Ab initio Study of Oxygen-Vacancy Ordering in Oxygen Conducting Ba2In2O5

Yueh-Lin Lee, Dane Morgan, Materials Science Program

Ba2In2O5 was first recognized as a fast ion conductor by Goodenough et al. in 1990. However, the coupling of oxygen-vacancy short- and long-range-order with fast ion conduction behavior is still unclear. We are using ab initio thermokinetic modeling to study this coupling.  The thermodynamic and structural properties of Ba2In2O5 are studied using the cluster expansion method based on ab initio energies. Monte Carlo methods are used to simulate the phase transitions and short-range-order.

At room temperature Ba2In2O5 adopts the orthorhombic brownmillerite-type structure, with empty oxygen sites (vacancies) aligned along the alternate <110> direction of the cubic perovskite structure (see the figure above). Upon heating the material partially disorders through a first-order transition to a tetragronal structure, and then completely disorders to cubic perovskite through a second-order transition. Preliminary simulation results (see the figure below) predict a first-order orthorhombic to tetragonal phase transition, followed by a second-order tetragonal to cubic phase transition, which is in good agreement with the experiments. The orthorhombic-tetragonal transition temperature is about 220 K too low compared to experiment (~1200 K, Speakman et al., Solid State Ionics, 149, p247), which is within the accuracy of the methods used here.  The tetragonal-cubic transition is about 700 K too high compared to experiment (~1350 K).  While this seems large, the small amount of configurational entropy is involved in this transition makes the transition temperature highly sensitive to small errors in the energetics.  Errors in the cluster expansion or physics that has not been included, e.g., vibrational contributions, could easily cause this effect. The source of the discrepancy is still being investigated.

Additional work analyzing the short-range-order derived from the Monte Carlo simulations is in progress.  The present model provides a foundation for further investigations of Ba2In2O5, such as addition of dopants and diffusion kinetics.

We gratefully acknowledge financial support from the National Science Foundation (NSF) MRSEC program, Division of Materials Research (DMR), award number 0079983.  We gratefully acknowledge computing support from the National Science Foundation (NSF) National Center for Supercomputing Applications (NCSA), award number DMR060007

• Publication

1. Ab Initio Study of Oxygen-Vacancy Ordering in Oxygen Conducting Ba₂In₂O₅; Lee, Y. -L.; Morgan, D. Materials Research Society Proceedings, 972,  0972-AA04-06, 2006