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Evidence for Oxygen Vacancies in Misfit-Layered Calcium Cobalt Oxide, [CoCa2O3]qCoO2 M. Karppinen,*,† H. Fjellvåg,†,‡ T. Konno,† Y. Morita,† T. Motohashi,† and H. Yamauchi† Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan, and Department of Chemistry, University of Oslo, Blindern, N-0315 Oslo, Norway Received March 25, 2004. Revised Manuscript Received May 5, 2004
We have synthesized samples of the misfit-layered [CoCa2O3]qCoO2 compound with three different oxygen contents and characterized them by both wet-chemical and thermogravimetric analyses for the determination of precise oxygen content, and by synchrotron X-ray diffraction for the precise determination of the magnitude of q. Combining the results from the structural and oxygen-content analyses it is concluded that for samples synthesized in air or a more reducing atmosphere there occur considerable amounts of oxygen vacancies in at least one of the three types of layers, CoO, CaO, and CoO2. Bulk character of the oxygen vacancies was confirmed from transport-property measurements.
Introduction The newly discovered misfit-layered cobalt oxides1,2 are the focus of current attention as many of them were recently revealed to exhibit excellent thermoelectric characteristics, i.e. high electrical conductivity, high Seebeck coefficient, and low thermal conductivity.3-6 These oxides containing hexagonal CoO2 layers coupled incoherently with square-planar metal oxide layers of rock-salt type show incommensurate mismatch along one of the two in-layer crystal axes, i.e., the b axis. The known phases obey a layer sequence of CoO2-AO(MO)m-AO with m ) 1 or 2 (M ) Co, Bi, etc.; A ) Ca, Sr, etc.) and are therefore typically approximated with a chemical formula [MmA2O2+m]qCoO2, where the “misfit parameter” q is determined by the ratio of the mutually incommensurate b axes of the two subsystems, CoO2 and MmA2O2+m, and is obtained from precise structural analysis. For functional oxides in general, oxygen nonstoichiometry is one of the key parameters to control physical properties. For the misfit-layered oxides nonstoichiometry in the overall oxygen content is not only highly possible but rather of an intrinsic nature, i.e., q is noninteger. On the other hand, each individual metal oxide layer has so far been simply assumed to be stoichiometric in terms of oxygen content, i.e., there exist no oxygen vacancies.
The simplest prototype misfit-layered cobalt oxide is found in the Ca-Co-O system. In this system, three ternary compounds, i.e., Ca3Co4O9,7,8 Ca2Co2O5,9 and Ca3Co2O6,7,8,10 are known to exist. These chemical formulas for the three phases all correspond to an average valence of cobalt, V(Co) ) 3.00. The Ca3Co4O9 phase is sometimes referred to as Ca9Co12O28 4 with V(Co) ) 3.17, as an apparent attempt to account for its oxygen nonstoichiometry. Even though the Ca3Co4O9 or Ca9Co12O28 phase had been recognized as a promising thermoelectric material earlier,4 the appropriate misfittype structure expression [CoCa2O3]qCoO2 was first revealed for the same compound in 2000.11,12 From a precise structural analysis based on X-ray and neutron powder diffraction data and a superspace group approach, Miyazaki et al.13 determined the value of q at 0.62, i.e., they revealed a stoichiometry of [CoCa2O3]0.62CoO2 for the compound. Assuming that the different metal oxide layers are individually stoichiometric in terms of oxygen content, V(Co) is calculated at 3.23. Recently, it was revealed that the [CoCa2O3]qCoO2 phase shows large-range variation in its oxygen content depending on the temperature and partial pressure of oxygen.14 However, absolute oxygen contents and corresponding V(Co) values have not been reported for any of the misfit-layered cobalt oxides. Another interesting
* To whom correspondence should be addressed. e-mail: karppinen@ msl.titech.ac.jp. † Tokyo Institute of Technology. ‡ University of Oslo. (1) Boullay, P.; Domenge`s, B.; Hervieu, M.; Groult, D.; Raveau, B. Chem. Mater. 1996, 8, 1482. (2) Boullay, P.; Seshadri, R.; Studer, F.; Hervieu, M.; Groult, D.; Raveau, B. Chem. Mater. 1998, 10, 92. (3) Terasaki, I.; Sasago, Y.; Uchinokura, K. Phys. Rev. B 1997, 56, R12685. (4) Li, S.; Funahashi, R.; Matsubara, I.; Ueno, K.; Yamada, H. J. Mater. Chem. 1999, 9, 1659. (5) Funahashi, R.; Matsubara, I.; Sodeoka, S. Appl. Phys. Lett. 2000, 76, 2385. (6) He´bert, S.; Lambert, S.; Pelloquin, D.; Maignan, A. Phys. Rev. B 2001, 64, 172101.
