Advanced Functional Oxides
Crystal Structure Refinement of the A-Site-Ordered Double-Perovskite Oxide PrBaCo2O6−δ
2020 Volume 61 Issue 8 Pages 1500-1502
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2020 Volume 61 Issue 8 Pages 1500-1502
Crystal structures of double perovskite cobalt oxide PrBaCo2O6−δ with a wide range of oxygen deficiency content δ (0 ≤ δ ≤ 0.85) have been analyzed by using Rietveld refinement of synchrotron X-ray powder diffraction data. The bond valence sum calculation indicates a monotonic reduction in Co valence with increasing δ. The atomic position of Co ion shifts toward the apical oxygen with whole occupation with increasing δ, together with the gradual decrease in the occupancy for the opposite apical oxygen with partial deficiency. This finding indicates that the local structure around Co ions as catalytically active sites is significantly varied by oxygen deficiency.
Double-perovskite-type cobalt oxides, RBaCo2O6−δ (R: rare-earth metal) have been extensively investigated due to their attractive properties such as magnetoresistance,1,2) metal-insulator transition,3,4) and electrocatalysis.5,6) These properties are predominated by the oxygen deficiency δ and structural environment around Co ions. The nominal valence of Co ions is derived from δ and varies from +3.5 (δ = 0) to +2.5 (δ = 1). The difference in ionic size and polarizability between R and Ba tends to form layered-type ordering of A-site ions and the oxygen deficiency is localized within the (R, O)-layer.5,7)
Maignan et al. reported that δ can be easily changed by using different heat treatment under appropriate atmospheres.8) However, the ordered stoichiometric perovskite, corresponding δ = 0 requires special synthesis conditions. To the best of our knowledge, the ordered stoichiometric perovskite RBaCo2O6 could only be synthesized for R = Y, La, Pr, and Nd.9–12)
A wide range of oxygen deficiency content and resulting Co valence states were obtained in PrBaCo2O6−δ (see the crystal structure in Fig. 1 drawn by using VESTA-3 program13)) by using various synthesis conditions.11,14–17) Seikh et al. synthesized the ordered stoichiometric PrBaCo2O6 by immersing PrBaCo2O5.8 in a sodium hypochlorite solution for 4 weeks.11) So far, it could not be obtained by direct solid-state reaction.
Schematic of crystal structure of PBCO6−δ. CoO6−δ octahedra/pyramids are illustrated.
Electrochemical catalytic activities in PrBaCo2O6−δ are associated with oxygen content and resulting local structures.18) Accordingly, to clarify the mechanism of this catalysis, a detailed investigation of the evolution of crystal structure with δ is needed.
In this paper, we systematically synthesized tetragonal PrBaCo2O6−δ (PBCO6−δ) samples with a wide range of oxygen deficiency content (0 ≤ δ ≤ 0.85) by using appropriate oxidation and reduction conditions including high-pressure method. Crystal structure refinement based on synchrotron X-ray powder diffraction (SXRD) data demonstrates that the atomic position of Co ion gradually shifts inside of CoO6 polyhedra with increasing δ. This finding serves to investigate local environment around catalytically active Co sites in lightly and heavily oxygen-deficient phases.
A pristine PBCO5.8 powder was prepared by a conventional solid-state reaction. Stoichiometric amount of metal oxides and carbonate, Pr6O11, BaCO3, and Co3O4 were mixed at a mole ratio of 1:6:4. The mixture was heated at 1100°C for 24 h in the air with intermediate grindings. A fully oxidized PBCO6 powder was synthesized by a high-pressure treatment of PBCO5.8. The PBCO5.8 powder was mixed with a KClO4 oxidizing agent and treated at 8 GPa and 1000°C for 30 min. A slightly oxidized PBCO5.9 powder was prepared by annealing PBCO6 in the air at 150°C. A slightly reduced PBCO5.7 powder was obtained by treating PBCO5.8 at 8 GPa and 500°C for 30 min without mixing KClO4. A strongly reduced PBCO5.15 powder was obtained by a reductive annealing method from PBCO5.8 powder. PBCO5.8 powder was pressed into a pellet and heated in a N2 flow at 1100°C for 12 h, followed by quench using liquid nitrogen.
The synchrotron X-ray powder diffraction (SXRD) measurements were conducted at room temperature using a Debye-Scherrer camera installed at BL02B2 beamline of SPring-8, Japan.19) Structure parameters were refined by Rietveld analysis using the Rietveld refinement program RIETAN-FP.20)
Figure 2(a) shows SXRD patterns for PBCO6−δ. All PBCO6−δ samples crystallized in the tetragonal double perovskite structures with the space group of P4/mmm (No. 123) consisting of the layered ordering of Pr3+ and Ba2+ ions. The stoichiometric PBCO6 sample with δ = 0 was successfully obtained by using high-pressure treatment in a short period (a few hours) compared to the conventional method.11)
(a) SXRD patterns for PBCO6−δ. (b) δ dependence of the unit cell volume (V) and lattice constants (a and c/2) for PBCO6−δ.
