2232
Chem. Mater. 1996, 8, 2232-2238
Topotactic Electrochemical Redox Reactions of the Defect Perovskite SrCoO2.5+x A. Nemudry,† P. Rudolf, and R. Scho¨llhorn* Institut fu¨ r Anorganische und Analytische Chemie, Technische Universita¨ t Berlin, D-10623 Berlin, Germany Received October 24, 1995. Revised Manuscript Received May 22, 1996X
The electrochemical and chemical oxidation of the defect perovskite SrCoO2.5 to the cubic perovskite SrCoO3 at ambient temperature in alkaline electrolyte are shown to be reversible processes with the appearance of intermediate compounds; the phase diagram can be described by two line phases SrCoO2.5 and SrCoO2.75 and one solid solution range SrCoO2.87+y with 0 < y < 0.13. The redox cycle displays a structural hysteresis under dynamic conditions. Several models are discussed in order to explain the unusual phenomenon of roomtemperature bulk oxygen transport.
Introduction Perovskites and perovskite-related materials are of interest with respect to their use, e.g., as solid electrolytes, mixed electronic/ionic conductors, electrodes, electrocatalysts, sensor systems, etc.1 Defect perovskites AMO3-z may exhibit large phase ranges up to z ∼ 0.7.2 At high oxygen stoichiometry (z ) 0) unusual valencies of transition-metal ions can be stabilized, e.g., Fe(IV) in SrFeO3,3 Co(IV) in SrCoO34 and Cu(III) in LaCuO3.5 Thermal synthesis of these compounds requires unusually high oxygen pressures, however. The lower phase range with the stoichiometry AMO2.5 (brownmillerite type) is characterized in structural terms by the existence of ordered oxygen vacancies which can be described as isolated parallel lattice channels. As compared to the isometric coordination of B atoms in the ideal perovskite lattice, there is a significant shift of the B atom toward a strongly distorted coordination in the brownmillerite structure type. Since oxygen ionic transport is a major materials property in some of the applications of these compounds, a considerable number of studies have been devoted to the determination of ionic conductivity, chemical diffusion coefficients, and activation energies for oxygen mobility.6-8 From these investigations it can be concluded as a rule that reasonable transport rates for practical purposes can be achieved only at higher temperatures above ca. 600 °C. † Permanent address: Russian Academy of Science, Institute of Solid State Chemistry, 630090 Novosibirsk, Russia. * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, July 1, 1996. (1) Steele, B. C. H. Mater. Sci. Eng. 1992, B13, 79. (2) Takeda, Y.; Kanno, R.; Takeda, T.; Yamamoto, O.; Takano, M.; Bando, Y. Z. Anorg. Allg. Chem. 1986, 540/541, 259. (3) MacChesney, J. B.; Sherwood, R. C.; Potter, J. F. J. Chem. Phys. 1965, 43, 1907. (4) Taguchi, H.; Shimada, M.; Koizumi, M. J. Solid State Chem. 1979, 29, 221. (5) Darracq, S.; Largeteau, A.; Demazeau, G.; Scott, B. A.; Bringley, J. F. Eur. J. Solid State Inorg. Chem. 1992, 29, 585. (6) Manthiram, A.; Kuo, J. F.; Goodenough, J. B. Solid State Ionics 1993, 62, 225. (7) Ishigaki, T.; Yamauchi, S.; Kishio, K.; Mizusaki, J.; Fueki, K. J. Solid State Chem. 1988, 73, 179. (8) Qiu, L.; Lee, T. H.; Liu, L.-M.; Yang, Y. L.; Jacobson, A. J. Solid State Ionics 1995, 76, 321.
