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Letter 3-#
Low Dimensional Oxygen Vacancy Ordering and Diffusion in SrCrO Phuong-Vu Ong, Yingge Du, and Peter V Sushko
J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b00355 • Publication Date (Web): 02 Apr 2017 Downloaded from http://pubs.acs.org on April 3, 2017
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Low Dimensional Oxygen Vacancy Ordering and Diusion in SrCrO! δ Phuong-Vu Ong,∗ Yingge Du, and Peter V. Sushko∗ Physical Sciences Division, Physical & Computational Sciences Directorate, Paci c Northwest National Laboratory, Richland, WA 99352, U.S.A. E-mail:
[email protected];
[email protected] Graphical TOC Entry
March 28, 2017 We investigate the formation mechanisms of vacancy-ordered phase and collective mass transport in epitaxial SrCrO lms using ab initio simulations within the density functional theory formalism. We show that as concentration of oxygen vacancy (V ) increases, they form one-dimensional (1D) chains that feature Cr-centered tetrahedra. Aggregation of these 1D V -chains results in the formation of (111)-oriented oxygen-de cient planes and an extended vacancy-ordered phase observed in recent experiments. We discuss atomic scale mechanisms enabling the quasi-2D V aggregates to expand along and translate across (111) planes. The corresponding lowest activation energy pathways necessarily involve rotation of Cr-centered tetrahedra, which emerges as a universal feature of fast ionic conduction in complex oxides. These ndings explain reversible oxidation and reduction in SrCrO at low-temperatures and provide insights into transient behavior necessary to harness ionic conductive oxides for high performance and low-temperature electrochemical reactors.
E
3δ
Reaction Co-ordinate
O
O
O
3δ
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Abstract
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The Journal of Physical Chemistry Letters
The Journal of Physical Chemistry Letters
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Perovskite-type oxides ABO3 have diverse structural phases with intriguing electronic and electrochemical properties, depending on their stoichiometry and the nature of their atomic constituents. The rich variety of structural and functional behaviors originate largely from the partially lled d -shells of the B -site transition metals, which participate in electron transfer processes and determine the strength of electron-lattice coupling and details of the ligand-metal complex structure. 13 In particular, these oxides can serve as parent phases to novel materials synthesized by soft-chemical approach at low temperatures. For example, AFeO2 phase (A=Ca or Sr) with square-planar oxygen coordination around Fe 2+ ions was synthesized by reducing perovskite SrFeO 2.875 or brownmillerite AFeO2.5 at 240-280 ◦ C, using CaH2 as a reducing agent. 4,5 Recently, it has been shown that a reversible topotactic transformation from cubic SrCrO 3 to rhombohedral SrCrO2.8 can be performed by redox reactions at relatively low temperatures (400-500 ◦ C) in epitaxial lms 6 and polycrystalline samples. 7 The reduced phase, SrCrO2.8 , exhibits an ordering of oxygen vacancies whereby one third of oxygen amount is lost from every fth (111)SrO3 plane, which, hence, transforms into (111)SrO2 plane; other oxygen ions rearrange to form a tetrahedral coordination shell around each Cr ion in the two adjacent (111)Cr planes. For simplicity, we refer to these quasi-two-dimensional (2D) structures as VO -planes. The low-temperature controlled and reversible topotactic transformation in this material implies the existence of fast mass transfer processes and, in turn, oers opportunities for applications in low-temperature solid oxide fuel cells, metal-air batteries, and other electrochemical reactors, where fast oxygen transport underpins the materials functionality. On the theoretical side, some progress has been made in understanding structural properties of the VO -planes in SrCrO2.8 and diusion mechanism of an isolated VO and a single extra O2− in a fully formed VO -plane. 6 However, still lacking is the atomic scale understanding of the formation and degradation mechanisms of these planes
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and the corresponding energetics, needed to control and harness these 2D defect structures. In this Letter, we investigate the formation and transport mechanisms of VO and their 1D and 2D aggregates in epitaxial SrCrO 3 δ using ab initio thermodynamics calculations. Figure 1(a) shows the dependence of tetragonality ratio c/a on VO concentration δ in SrCrO3 δ . With increasing δ, dimensionality of vacancy cluster changes from 0D, e.g., single vacancy, to 1D and then to 2D, which also induces the formation of tetrahedrally coordinated Cr. To rationalize the latter we turn to the JahnTeller theorem and note that each VO produces two electrons that convert two formally Cr 4+ ions from electronically degenerate d2 con guration to non-degenerate d3 con guration (Cr3+ ions) and, thus, stabilize them in the octahedral coordination [Figure 1(a)]. In addition, at high local concentrations, VO provide su cient space for oxygen neighbors of Cr 4+ ions to rearrange and form tetrahedral environment so as their d2 electronic con guration also becomes non-degenerate. Oxygen vacancies at the axial sites (in the O-Cr-O chains along the c-axis) and equatorial sites (in the CrO2 planes perpendicular to the c-axis) are denoted Va and Ve , respectively. Atomic con gurations of a Va − Ve vacancy pair, i.e., divacancy, and of several 1D VO -chains are shown in Fig. 1(b). The c/a ratio is found to increase linearly with increasing vacancy concentration. This is in agreement with experiment results, which show that outof-plane lattice parameter of epitaxial SrCrO 3 δ lms increases under reducing conditions: the observed c/a ratio changes from ∼1 to 1.016 1.026, depending on the substrate, as δ increases from 0.0 to 0.2. 6 We note that for the bulk SrFeO3 δ and CaFeO3 γ , topotactic reduction induces contraction of the out-of-plane lattice parameter and concomitant expansion of the in-plane lattice parameter, so as the c/a ratio decreases to 0.8700.985. 4,5,8,9 Thus, formally stoichiometric crystal lattices with similar structures, such as SrCrO 3 and SrFeO3 , respond dierently to accumulation of oxygen vacancies and, accordingly, promote dierent arrangements and preferential concentrations
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1.06 1.04
[
single
pair ] 0D
1D
2D
(b)
Symbols
Full: Va 1.02 Open: Ve Plus: mixed
Cr Va-Ve pair
1.00 0.98
Cr4+ (d 2)
0.96
t2g eg
0.94 0.00
0.05
0.10
0.15
in SrCrO 3-
Cr3+ (d 3)
eg t2g
0.20
0>