(7) Brisi, C.; Rolando, P. Ann. Chim. (Rome) 1968, 58, 676. (8) Woermann, E.; Muan, A. J. Inorg. Nucl. Chem. 1970, 32, 1455. (9) Vidyasagar, K.; Gopalakrishnan, J.; Rao, C. N. R. Inorg. Chem. 1984, 23, 1206. (10) Fjellvåg, H.; Gulbrandsen, E.; Aasland, S.; Olsen, A.; Hauback, B. C. J. Solid State Chem. 1996, 124, 190. (11) Masset, A. C.; Michel, C.; Maignan, A.; Hervieu, M.; Toulemonde, O.; Studer, F.; Raveau, B.; Hejtmanek, J. Phys. Rev. B 2000, 62, 166. (12) Miyazaki, Y.; Kudo, K.; Akoshima, M.; Ono, Y.; Koike, Y.; Kajitani, T. Jpn. J. Appl. Phys. 2000, 39, L531. (13) Miyazaki, Y.; Onoda, M.; Oku, T.; Kikuchi, M.; Ishii, Y.; Ono, Y.; Morii, Y.; Kajitani, T. J. Phys. Soc. Jpn. 2002, 71, 491. (14) Shimoyama, J.; Horii, S.; Otzschi, K.; Sano, M.; Kishio, K. Jpn. J. Appl. Phys. 2003, 42, L194.
10.1021/cm049493n CCC: $27.50 © 2004 American Chemical Society Published on Web 06/12/2004
Oxygen Vacancies in Misfit-Layered Calcium Cobalt Oxide
question yet to be clarified is how the oxygen-content variation is reflected in crystal structure and in physical properties. In the present contribution, we show that samples synthesized in air or under more reduced conditions contain significant amounts of oxygen vacancies in one or more of the three types of layers, CoO, CaO, and CoO2, of the [CoCa2O3]qCoO2 phase. This is reached by combining results of precise wet-chemical and thermogravimetric analyses for the oxygen content with information on the crystal lattice as derived from synchrotron X-ray diffraction measurements. Experimental Section Sample Preparation. A single-phase polycrystalline sample of [CoCa2O3]qCoO2 was obtained through solid-state reaction from an appropriate powder mixture of CaCO3 and Co3O4: the mixture was fired twice at 900 °C for 20 h in air, first in powder form and then as a pellet, and after the second firing the sample was furnace-cooled to room temperature. The ratio of Ca/Co for the synthesized sample was determined at 3:3.95(5) by means of inductively coupled-plasma atomic-emission spectroscopy (ICPAES; Seiko Instruments SPS-1500VR) analysis. A portion of this as-air-synthesized Ca3Co3.95O9+δ sample was subjected to postannealing to either increase or decrease the oxygen content. The oxygenation annealing was performed in a 90-atm O2-gas atmosphere (at 400 °C for 24 h), and the deoxygenation was performed in pure N2 gas. The latter treatment was carried out in a thermobalance (MAC Science TG/DTA 2000 S) to thermogravimetrically (TG) follow the oxygen depletion process in situ: a sample of ca. 100 mg was heated with a rate of 2 °C/min to 600 °C, kept isothermally at that temperature for 20 h, and then cooled with a rate of 20 °C/min to room temperature. Oxygen Content Determination. The average valence of cobalt, V(Co), and thereby the overall oxygen content, δ, were determined for all the three Ca3Co3.95O9+δ samples by iodometric titration. For two of the samples, the result was confirmed by cerimetric titration. Both the titration methods are based on dissolution of the sample (10-20 mg) in an acidic solution (1 M HCl solution) and subsequent reduction of CoIII and CoIV with I- (iodometric titration) or Fe2+ (cerimetric titration). The experimental details were the same as described in ref 15. Several parallel experiments confirmed reproducibility of about (0.005 (in terms of the δ value) for each of the methods and showed agreement of better than (0.01 between results obtained by the two titration methods for the same sample. Structural Characterization. High-resolution X-ray powder diffraction data were collected for the three samples at the Swiss-Norwegian Beam Line (BM01) at ESRF in Grenoble. The sample was held in a 0.5-mm rotating borosilicate capillary. Intensity data were collected at 298 K between 2θ ) 1.518 and 46° using a step length of ∆(2θ) ) 0.003°. The wavelength of the X-rays was 0.50018 Å (calibrated by means of a Si standard from NIST). The data were subjected to profile fitting by means of the Jana-program16 system. The background was fitted with a 10-member Legendre polynomial, and the peak shape was described by a Lorentzian function including peak asymmetry and nonisotropic peak broadening assuming a cylindrical crystallite shape. Physical-Property Characterization. To demonstrate that the changes in the oxygen content occur throughout the sample bulk, the samples were additionally characterized by resistivity (F; using a four-point-probe apparatus) and thermoelectric power (S; using a steady-state technique) measurements.17 (15) Karppinen, M.; Matvejeff, M.; Saloma¨ki, K.; Yamauchi, H. J. Mater. Chem. 2002, 12, 1761. (16) Petriceˆk, V.; Dusek, M. JANA2000 Crystallographic Computing System; Inst. Phys. Acad. Sci.: Prague, Czech Republic, 2000.