S0897-4756(95)00504-7 CCC: $12.00
Under these aspects it is most remarkable that in a series of publications the anodic oxidation of perovskite and K2NiF4 type oxometallates at ambient temperature has been reported, e.g., La2CuO4 f La2CuO4+x, SrFeO2.5 f SrFeO3, SrCoO2.5 f SrCoO3.9-11 For La2CuO4 it has been shown that reactions of this type are reversible12 and that chemical oxidation and reduction can be performed as well.13,14 The principal point here is that these processes obviously are correlated with the unusual phenomenon of oxygen ionic transport at room temperature. It is, therefore, most important to understand the mechanism of the low-temperature oxygen ionic transport and the changes in structure and composition occurring in the course of the reaction process. We have performed recently a study on the SrCoO2.5+x system and report here on the phase diagram of the electrochemical oxidation of this defect perovskite, on the reversibility of the process and on the possibility of chemical oxidation and reduction. Experimental Section SrCoO2.5 was prepared by standard solid-state techniques. Stoichiometric amounts of SrCO3 and Co3O4 (analytical grade) were ground and calcined in air at 900 °C for 12 h. The material samples were pelletized and heated in air at temperatures of 1250 (24 h) and 1000 °C (12 h) and quenched in liquid N2. Powder X-ray diffraction data were measured with a Siemens D 5000 diffractometer using Cu KR radiation. The starting compound was orthorhombic with the lattice parameters a ) 5.465(1), b ) 15.727(2), c ) 5.569(1) Å and the terminal product of the oxidation was cubic with a ) 3.830(1) Å, in agreement with brownmillerite and perovskite structures, respectively, as described earlier.11,15 X-ray diffraction and EDX measurements showed the absence of impurity phases in both compounds. According to iodometric titration (9) Wattiaux, A.; Park, J. C.; Grenier, J. C.; Pouchard, M. C. R. Acad. Sci. Ser. 2 1990, 310, 1047. (10) Wattiaux, A.; Fournes, L.; Demourgues, A.; Bernaben, N.; Grenier, J. C.; Pouchard, M. Solid State Commun. 1991, 77, 489. (11) Bezdicka, P.; Wattiaux, A.; Grenier, J. C.; Pouchard, M.; Hagenmuller, P. Z. Anorg. Allg. Chem. 1993, 619, 7. (12) Rudolf, P.; Paulus, W.; Scho¨llhorn, R. Adv. Mater. 1991, 3, 438. (13) Rudolf, P.; Scho¨llhorn, R. J. Chem. Soc., Chem. Commun. 1992, 16, 1158. (14) Takayama-Muromachi, E.; Sasaki, T.; Matsui, Y. Physica C 1993, 207, 97. (15) Grenier, J. C.; Ghodbane, S.; Demazeau, G.; Pouchard, M.; Hagenmuller, P. Mater. Res. Bull. 1979, 14, 831.
© 1996 American Chemical Society
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Chem. Mater., Vol. 8, No. 9, 1996 2233
Figure 2. Anodic oxidation of SrCoO2.5: potential step scanning mode; ∆n ) charge transferred per potential step. Figure 1. Galvanostatic anodic oxidation and subsequent cathodic reduction of the brownmillerite type defect perovskite SrCoO2.5 at 300 K in 1 M aqueous KOH electrolyte: change of potential E with charge transfer n; the correlation between the oxygen stoichiometry index x and the charge-transfer value n is x ) n/2. the samples had an oxygen stoichiometry corresponding to SrCoO2.47(1) and SrCoO3.00(1), respectively. Electrochemical experiments were performed at room temperature in two modes: galvanostatic and potential step scanning techniques (three electrode cell, 1 N KOH electrolyte) with polycrystalline materials pressed into Pt grids along with 1% mass of Teflon as working electrodes. In situ X-ray diffraction measurements were carried out in specially designed three-electrode cells with Hg/HgO as the reference electrode. Current densities in galvanostatic experiments are given in terms of current per mass unit of working electrode material and varied between 3 and 10 µA/mg. Chemical oxidation was performed by alkaline hypobromite solution as described previously by Rudolf et al.13 Chemical reduction was carried out with 15% aqueous H2O2.
