Issue 5, 2022

Multi-scale chemo-mechanical evolution during crystallization of mixed conducting SrTi0.65Fe0.35O3−δ films and correlation to electrical conductivity

Abstract

Recent work has demonstrated a low-temperature route to fabricating mixed ionic/electronic conducting (MIEC) thin films with enhanced oxygen exchange kinetics by crystallizing amorphous-grown thin films under mild temperatures, eluding conditions for deleterious A-site cation surface segregation. Yet, the complex, multiscale chemical and structural changes during MIEC crystallization and their implications for the electrical properties remain relatively unexplored. In this work, micro-structural and atomic-scale structural and chemical changes in crystallizing SrTi0.65Fe0.35O3−δ thin films on insulating (0001)-oriented Al2O3 substrates are observed and correlated to changes in the in-plane electrical conductivity, measured in situ by ac impedance spectroscopy. Synchrotron X-ray absorption spectroscopy at the Fe and Ti K-edges gives direct evidence of oxidation occurring with the onset of crystallization and insight into the atomic-scale structural changes driven by the chemical changes. The observed oxidation, increase in B-site polyhedra symmetry, and alignment of neighboring B-site cation coordination units demonstrate increases in both hole concentration and mobility, thus underpinning the measured increase of in-plane conductivity by over two orders of magnitude during crystallization. High resolution transmission electron microscopy and spectroscopy of films at various degrees of crystallinity reveal compositional uniformity with extensive nano-porosity in the crystallized films, consistent with solid phase contraction expected from both oxidation and crystallization. We suggest that this chemo-mechanically driven dynamic nano-structuring is an additional contributor to the observed electrical behavior. By the point that the films become ∼60% crystalline (according to X-ray diffraction), the conductivity reaches the value of dense, fully crystalline films. Given the resulting high electronic conductivity, this low-temperature processing route leading to semi-crystalline hierarchical films exhibits promise for developing high performance MIECs for low-to-intermediate temperature applications.

Graphical abstract: Multi-scale chemo-mechanical evolution during crystallization of mixed conducting SrTi0.65Fe0.35O3−δ films and correlation to electrical conductivity

Article information

Article type
Paper
Submitted
30 jul 2021
Accepted
19 okt 2021
First published
20 okt 2021

J. Mater. Chem. A, 2022,10, 2421-2433

Author version available

Multi-scale chemo-mechanical evolution during crystallization of mixed conducting SrTi0.65Fe0.35O3−δ films and correlation to electrical conductivity

H. B. Buckner, Q. Ma, J. Simpson-Gomez, E. J. Skiba and N. H. Perry, J. Mater. Chem. A, 2022, 10, 2421 DOI: 10.1039/D1TA06455J

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