- Email: [email protected]
Crystal structure and thermoelectric properties of the misfit-layered cobalt oxides
Solid State Ionics 172 (2004) 463 – 467 www.elsevier.com/locate/ssi
Crystal structure and thermoelectric properties of the misfit-layered cobalt oxides Yuzuru Miyazaki * Department of Applied Physics, Graduate School of Engineering, Tohoku University, Aramaki Aoba, Aoba, Sendai 980-8579, Japan Received 9 November 2003; received in revised form 20 January 2004; accepted 24 January 2004
Abstract On the basis of the crystal chemistry and electronic structure of g-NaxCoO2, we explored many cobalt-based oxide systems and encountered the compound ‘‘Ca3Co4O9’’. Through the detailed structure analyses, the cobalt oxide has been revealed to be a misfit-layered compound, which consists of two interpenetrating monoclinic subsystems. The first subsystem [CoO2] has a CdI2-type triangular lattice while the second subsystem [Ca2CoO3] is built up from an ordered three-layered NaCl-type (RS) block. The structure formula can be expressed as [Ca2CoO3]pCoO2, where p stands for the b-axis ratio, bCoO2/bRS f 0.62. A typical polycrystalline sample of the compound exhibits reasonable thermoelectric (TE) performance with thermopower S = 130 AV K 1, resistivity = 15 mV cm and thermal conductivity j = 1 W m 1 K 1 at 300 K. The resulting figure-of-merit Z (S2/qj) = 1.1 10 4 K 1 is comparable to that of polycrystalline samples of the g-phase, indicating that the present misfit cobalt oxide is a potential candidate for a thermoelectric material. D 2004 Elsevier B.V. All rights reserved. PACS: 61.44.Fw; 72.15.Jf Keywords: Thermoelectrics; Misfit-layered compounds; Superspace group; Crystal structure; Modulated structure
1. Introduction Layered cobalt oxides have attracted much attention as candidates for thermoelectric (TE) materials since the discovery of high TE performance in g-NaxCoO2 by Terasaki et al. [1]. In order to determine the essential structural components for the production of superb TE performance, we studied the Ca – Co –O system and discovered that the compound ‘‘Ca3Co4O9’’ exhibits large thermopower S, as well as low electric resistivity q and low thermal conductivity j [2], which are necessary for a good TE material. Through the conjoint structure analyses using electron, Xray and neutron diffraction techniques, we unexpectedly found that the compound has a misfit-layered structure. After this discovery, a number of misfit-layered cobalt oxides have been found and most of them show reasonable TE properties [3 – 12]. Since all the misfit compounds possess a CdI2-type CoO2 triangular lattice in their structures, the structure unit should play an important role for realising large TE power factor, S2/q. In this article, we will briefly review the crystal chemistry and TE properties of the * Tel.: +81-22-217-7971; fax: +81-22-217-7982. E-mail address: [email protected] (Y. Miyazaki). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.01.046
misfit-layered cobalt oxides, mainly focused on those of [Ca2CoO3]pCoO2. A structure classification method is also demonstrated for the better understanding of such complicated incommensurate compounds. 2. Crystal structure of ;-NaxCoO2 In Fig. 1, we show the reexamined crystal structure of gNaxCoO2 [13]. The compound possesses a CdI2-type CoO2 triangular sheet, in which the CoO6 octahedra are rhombohedrally distorted. The most remarkable finding is the splitting of the Na site into Na(1) and Na(2). Each Na site has three equivalent positions and both the Na atoms are statistically distributed over the sites with the occupancies of gNa(1) = 0.21(1) and gNa(2) = 0.49(1), respectively. Numbers in parentheses represent estimated standard deviations of the last significant digits. Then, the structure formula can be expressed as g-Na0.70(1)CoO2 based on the refined Na occupancies. Both the Na atoms form trigonal NaO6 prisms but the Na(1) site is sandwiched by the Co sites with the cation– ˚ , which is much cation (Na(1) – Co) distance of ca. 2.74 A ˚ . In addition, closer than the Na(2) – Co distance of ca. 3.04 A ˚ apart from the Na(2) site. It the Na(1) site is only ca. 1.24 A
464
Y. Miyazaki / Solid State Ionics 172 (2004) 463–467
large thermopower can be also realised near the compositional range because thermopower is roughly expressed as S f (B lnr(E)/BE)E = EF, where r(E) is proportional to the area of DOS at EF. Hence, large thermopower accompanied by low resistivity can be achieved for the compositional region where n+ is between 3.1 and 3.3. In fact, single crystalline g-NaxCoO2 (x = 0.70) exhibits a metallic conduction with low resistivitiy of q f 200 AV cm as well as large thermopower of S f 100 AVK 1 at room temperature, yielding excellent TE power factor of S2/q f 5 10 3 WK 1 m 2 [1].