Results The variation of the potential of the working electrode material SrCoO2.5 with the charge transfer in galvanostatic mode during the oxidation and reduction process are given in Figure 1. After a charge transfer of n ) 1e-/SrCoO2.5, oxygen evolution is observed and lattice parameters and analytical composition remain constant. The maximum transfer of one electron per formula unit of the starting material is confirmed by iodometric titration values. Similarly, upon reduction of the terminal product of anodic oxidation, again a maximal value of 1e- is observed as confirmed by iodometric titration and by measurement of the changes in lattice parameters. The terminal oxidation product corresponds thus with the perovskite SrCoO3, while the product of the subsequent reduction is identical with the brownmillerite phase SrCoO2.5 except for a minor broadening in the line width of the X-ray pattern. A strong color change is observed upon oxidation: SrCoO2.5 has a gray black appearance while the final reaction product SrCoO3 has a metallic golden bronze color. The gross reaction process appears thus to be entirely reversible and can be described formally by the equation
SrCoO2.5 + 1/2O2- a SrCoO3 + ebrownmillerite perovskite orthorhombic cubic
(1)
The change in potential in the course of oxidation and reduction with plateau regions and ranges with a slope in potential suggests that there are intermediate phases in this reaction. This is also supported by potential step scanning measurements which suggest the existence of two-phase regions at ca. +270 and at ca. +420 mV (Figure 2) and by the X-ray patterns obtained at different oxidation states as indicated in Figure 3. Chemical oxidation was carried out by treatment of powdered SrCoO2.5 in alkaline solution of sodium hypobromite at t ) 4 °C for 12 h with intensive stirring. After this time quantitative conversion was confirmed by X-ray data and wet analysis to the perovskite SrCoO3.0. The reaction proceeds due to the excess of oxidant in a pseudo-two-phase range; sintered pellets (90% theoretical density) show optically a defined interface golden metallic (SrCoO3.0)-gray black (SrCoO2.5) on a fresh fracture face after partial reaction. Chemical reduction of SrCoO3.0 in 15 wt % aqueous solution of H2O2 yields quantitatively SrCoO2.5; the process is accompanied by a cobalt-catalyzed intensive exothermic decomposition of the reducing agent. The reaction rate of reduction is about 2 times faster as compared to oxidation. For a more detailed study on the reaction mechanism we performed in situ X-ray diffraction measurements in galvanostatic mode with an adapted three-electrode cell with simultaneous registration of potential change, charge transfer, and diffraction data. X-ray patterns obtained in the course of the oxidation reaction are given in Figure 4. Electrochemical and structural data for the oxidation process related to the charge transfer values are given in Figure 5. Three different regions can be distinguished during the reaction. Region I (0 < n < 0.5) is characterized by a potential plateau and the coexistence of two phases, i.e., the orthorhombic starting compound SrCoO2.5 and a phase C which could be indexed with cubic symmetry. The line intensities vary with charge transfer n as expected for a two-phase range while the lattice parameters remain constant. Lattice parameters for C which corresponds in terms of stoichiometry to SrCoO2.75 are listed in Table 1. The structural data obtained for C are in good agreement with ex situ measurements on samples obtained via galvanostatic and potentiostatic preparation with a charge transfer of 0.5e-/SrCoO2.5 and checked by iodometric titration. The data for these samples are listed in Table 1.
2234 Chem. Mater., Vol. 8, No. 9, 1996
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Figure 3. X-ray powder diffractograms of oxocobaltates SrCOO2.5+x: (a) starting material; samples (b)-(d) obtained by electrochemical oxidation: (b) SrCoO2.75, cubic; (c) SrCoO2.87 tetragonal; (d) SrCoO3.0 cubic.
Figure 4. In situ measurement of the anodic oxidation of SrCoO2.5: sections of X-ray powder diagrams. Bottom, starting material; top, terminal phase; lines from the platinum grid support (Pt) were used as standard for correction of line position and determination of changes in line width; time resolution 1.37 h.
