Project
E-Field and strain enhanced anti-Frenkel defect formation and oxygen diffusion in brownmillerite Sr2Fe2O5
The brownmillerite material Sr2Fe2O5 undergoes a topotactic phase transition between the insulating brownmillerite (BM) phase and a conducting perovskite (PV) phase by applying an electric field. Since this phase transition can be induced by the application of an external electric field, Sr2Fe2O5 is attracting attention for resistive-switching random access memory (ReRAM) devices. Key process for the phase transition and thus, the switching kinetics, is the diffusion of oxygen point-defects. Detailed knowledge is, therefore, required for optimising the memresistive device’s performance. The aim of this project is a comprehensive investigation of the oxygen-ion transport in brownmillerite Sr2Fe2O5 using molecular dynamics (MD) simulations. MD simulations allow not only a thorough understanding of the atomistic processes and mechanisms, but also a quantification of the transport processes. Moreover, effects of an electric field can be investigated independently from other effects.
Project Details
Project term
April 1, 2024–March 30, 2025
Affiliations
RWTH Aachen University
Institute
Institute of Physical Chemistry
Principal Investigator
Methods
For the MD simulations, we used the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) software and empirical pair potentials (Buckingham potentials) that were adapted specifically to the SrFeO3–δ system. A large supercell (total number of ions: 98.784) of 14×14×14 unit cells of orthorhombic Sr2Fe2O5 (space group Ibm2) ensured an investigation also of rarely formed anti-Frenkel pairs. The system required long equilibration runs of ≈ 1 ns, and production runs proceeded for an additional 2 ns. Investigation of a stoichiometric system (Sr2Fe2O5) and two non-stoichiometric systems with oxygen excess (Sr2Fe2O5+0.05) or oxygen deficiency (Sr2Fe2O5-0.05) allowed us to achieve a deep and detailed description of the system and to study diffusion processes by means of oxygen interstitials or oxygen vacancies independently from each other. From the field-independent simulations, three major results emerge. First, oxygen diffusion in the BM phase is not confined to the (so-called) one-dimensional “oxygen vacancy channels” (OVC) as suggested in the existing literature. Instead, it occurs twodimensionally by interstitial and interstitialcy mechanisms within the FeO4 layers of the BM structure. Second, the simulations indicate that in the BM structure oxygen-vacancy diffusion is faster than oxygen-interstitial diffusion. This vacancy diffusion process is also two-dimensional, but restricted to the FeO6 layers. Third, we find similar activation enthalpies of vacancy diffusion for the BM and PV phases, which indicates essentially the same diffusion mechanism.
Results
A comparison, here, with experimental data reveal good agreement between our diffusion data for the PV phase and experimental data. This strongly implies a satisfactory description of diffusion processes by our MD simulations. As for the field-dependent simulations, a detailed analysis is still in progress. Preliminary insights suggest that the two diffusion mechanisms (interstitial/interstitialcy and vacancy) respond quite differently to the applied electric field, but can be described with small adaptions of existing models. Additionally, while the electric field significantly impacts the appearance of the phase transition, it does not substantially affect the formation of anti-Frenkel pairs. This suggests that there is no direct correlation between the number of anti-Frenkel pairs and the initiation of the phase transition.
Discussion
Our project provides a fundamental basis to all those who want to study, model, develop or tune brownmillerite materials for ReRAM devices, and we provide, more generally, a more detailed understanding of ion transport processes in vacancy-ordered perovskite phases.
Additional Project Information
DFG classification: 302-02 Physical Chemistry of Solids and Surfaces, Material Characterisation
Cluster: CLAIX
Publications
Sonja Ambaum, Neil L. Allan, Regina Dittmann, Roger A. De Souza, Oxygen Diffusion
in Brownmillerite Sr2Fe2O5 Is Two-Dimensional: Results from a Molecular Dynamics
Study, Chemistry of Materials, Volume 36, 2039-2048, 2024, 10.1021/acs.chemmater.3c03032.
Sonja Ambaum, Stine Spinger, Roger A. De Souza, Electric Field Effects on brownmillerite
Sr2Fe2O5 and perovskite SrFeOx: Haven ratio, non-linear enhancement of oxide-ion
mobilities, and phase transition thermodynamics, in preparation.
Bachelor thesis: Stine Spinger, E-Feld-getriebene Sauerstoffdefektbildung und -migration
in Sr2Fe2O5, Aachen 2023.