4. Misfit-layered cobalt oxide [Ca2CoO3]pCoO2 4.1. Crystal structure
Fig. 1. Reexamined crystal structure of g-NaxCoO2 [13].
would be reasonable that such an energetically less-stable Na(1) site has lower occupancy than that of the Na(2) site. Then, the Na vacancies are statistically distributed in the Nalayers. Such disordering of Na and vacancies should be responsible for low n, being necessary for a TE material.
3. Electronic structure Let us consider the electronic states of the CoO2 sheet. As is well known, the 3d orbital of an octahedrally coordinated Co ion splits into doubly degenerated upper eg and triply degenerated lower t2g levels as shown in Fig. 2(a). The t2g levels further split into another doubly degenerated egV levels and a non-degenerated a1g level due to the rhombohedral distortion of the octahedron (Fig. 2(b)). From the charge neutrality, the Co ions are either Co3 + or Co4 + in g-NaxCoO2 (x = 0.70) and their nominal ratio is Co3 +: Co4 + = 3:7. Since the magnetic susceptibility measurement confirmed that both the Co ions are in the low-spin (LS) state, the electronic configuration of each ion is Co3 +(egV)4 (a1g)2 and Co4 +(egV)4(a1g)1, respectively. According to a recent band calculation of g-NaxCoO2 [14], the density of states (DOS) near the Fermi level (EF) consists of the narrow (localised) a1g band and the broad (itinerant) egV band as shown in (Fig. 2(c)). The height of EF depends on the nominal valence state of the Co ions, Con +, which can be controlled by x. For the case of x = 1 (n+ = 3.0), EF is located at the upper edge of the a1g band, and therefore, poor electric conduction is expected. With decreasing x (equivalent to the hole-doping), the n+ parameter gradually increases towards Co4 + and EF crosses the bands where egV and a1g are hybridised. Good electronic conduction is then expected for such a mixed valent compound. In addition,
Since g-NaxCoO2 has a serious problem of the Na+ ion’s diffusion to deteriorate TE performance, we tried to search a new potential cobalt oxide and encountered ‘‘Ca3Co4O9’’. At that time, neither precise structure analyses nor physical properties of the compound were reported. After careful sample preparation, we found that the single-phase metallic composition is Ca/Co = 3.00:3.92(1) when the samples were prepared under 1 atm of O2. The determined oxygen content of 9.34 (2) is also different from the proposed formula, implying that the compound has a rather complicated structure. The structure model was constructed on the basis of our preliminary structure analyses [15] and we assigned the 2.82 ˚ cell (b1) as subsystem 1 and the 4.55 cell (b2) as A subsystem 2. Neutron and X-ray diffraction data were then simultaneously refined using a Rietveld program PREMOS [16], adopting a superspace group of C2/m(1p0)s0, where the first subsystem has C2/m symmetry while the latter has C21/m symmetry. The p parameter, being a b-axis ratio b1/b2 ( u bCoO2/bRS), was refined to be 0.6196(1). In Fig. 3, we show the refined fundamental structure of ‘‘Ca3Co4O9’’. The structure consists of a CdI2-type CoO2 triangular lattice (subsystem 1) and a three-layered rock-salt (RS)-type [Ca2CoO3] block (subsystem 2), stacked along the c-axis.
Fig. 2. Splitting of the 3d energy levels of the Co ion under octahedral coordination of the O ions (a) and that under rhombohedrally distorted CoO6 octahedron (b). The density of states (DOS) near the Fermi surface is schematically shown in (c).
Y. Miyazaki / Solid State Ionics 172 (2004) 463–467
465
Fig. 3. The refined fundamental structure of [Ca2CoO3]0.62CoO2 projected in perspective from the b-axis (left) and parallel to the a-axis (right) [17].