Regions II (0.5 < n < 0.75) and III (0.75 < n < 1.0) appear to be characterized by a continuous change in cubic lattice parameters and significant changes in line width with a continuous increase of the potential with different slope. A detailed analysis of the width and shape of the reflections demonstrates that there is a maximum line width at n ∼ 0.7 for the (002) reflection (Figure 5c); simultaneously the line width of the (111) reflection remains constant within the error margin. It can be assumed in agreement with arguments discussed below that region II is a two-phase range with the formation of a tetragonal phase that has lattice parameters close to those of the lower phase boundary of the
terminal solid solution which results in a low resolution of the X-ray lines and the simulation of a continuous one-phase transition. This is also supported by analysis of a sample obtained in potentiostatic mode at E ) +420 mV which has a composition close to SrCoO2.87 (equivalent to an occupation of three-quarters of the available channel vacancies) and displays split reflections indicating a tetragonal structure (Table 1, Figure 3). Region III is characterized by a small continuous change in potential and a decreasing line width of the (002) reflection. This can be interpretated as the continuous disappearance of a small tetragonal distortion with a reduction in apparent line width and the formation of
Topotactic Electrochemical Redox Reactions
Chem. Mater., Vol. 8, No. 9, 1996 2235
a continuous change in potential is observed as well as a continuous change in lattice parameters with a transition from cubic to tetragonal with a lower phase limit corresponding to SrCoO2.87 (T* phase). It is to be noted that despite the similarity in symmetry and composition there is a clear difference in unit-cell parameters between T and T* (Table 1). In region II* with 0.25 < n < 0.5 a potential plateau is observed that is in agreement with the structural data in this range which show the coexistence of T* and an intermediate cubic phase C* with constant lattice parameters in a two-phase range. The line phase C* ) SrCoO2.75 has again a slightly larger unit cell as compared to C (Table 1). The third region I* corresponds as expected to a twophase range with the coexistence of C* and the brownmillerite type phase SrCoO2.5 (Figures 6 and 7); further reduction is associated with the discharge of hydrogen at the cathode while lattice parameters and oxidation state of cobalt remain constant. The idealized phase diagram is thus to be described by two two-phase ranges and a one-phase range, i.e., by two line phases SrCoO2.5 and SrCoO2.75, respectively, and one nonstoichiometric phase SrCoO2.87+y with the upper composition limit SrCoO3.0. The reaction is reversible; a structural hysteresis phenomenon is observed, however, in terms of slightly different lattice parameters in the intermediate region. A scheme of the structural transformations is given in Figure 8. The potential/charge transfer curve for the reduction process exhibits a better correlation in terms of potential changes with structural data as compared to the oxidation half-cycle which is most likely due to a reduced influence of kinetics.
Figure 5. In situ anodic oxidation of SrCoO2.5: (a) potential E vs charge transfer n, (b) change of unit-cell parameters with n, (c) variation of line width of the 002 reflections (“cubic phase”) vs charge transfer n.
