Structural Disorder and Electronic Structure in Alloyed SrTiO3/SrFeO2.5 Compounds: A Theoretical Study

TL;DR

This theoretical study develops a cluster expansion model to understand the disorder and electronic properties of SrTiO3/SrFeO2.5 alloys, revealing how cation disorder influences conductivity and band gap evolution.

Contribution

The paper introduces a cluster expansion Monte Carlo approach to model disordered MIEC alloys, enabling prediction of atomic configurations and electronic structures across compositions.

Findings

Band gap evolution matches experimental data.

Electronic conductivity is enhanced by Fe/Ti cation disorder.

The framework can be applied to other MIEC systems.

Abstract

Many mixed ionic/electronic conductors (MIECs) applied in fuel cell electrodes can be considered as alloys between perovskite oxides and ordered oxygen vacancy compounds. For example, in the model MIEC (STF), low oxygen diffusion barrier exist in SrTiO3 lattice, when it has been mixed with SrFeO2.5 with intrinsic oxygen deficiency, the ionic conductivity can be greatly improved. Meanwhile, the electronic conductivity can be optimized by controlling the defect chemistry of the alloy. However, the configurational space is too large in such alloys so that it is difficult for direct atomic modeling, which hinders in-depth understanding and predictive modeling. In this work, we present a cluster expansion model to describe the energetics of the disordered SrTiO3/SrFeO2.5 alloy within the full solid solution composition space Sr(Ti1-x,Fex)O3-0.5x (0<x<1). Cluster expansion Monte Carlo simulations have been performed to search the lowest energy atomic configurations and investigate the origin of lattice disorder. With representations of realistic configurations, the electronic structures of such alloys at different stoichiometry have also been examined. We find that the band gap evolution with composition calculated using our atomic model is consistent with experiment measurement. Meanwhile, the band edge analysis elucidate that electronic conductivity within such alloy can be facilitated by the Fe/Ti cation disorder. Taking SrTiO3/SrFeO2.5 alloy as an example, the generalized computational framework applied here can be extended to other relevant MIEC material systems.