# Mechanism of Topotactic Reduction-Oxidation Between Mg-Doped SrMoO3 Perovskites and SrMoO4 Scheelites, Utilized as Anode Materials for Solid Oxide Fuel Cells

## TL;DR

This paper explains how a specific oxide material can switch between two crystal structures, which is important for its use in fuel cells.

## Contribution

The study reveals a direct topotactic transformation between perovskite and scheelite phases without intermediate steps.

## Key findings

- The perovskite SrMo0.9Mg0.1O3-δ transforms topotactically into a scheelite phase upon oxidation.
- Oxygen vacancies are present in both oxidized and reduced phases, contributing to mixed ionic-electronic conductivity.
- The crystal structures of perovskite and scheelite phases were confirmed as cubic and tetragonal, respectively.

## Abstract

Recently, we have described SrMo1-xMgxO3-δ perovskites (x = 0.1, 0.2) as excellent anode materials for solid oxide fuel cells (SOFCs), with mixed ionic and electronic conduction (MIEC) properties. After depositing on the solid electrolyte, they were annealed for sintering at high temperatures (typically 1000 °C), giving rise to oxidized scheelite-type phases, with SrMo1-xMgxO4-δ (x = 0.1, 0.2) stoichiometry. To obtain the active perovskite phases, they were reduced again in the working anode conditions, under H2 atmosphere. Therefore, there must be an excellent reversibility between the oxidized Sr(Mo, Mg)O4-δ scheelite and the reduced Sr(Mo, Mg)O3-δ perovskite phases. This work describes the topotactical oxidation, by annealing at 400 °C in air, of the SrMo0.9Mg0.1O3-δ perovskite oxide. The characterization by X-ray diffraction (XRD) and neutron powder diffraction (NPD) was carried out in order to determine the crystal structure features. The scheelite oxides are tetragonal, space group I41/a (No. 88), whereas the perovskites are cubic, s.g. Pm-3m (No. 221). The Rietveld refinement of the scheelite phase from NPD data after annealing the perovskite at 400 °C and cooling it down slowly to RT evidences the absence of intermediate phases between perovskite and scheelite oxides, as well as the presence of oxygen vacancies in both oxidized and reduced phases, essential for their performance as MIEC oxides. The topotactical relationship between both crystal structures is discussed.