Direct Observation of Solid-Phase Crystallization of an Amorphous Complex Oxide in the Transmission Electron Microscope
Authors: Wardini Jenna L. , Harrington George F. , Kemp Dennis , De Souza Roger A. , Bowman William J 
Published: February 2024

Many complex metal oxides possess intricate structure-function relations, resulting in a wide range of tunable functional properties. The introduction of disorder through amorphization provides additional opportunities for tuning the properties, particularly in the area of electrochemical energy storage and conversion [1]. Consequently, amorphous and partly crystalline complex oxides have garnered growing interest in recent years, with two primary goals: to harness their unique functional properties or to employ them as a chemically-flexible, far-from-equilibrium, thin-film processing platform that offers novel synthesis opportunities [2] due to the minimal energy input required to initiate crystallization and other structural relaxation processes.
Relevant to both these aims, the processing-structure-function relationship for amorphous complex oxides is examined by in situ solid-phase crystallization (SPC) in the (scanning) transmission electron microscope ((S)TEM), focusing on perovskite-like (ABO3) mixed ion-electron conducting (MIEC) amorphous complex oxides which make promising low-temperature solid-oxide cell (SOC) electrodes [3] due to the widely observed disorder-mediated oxygen surface-exchange [4]. Key aspects of MIEC amorphous complex oxide processing, such as the evolution of structure and chemistry with temperature and the corresponding effect on conductivity are detailed in the amorphous La0.8Sr0.2MnO3-δ (LSM) system. Further, approaches to direct amorphous complex oxide processing towards desirable synthesis products based on these insights are discussed.
Here, we elucidate down to the atomic scale the development of structural characteristics such as porosity, crystallized fraction, crystallite tension, size, curvature, and surface termination, with the aim to establish approaches to tune surface functionality in partly crystalline MIECs by heating or electron beam irradiation [5]. In situ electrical measurements are used to correlate these changes with electrical behavior, revealing that the onset of crystallization is correlated with an increase in electronic conductivity by small polaron hopping, attributed to oxygen-metal-oxygen bond alignment. Further, in the absence of epitaxial constraints, an immiscibility-driven self-organization of amorphous heterointerfacial structures manifests by spinodal decomposition, that is likely to be broadly relevant in low-temperature processing of amorphous complex oxides [6]. Thus, processing from an amorphous complex oxide precursor could be a method to establish uniformly-dispersed nanoscale heterointerfaces of tunable characteristic length scale throughout a thin film volume, while also enabling direct tuning of the porous mesostructure by electron beam irradiation. Overall, case studies of amorphous complex oxide crystallization are limited to relatively few chemical systems and processing conditions. Thus, extending this body of work to explore a broader processing space, and extracting fundamental insights to establish process-structure-function relationships are prerequisites to realizing the full potential of partly crystallineMIEC systems, and provides guidance for processing amorphous complex oxides generally.

Acknowledgments

The authors acknowledge the UC Irvine School of Engineering new faculty setup funds, the National Science Foundation CAREER award (DMR-2042638) and the UC Irvine Materials Research Institute (IMRI).

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