Discussion In a comparison with the data obtained by electrochemical reaction at ambient temperature and pressure with those reported for samples prepared by thermal synthesis at high oxygen pressures we find some differences. Taguchi et al.4 describe the existence of a nonstoichiometric phase region SrCoO3-x (0.05 < x < 0.26) with cubic structure. For SrCoO2.74 which represents the lower end of the phase range (prepared at 350 °C, p(O2) ) 50 bar) a ) 3.845 Å was found, which is close to the lattice parameters for the intermediate cubic line phase C (SrCoO2.75) found upon electrochemical oxidation (compare Table 1). The change in a axis between the phase limits for thermal preparation as reported by Taguchi was found to be similar to the change in lattice parameters found upon electrochemical oxidation if our X-ray line patterns are indexed in cubic symmetry (Figure 5b). The cubic lattice parameter
a cubic perovskite type terminal product SrCoO3. We conclude, therefore, that region II is a two-phase region with the coexistence of the intermediate phases C and T (the latter corresponding to the lower phase limit of the nonstoichiometric phase P) while region III is a one phase range SrCoO2.87+y with 0 < y < 0.13 (phase P). The potential/charge transfer curve (Figure 5a) shows in fact a small potential plateau in range II which is kinetically distorted toward the border regions. To study the reversibility and possible hysteresis of the redox cycle, we performed an in situ study on the reduction of SrCoO3.0. Figures 6 and 7 show X-ray patterns and changes of lattice parameters and potential with the charge transfer. As in the case of the oxidation reaction the reduction process can again be divided into three regions. In region III* (0 < n < 0.25)
Table 1. Structural and Analytical Data for Oxocobaltates Obtained via Electrochemical Synthesis (n ) Charge-Transfer Value (e-/f.u.)) oxidation in situ (galvanostatic) compound
reduction
ex situ (galvanostatic)
ex situ (potentiostatic) titration
n
a, b, c (Å)
titration
a, b, c (Å)
SrCoO2.5
0.0
SrCoO2.47(1)
SrCoO2.75 SrCoO2.875
0.5 0.75
5.468(3) 15.756(3) 5.561(3) 3.843(3)
SrCoO2.74(1)
5.465(1) 15.727(2) 5.569(1) 3.843(1)
SrCoO3.0
1.0
3.829(3)
SrCoO3.00(1)
3.830(1)
SrCoO2.76(1) SrCoO2.86(1)
a, b, c (Å)
3.842(1) 3.828(1) 3.838(1)
in situ (galvanostatic) n
a, b, c (Å)
1.0
5.460(3) 15.716(3) 5.564(3) 3.867(3) 3.829(3) 3.853(3) 3.829(3)
0.5 0.25 0.0
2236 Chem. Mater., Vol. 8, No. 9, 1996
Nemudry et al.
Figure 6. In situ cathodic reduction of SrCoO3: sections of diffractograms with different charge transfer n. Top, SrCoO3 (n ) 0); bottom, terminal phase SrCoO2.5 (n ) 1); time resolution 1.29 h. The reflection at 2θ ) 44.42° can be indexed on the base of a brownmillerite type lattice as an intermediate superstructure with doubled a axis.
Figure 8. Scheme for electrochemical redox transformation cycle of strontium oxocobaltate.
Figure 7. In situ reduction of SrCoO3.0: potential E and lattice parameters vs charge transfer n.
found at the upper phase limit SrCoO2.95 (prepared at 300 °C, p(O2) ) 2 kbar) is a ) 3.837 Å; a linear extrapolation to the composition SrCoO3.04 yields a ) 3.836 Å which is slightly higher as compared to the value found for this stoichiometry by electrochemical preparation (Table 1). Takeda et al.2 reported on a solid solution range SrCoOz with 2.42 < z < 2.52 for the brownmillerite phase. Under oxygen pressure these
authors also noticed only the formation of cubic perovskites. A phase with the composition SrCoO2.8 was indexed by Takeda et al., however, with tetragonal symmetry (a ) 10.86 Å ) 2‚x2‚3.839 Å, c ) 7.657 Å ) 2‚3.828 Å) which is in good agreement with the data for SrCoO2.87 obtained in this work (Table 1). Bezdicka et al.11 used the potentiostatic technique at high potential for the oxidation of SrCoO2.5 at 300 K and obtained thus the cubic end phase SrCoO3.0 with a ) 3.835 (2) Å which is somewhat higher as compared to the corresponding value given in Table 1. We observed, however, that upon extended storage in air or washing with distilled water of SrCoO3.0 the stoichiometry is changing to SrCoO2.96 with a simultaneous change in lattice parameter from a ) 3.829 Å to a ) 3.834 Å, the latter value being identical with the lattice parameter reported by Bezdicka et al.11 We conclude that although the ideal
