando chemical expansion or "breathing" of such oxide films.

Chemical lattice strain in nonstoichiometric oxides: an overview

“…171 This was explained by the chemical expansion due to the defect formation relieving the stress, 172 and is closely related to the anisotropic – in directions parallel and normal to the interface – chemical strain observed with in situ TEM, scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS) and optical strain measurements in Ce 1– x Pr x O 2– δ thin films on a YSZ substrate. 172,173 The anisotropic character of chemical expansion was also found in perovskite-based epitaxial thin films using XRD. 174 Both experimental and computational studies 175,176 showed that the nature of the substrate determines the presence of compressive or tensile in-plane strain in perovskite La 1– x Sr x CoO 3− δ films, which impacts the oxygen vacancy concentration therein, influencing such properties as the oxygen evolution reaction (OER) activity, 175 oxygen surface exchange and diffusion.…”

In Situ Method Correlating Raman Vibrational Characteristics to Chemical Expansion via Oxygen Nonstoichiometry of Perovskite Thin Films

“…This leads to an overestimation of the expansivity, since the βC ≈ 3αC holds only in cases of isotropic expansion. The films can, however, be partially or fully compensated for in the out of plane expansion, [13] Figure 6b.…”

“…This requires control over the thin film structure-property relationships such as fast reaction kinetics, ionic and electronic conductivities, [1,9] stoichiometry, [10] optical transmittance [11] and/or mechanical properties. [12,13] The small sample volumes, large interfacial areas and substrate confinement of the thin films often result in substantially altered properties when compared to macroscopic bulk Sr(Ti,Fe)O3-y specimens, processed by powder pressing and sintering at higher temperatures. [14,15] For example, when processed as a thin film, Sr(Ti,Fe)O3-y reveals a higher optical transmittance [11] and lower conductivity, [9] e.g.…”

In situ and operando characterisation techniques for solid oxide electrochemical cells: recent advances

“…© 2014, The Royal Society of Chemistry. (d) In situ phase contrast TEM images of PrxCe1-xO2-YSZ interface at 300°C and 650°C (upper panels) and temperature evolution of interfacial fringe width, W and periodicity, 𝜆 (lower panel) [20]. © 2018, American Chemical Society.…”

“…A beautiful example of the use of in situ TEM combined with scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) was reported for the study PrxCe1−xO2−δ (PCO) deposited as a thin film on YSZ [20]. By performing a series of consecutive experiments at three different temperatures (room temperature, 300 °C and 650°C) in oxidizing and reducing conditions, the authors were able to characterize chemical strains and changes in oxidation state in PCO lamella cross sections.…”