  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.