Topotactic Electrochemical Redox Reactions
terminal stoichiometry SrCoO3.0 can be obtained via electrochemical preparation at high potentials, the stability at this composition is low due to the high cobalt oxidation state. There are several questions which need to be answered for an understanding of the reaction process described: (i) which transport mechanism is responsible for the apparent unusual fast oxygen transport at room temperature, (ii) what is the reason for the formation of an intermediate phase C/C* with cubic structure, and (iii) why do phases C/C* and T/T* which are close to an average oxidation state of +4 for cobalt have markedly different unit-cell parameters? High reactivity of solids in topotactic reactions can be a consequence either of high mobility of guest species or of structure-specific defects. We will discuss here three different models concerning these aspects. (i) The advent of oxocuprate superconductors of the YBa2Cu3O7 type introduced new physics but also some most interesting perspectives in terms of solid-state chemistry. It was observed that oxygen can be intercalated reversibly into the lattice matrix at unusually low temperatures of 300-400 °C.16-20 An attempt to explain the low-temperature oxygen anion transport has been made on the base of the high oxidation state of copper in these phases and the model of a critical valence state with participation of O•- bonding states. Since the latter ion has a lower effective ionic radius and reduced Coulomb interaction with the lattice due to the reduced ionic charge, it was already proposed earlier to explain the high mobility of oxygen in oxocuprates by the lower activation energy for site change of O•- ions.18 A similar argument would hold for related oxometallates with metal ions in rather high valence states, e.g., Fe3+/Fe4+, Co3+/Co4+. In the present case the formation of phase C with cubic symmetry could be explained by a transition from the brownmillerite phase with oxygen vacancies ordered along [101] lattice channels to SrCoO2.75 with a statistical disorder of the residual 50% anion vacancies as a consequence of an effective charge exchange between O2- matrix ions and O•- “guest ions” and three dimensional diffusion of equivalent anions in consequence. Further increase in oxygen content would lead then to structural disorderorder transitions with formation of the tetragonal T phase related, e.g., to magnetic transitions and/or changes in the band structure. This model would not readily explain, however, the difference between C/C* and T/T* structures. (ii) An alternative model we propose here is based on a two-stage process involving hydroxyl ions present in the electrolyte. As compared to the direct intercalation (16) Michel, C.; Raveau, B. Rev. Chim. Miner. Fr. 1984, 21, 407. (17) Eickenbusch, H.; Paulus, W.; Gocke, E.; March, J. F.; Koch, H.; Scho¨llhorn, R. Angew. Chem., Int. Ed. Engl. 1987, 26, 1188. Eickenbusch, H.; Paulus, W.; Scho¨llhorn, R.; Schlo¨gl, R. Mat. Res. Bull. 1987, 22, 1505. (18) Scho¨llhorn, R. Angew. Chem., Int. Ed. Engl. 1988, 27, 1392. (19) Chemistry of high-temperature oxide superconductors. In New Directions in Solid State Chemistry; Rao, C. N. R., Gopalakrishnan, J., Eds.; Cambridge University Press: Cambridge, 1989; p 475. (20) Kishio, K.; Hasegawa, T.; Suzuki, K.; Kitazawa, K.; Fueki, K. In High Temperature Superconductor: Relationships between Properties, Structure, and Solid-State Chemistry; Jorgensen, J. D., Kitazawa, K., Tarascon, J. M., Torrance, J. B., Eds.; Materials Research Society: Pittsburgh, PA, 1989; p 91. Post, M. L.; Pleizier, G. J. Solid State Chem. 1993, 107, 166. De Fontane, D.; Ceder, G.; Asta, M. Nature 1990, 343, 544.
