Understanding Cation Ordering and Oxygen Vacancy Site Preference in Ba3CaNb2O9 from First-Principles
Authors: Ding Hepeng , Virkar Anil V. , Liu Feng , Wang Siwei , Chen Fanglin (Frank) 
Published: February 2020

Ba3Ca1.18Nb1.82O9-δ (BCN18) is a complex perovskite, which exhibits excellent proton conduction when hydrated. It is also known to be more stable compared other proton conductors such as doped BaCeO3 and doped SrCeO3. BCN18is thus a potential electrolyte for fuel cells and for hydrogen separation. It is generally recognized that efficacy of proton transport at least partly depends on the local atomic environment, which implies that B-site cation ordering and oxygen vacancy occupation site may play important roles, making the study of the B-site cation ordering and oxygen vacancy position of interest.
Earlier studies1,2 have shown the important role played by electrostatic effects between different ionic species in driving the ordering process. Some recent works3,4, however, have suggested the ordering is related to the large ionic radius difference (~50%) between Ca2+ (1.14 Å) and Nb5+ (0.78 Å).
Using density functional theory calculations, we investigate the physical mechanism underlying the formation of the B-site cation ordering and the oxygen vacancy site selection in Ba3CaNb2O9. We found that either cation site exchange or oxygen vacancy formation induces negligible lattice strain. This implies that the ionic radius plays a minor role in governing these two processes.
We also found that the electrostatic interactions are dominant in the ordering of mixed valence species on one or more sites; the ionic bond strength is identified as the dominant factor in governing both the 1:2 B-site cation ordering along the <111> direction and the oxygen vacancy site preference in Ba3CaNb2O9. Starting from the 1:2 fully ordered atomic structure, we exchange one of the Ca atoms with one of its surrounding Nb atoms to study the B-site cation ordering in BCN. As shown in Fig. 1, there are several possible exchange types, denoted as 1-1x, 1-1y, 1-1z, 2-2x, 2-2y, 2-2z, and 2-2z’, respectively. Specifically, the cation ordering can be rationalized by the preference of mixed Ca-O-Nb bonds over the combination of Ca-O-Ca and Nb-O-Nb bonds; while oxygen vacancy prefers a site to minimize the electrostatic energy and to break the weaker B-O-B bond, as shown in Fig. 2.

Funded by DOE EFRC Grant Number DE-SC0001061 as a flow through from the University of South Carolina.

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Fig. 1 (a) Demonstration of the Ca/Nb exchange types, where 1 and 2 are Ca atoms, 1x, 1y, 1z, 2x, 2y, 2z, and 2z’ are the exchange processes involving Nb atoms. (b) Relative system energy (exchange energy) upon cation exchange. “Initial” denotes the 1:2 ordered structure, while the others denote the different exchanged structures.
Fig. 2 (a) Relative energies for oxygen vacancy formation sites vs. the number of Ca atoms in the NNN layer. Black squared denotes the oxygen vacancy formation in AO layers, and red circled denotes the oxygen vacancy formation in BO2 layers. (b) O ion effective charge vs. the number of Ca atoms in the NNN layer. The number of Ca in the NN layer is fixed at 1.