The δ values were obtained from the refinement of occupancy factors at the apical oxygen site [O(1) site] in (Pr, O)-layer. A plot of cell volume and lattice parameters versus δ is displayed in Fig. 2(b). The unit cell volume increased monotonically as increasing δ, which is attributed to the increase in ionic radii of Co ions with the nominal valence decrease from +3.5 (δ = 0) to +2.6 (δ = 0.90). The a-axis length increased with increasing δ whereas the c-axis length decreased. The simultaneous elongation of the a-axis and shrinkage of the c-axis are 1.8% and 0.91%, respectively, thus the tetragonality [= (c/2)/a] was enhanced from 0.988 (δ = 0) to 0.962 (δ = 0.90). The anisotropic distortion is associated with the local structures around Co ions, as shown later.
The metal-oxygen bond lengths calculated from the Rietveld refinement results are associated with the above anisotropic structural evolution. The Co–O bonds are distinguished between Co–O(1), Co–O(2), and Co–O(3), as shown in Fig. 3(a). The Co–O(3) (equatorial) bond was elongated by 3.0% from δ = 0 to δ = 0.90. By contrast, the Co–O(1) and Co–O(2) (axial) bonds were shrunk by 1.6% and 0.28%, respectively. The increase (decrease) in a-axis (c-axis) with increasing δ is basically derived from the anisotropic distortion for CoO6 octahedra.
(a) Variation of Co–O bond lengths as a function of δ. (b) δ dependence of the BVS of Co for PBCO6−δ. The BVS value was calculated using the following parameters: b0 = 0.37 Å and r0 = 1.70 Å.22) Variation of (c) O(3)–Co–O(3) bond angles and (d) distance between Co and equatorial plane for CoO6 octahedron for PBCO6−δ as a function of δ. (e) Schematic of structure evolution of CoO6 octahedral structure for PBCO6−δ. Arrows represent the directions of atomic shifts along c-axis with increasing δ.
The bond valence sum (BVS)21) values for Co ions were calculated from the Co–O bond lengths and occupancy factor g(O1): BVS(Co) = ΣbCo–O(j) = g(O1)bCo–O(1) + bCo–O(2) + 4bCo–O(3), where oxygen deficiency at O1 site was included. The BVS value monotonically decreased from 3.18 (δ = 0) to 2.42 (δ = 0.90), confirming the decrease in Co valence (Fig. 3(b)). The systematic discrepancy between BVS and nominal valence, in which the former is about 0.3 smaller than the latter, is attributed to possible uncertainty of the bond valence parameters adopted.
The local structure for CoO6−δ octahedron/pyramid changed in accordance with oxygen deficiency. The O(3)–Co–O(3) bond angle became smaller with increasing δ (Fig. 3(c)). This indicates that the Co position shifts from the ideal in-plane to distorted out-of-plane from the equatorial plane. The distance between Co ion and equatorial plane consisting four O(3) ions increased from ∼0.13 Å (δ = 0) to ∼0.34 Å (δ = 0.90) simultaneously with the narrowing of O(3)–Co–O(3) bond angle (Fig. 3(d)). As a result, the Co ion is located closer to the apical O(2) site in the formation of the CoO5 square pyramid (Fig. 3(e)).
The above-described crystal structure analysis for the layered double perovskite cobalt oxide PBCO6−δ implies significant change in electrochemical properties. Together with the reduction in Co valence, Co ions move away from equatorial planes and shift inside the square pyramids with decreasing the average coordination number from 6 to 5. This local structural change appears to make Co ions disadvantageous as adsorptive sites during electrochemical reactions. Hence, Co ions in PBCO6−δ with lower oxygen deficiency content are presumed to be more active for electrochemical catalysis.
The ordered tetragonal double perovskite oxides PBCO6−δ with a wide range of oxygen deficiency (0 ≤ δ ≤ 0.85) were synthesized. Structural changes induced by oxygen deficiency content make significant shift of the central Co ion toward the CoO5 square pyramid. These shifts and valence changes of Co ions may affect the catalytic property during adsorption for intermediates. Further study on electrochemical activity is in progress.
The synchrotron radiation experiments were performed at SPring-8 under the approval of the Japan Synchrotron Radiation Research Institute (proposal number 2019A1476 and 2019B1420). This work was supported by JSPS KAKENHI (grant number 18H03835 and 19H02438).