Chem. Mater., Vol. 8, No. 9, 1996 2237
Figure 9. Partial thermal reduction of brownmillerite (SrCoO2.5), phase C (SrCoO2.75), and phase P (SrCoO3.0) in hydrogen atmosphere: thermogravimetric data.
of O2- ions, the insertion of OH- should result in a strong reduction of activation energy for site change due to the reduced effective anion charge and a higher ionic mobility. In the first stage of this model the intercalation reaction proceeds by the uptake of OH- ions into the lattice channels of up to a complete filling of the vacancies available according to eq 2 in a two-phase
SrCoO2.5 + 0.5OH- f SrCoO2.5(OH)0.5 + 0.5e- (2) range. The resulting structure (phase C) should have cubic lattice symmetry due to proton disorder, i.e., exchange processes between “guest” OH- ions and lattice O2- ions:
*O2- + OH- a *OH- + O2-
(3)
The second stage of this model would correspond to an electron/proton transfer process under formation of the terminal perovskite phase:
SrCoO2.5(OH)0.5 f SrCoO3 + 0.5 H+ + 0.5e- (4) This model would explain reasonably well the lowtemperature ionic transport properties; it turned out, however, that it is at variance with thermal analysis data. This reaction scheme would require obviously that no measurable difference is to be observed in weight loss upon partial thermal reduction of C and P, respectively, in hydrogen atmosphere to SrCoO2.5 (eqs 5 and 6). The experiment shows, however, that there
SrCoO2.5(OH)0.5 + 1/4H2 f SrCoO2.5 + 0.5H2O SrCoO3 + 1/2H2 f SrCoO2.5 + 0.5H2O
(5) (6)
is a significant weight difference in agreement with the model of simple oxygen intercalation/deintercalation (Figure 9). It is still possible, however, that the reduction process of SrCoO3 starts by electron/proton transfer and is followed by a subsequent deintercalation of OHgroups. This could explain the difference between C/C* and T/T* structures. (iii) A different explanation for the high reactivity of the strontium oxocobaltate at room temperature and the formation of an intermediate cubic phase C can be based on specific host defects which exist or can arise in the course of the intercalation process and which would increase the inner surface of the solid, e.g., by evolution of a microdomain structure. It is known that defect perovskite structures have a tendency to form microdomains with increasing temperature or increasing
2238 Chem. Mater., Vol. 8, No. 9, 1996
oxygen content.21-23 It is assumed that phases having the brownmillerite type structures may have domains with a size of 50-500 Å with statistical disorientation which results in an apparent cubic unit cell symmetry if averaging diffraction techniques are used for structure determination.21-23 Highly defective domain walls could then represent diffusion pathways with reduced activation energy for oxygen ionic transport and small diffusion length for the undisturbed bulk regions within the domains. Even if the intrinsic oxygen anion diffusion coefficient is low, rapid apparent reaction kinetics should consequently be possible under these conditions. In those terms phase C (SrCoO2.75) could represent a microdomain disordered pseudocubic phase and the difference between C/C* and T/T* phases could be (21) Grenier, J. C.; Ea, N.; Pouchard, M.; Hagenmuller, P. J. Solid State Chem. 1985, 58, 243. (22) Nakayama, N.; Takano, M.; Inamura, S.; Nakanishi, N.; Kosuge, K. J. Solid State Chem. 1987, 71, 403. (23) Adler, S.; Russek, S.; Reimer, J.; Fendorf, M.; Stacy, A.; Huang, Q.; Santoro, A.; Lynn, J.; Baltisberger, J.; Werner, U. Solid State Ionics 1994, 68, 193.
Nemudry et al.
explained, e.g., as the formation of microdomains with different size and defect structure in the course of oxidation and reduction phases, respectively. It is interesting to note that apparent oxygen ionic transport in reversible systems at room temperature has been observed so far only for oxometallates with structures related to the perovskite and K2NiF4 type. We have been able to demonstrate for the case of strontium oxocobaltate the reversibility of the process, the appearance of intermediate phases and a hysteresis in the redox cycle. Different models have been discussed with respect to an interpretation of the unusual oxygen mobility; a decision in favour of one of these models cannot be made at present, however.
Acknowledgment. This work has been supported by the Bundesministerium fu¨r Forschung und Technologie, Germany. CM950504+