Chem. Mater., Vol. 16, No. 14, 2004 2791 Table 1. Oxygen Content (δ) and the Average Valence of Cobalt, V(Co), as Determined for Three Ca3Co3.95O9+δ Samples by Two Independent Wet-Chemical Redox Analysis Methods sample
V(Co)
δ
90-atm-O2-annealed as-air-synthesized N2-600 °C-annealed
iodometry: iodometry: cerimetry: iodometry: cerimetry:
0.29(1) 0.24(1) 0.23(1) 0.00(1) 0.00(1)
3.18 3.16 3.04
Results and Discussion The synchrotron X-ray diffraction data confirmed that the three misfit-layered Ca3Co3.95O9+δ samples were of single phase. For the as-air-synthesized sample, several parallel iodometric and cerimetric titration experiments revealed, respectively, δ ) 0.24(1) and 0.23(1). Upon the 90-atm O2 annealing, oxygen content of this sample increased to δ ) 0.29(1) as determined by means of iodometric titration. On the other hand, postannealing of the same sample in N2 gas at 600 °C using a thermobalance resulted in an overall weight loss of 0.67 wt%, thereby reducing the amount of oxygen per “Ca3Co3.95O9+δ formula unit” with ∆δ ) 0.21(3). In agreement with this TG result, both the iodometric and cerimetric titration analyses revealed δ ) 0.00(1) for the N2-annealed sample. It should be noted that in our preliminary TG experiments it was seen that when heated beyond 750 °C in N2, the [CoCa2O3]qCoO2 phase decomposed to yield a mixture of Ca3Co2O6 and CoO, whereas for a yet single-phase sample obtained through deoxygenation at 750 °C, δ was determined at -0.05(1). Therefore, the oxygen content of the present 600 °Cdeoxygenated sample is not exactly, but close to, the minimum oxygen content tolerated for the [CoCa2O3]qCoO2 structure. For the presently studied three samples, the values of δ and V(Co) as calculated from the results of wet-chemical redox analyses are summarized in Table 1: V(Co) varies in the range of 3.04 to 3.18. Selected regions of the synchrotron X-ray diffraction patterns are displayed for the most oxidized and the most reduced sample in Figure 1. The profile fitting of the diffraction data was carried out on the basis of the composite crystal model described by Miyazaki et al.13 The differences in the patterns are small, nevertheless easily recognizable. The lattice parameters, a, b1, b2, c, and β, for the two subsystems, CoO2 (subsystem 1) and CoCa2O3 (subsystem 2), and the misfit parameter, q (≡ b1/b2), are given in Table 2. The parameters for the asair-synthesized sample are very close to those previously reported13 for an O2-synthesized sample (in parentheses): a ) 4.8323 (4.8339) Å, b1 ) 2.8249 (2.8238) Å, b2 ) 4.5657 (4.5582) Å, c ) 10.843 (10.844) Å, β ) 98.198 (98.14)°, and q ) 0.6187 (0.6195). As can be seen from Table 2, on going from the deoxygenated to the oxygenated sample the c axis shrinks whereas the a axis remains unchanged. The b axis parameters for the two subsystems behave oppositely: for the CoO2 subsystem of the composite structure b1 decreases with increasing oxygen content, whereas b2 for the CoCa2O3 subsystem increases. These changes are, however, very small. Therefore, the misfit parameter q varies only slightly (17) Motohashi, T.; et al.; unpublished at Tokyo Institute of Technology, 2004.
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Table 2. Refined Unit Cell Dimensions of the Two Substructures for Three Ca3Co3.95O9+δ Samples of the [CoCa2O3]qCoO2 Phase with Different Oxygen Contentsa sample
compositon from chemical analysis
a [Å]
b1 [Å]
b2 [Å]
c [Å]
β
q
composition from q
90-atm-O2-annealed as-air-synthesized N2-600 °C-annealed
Ca3Co3.95O9.29 Ca3Co3.95O9.24 Ca3Co3.95O9.00
4.8323(1) 4.8323(1) 4.8326(1)
2.8229(1) 2.8249(1) 2.8269(1)
4.5689(1) 4.5657(1) 4.5610(1)
10.841(2) 10.843(2) 10.867(2)
98.156(3) 98.198(3) 98.282(3)
0.6176 0.6187 0.6199
Ca3Co3.928O9.358 Ca3Co3.924O9.348 Ca3Co3.920O9.341
a Also given is the “chemical composition” as calculated from the structurally determined q value with an assumption that no vacancies exist in the individual CaO, CoO, and CoO2 layers.
Figure 2. Thermoelectric power (S) versus temperature for the presently studied two Ca3Co3.95O9+δ samples with δ ≈ 0.24 and 0.29. Additionally shown are data for a δ ≈ 0.07 sample. The inset shows the temperature (T) dependence of resistivity (F) for δ ≈ 0.00, 0.24, and 0.29.
Figure 1. Two selected regions of synchrotron X-ray powder diffraction patterns for Ca3Co3.95O9+δ, δ ≈ 0.00 (upper pattern in both pattern regions) and 0.29 (lower pattern in both pattern regions), of the misfit-layered [CoCa2O3]qCoO2 phase. For the lower-angle part of the diffraction patterns the major reflections of the [CoCa2O3]qCoO2 phase are indicated by their hklm indices; for the higher-angle part the indices are not given because of the many overlapping reflections.
for the three samples, decreasing with increasing oxygen content. Using the q value, we may calculate chemical compositions of the samples by first simply assuming that each individual layer is free of cation and oxygen vacancies. For the sake of comparison, we convert the [CoCa2O3]qCoO2 formulation into the form Ca3Co4-yO9+δ. Results are given in Table 2. From the structural data the Ca/Co ratio is obtained at 3:3.92(1) for all three samples. This ratio is very close to that determined from the ICPAES analysis, i.e. 3:3.95(5). On the other hand, from Tables 1 and 2 it is obvious that the values for oxygen content as derived from the structural data (with the assumption that each layer is free from oxygen vacancies) are shifted from those determined by means of wet-chemical analyses. Furthermore, the small variation in q among the three samples is not enough to
account for the changes in the overall oxygen content as detected by both wet-chemical and TG analyses. This situation is explained only if oxygen vacancies are present in one or both of the two subsystems. Hence, it is plausible that not all the three layers, CoO, CaO, and CoO2, are oxygen stoichiometric. Unfortunately the present analysis does not yield the location of the oxygen vacancies. However, just as the first approximation we suppose that all vacancies are in the CoO layer of the CoCa2O3 block; the vacancy concentration in that layer would just be a few % for the 90-atm-O2-annealed sample, but around 20% for the N2-600 °C-annealed sample, possibly indicating tendency toward CoO4tetrahedral entities. Here a detailed crystal structure analysis is warranted to precisely clarify the position and occupancy of each oxygen atom, primarily based on neutron diffraction. To demonstrate the dependence of transport properties on the overall oxygen content of the sample (as determined by means of chemical analyses), we show in Figure 2 the temperature-dependent thermoelectric power for two (δ ≈ 0.24 and 0.29) of our three samples. Additionally shown are data for a Ca3Co3.95O9+δ sample with δ ≈ 0.07 obtained from the as-air-synthesized sample through deoxygenation in N2 at 500 °C (i.e., at a somewhat lower temperature than the δ ≈ 0.00 sample).17 The inset of Figure 2 depicts the resistivity versus temperature behavior for all the three presently studied samples of δ ≈ 0.00, 0.24, and 0.29. The prominent response of transport properties on the variation in oxygen content (concentration of oxygen vacancies) manifests the fact that the oxygen losses occur within the sample bulk.
Oxygen Vacancies in Misfit-Layered Calcium Cobalt Oxide
Chem. Mater., Vol. 16, No. 14, 2004 2793
In conclusion, we have shown evidence that at least one of the individual layers, CoO, CaO, and CoO2, in the misfit-layered calcium cobalt oxide, [CoCa2O3]qCoO2, is not oxygen stoichiometric but contains considerable amounts of oxygen vacancies at reduced conditions. By turning to high-oxygen-pressure annealing the concentration of oxygen vacancies is reduced below that of airsynthesized samples.
at the Materials and Structures Laboratory, Tokyo Institute of Technology. Financial support was received through a grant from Hayashi Memorial Foundation for Female Natural Scientists and also through Grants-inAid for Scientific Research (15206002 and 15206071) from the Japan Society for the Promotion of Science. The opportunity to collect powder diffraction data at SNBL at ESRF, Grenoble, is gratefully acknowledged.
Acknowledgment. The present work was primarily accomplished during the Visiting Professorship of H.F.
CM049493N