Using the p parameter, the structure formula can be represented as [Ca2CoO3]pCoO2. In the real structure, displacive modulation of the atomic sites occurs through mutual potential interaction between the subsystems. This is not shown in the figure but such a modulation is most significant in the Co – O layer of the RS-type subsystem. Small modulation is also observed in the Ca– O layers and the CoO2 conduction sheets [17].
contribution from the electric term je, the phonon term jph of [Ca2CoO3]0.62CoO2 should be smaller because both the subsystems are incommensurate to each other,
4.2. Thermoelectric properties Since the compound also possesses a CoO2 conduction sheet and the nominal Co valence of Co+ 3.23, good TE properties are expected. In Fig. 4, we show the temperature dependence of q (a), S (b) and j (c) below 300 K. As a reference, corresponding data of a polycrystalline sample of g-Na0.70CoO2 are plotted in each panel [18]. Both the samples show metallic q(T) behaviour but the magnitude of q(300 K) = 15 mV cm, of [Ca2CoO3]0.62CoO2 is five times larger than that of the g-phase, implying a lower carrier concentration. Below 60 K, q(T) of the Ca-phase turns to increase due to the possible occurrence of a spin density wave (SDW) transition [19]. A small kink around 18 K is attributed to a ferrimagnetic transition, which is originated from the inequivalent (modulated) magnetic sublattices of Co ions in the RS-type subsystem. Despite its metallic behaviour, a relatively large positive ( p-type) value of S f 130 AV K 1 is observed at 300 K. This performance surpasses that of the g-phase, more than 1.6 times larger at 300 K and almost twice as large at 100 K. The observed S(T) behaviour, i.e., both magnitude and T-dependence, closely resembles that of a slightly holedoped cuprate system, suggesting that magnetically correlated electrons play an important role for producing large thermopower. We observed qualitatively similar j(T) behaviour to that of the g-phase but the magnitude of the Ca-phase is considerably smaller. For example, j(290 K) of f 1 W m 1 K 1 is approximately 30% smaller than that of the g-phase. Although, we are not yet able to determine the
Fig. 4. Temperature dependence of the resistivity q (a), thermopower S (b) and thermal conductivity j (c) of [Ca2CoO3]0.62CoO2 and those of its reference compound g-Na0.70CoO2.
466
Y. Miyazaki / Solid State Ionics 172 (2004) 463–467
and thus, very few lattice waves can propagate along the c-axis. Using q, S and j values, we can evaluate the TE figureof-merit, Z = S2/qj = 1.1 10 4 K 1 and the dimensionless figure-of-merit ZT = 0.035 at 300 K, which are approximately equal to the polycrystalline samples of g-Na0.70CoO2. Among the TE parameters, only the q value is unacceptably large for the present compound and it must be lowered for the practical application. In order to reduce q while maintaining S and j, we are currently studying the carrier-doping effect into the RS-type [Ca2CoO3] subsystem by a partial substitution of the Ca ions.
5. Structure classification method Up to now, similar misfit cobalt oxides have been discovered in the Bi –Sr – Co –O [3,4], Tl– Sr –Co – O [5], Pb – Sr – Co –O [6] and Ca – Co –Cu – O [7] systems and all the compounds show good TE properties. However, due to the lack of a global classification method, the hithertodiscovered misfit cobalt oxides have been expressed in different ways. For example, the compound [Ca2CoO3]p CoO2 is expressed as Ca3Co4O9 [8], Ca9Co12O28 [9], Ca2Co2O5 [10], etc. As for the Bi-based cobalt oxides, numerous formulas, such as Bi2Sr3Co2O9 [11] and Bi2Sr2 Co2Ox [12], were used to express the identical phase. These inconsistencies are derived from the difficulty in describing such incommensurate compounds by the rational chemical formulas. Inconveniently, these cobalt oxides change their composition by the possible partial substitution and/or their nonstoichiometry, because the chemical formula directly depends on the b-axis ratio, p = bCoO2/bRS. All the misfit cobalt oxides discovered so far contain alkali-earth elements and other metallic element(s) in the RS-type subsystem. For comprehensive understanding, it is more convenient to describe the compound as [A2MxO2 + x]pCoO2, where A and M stand for alkali-earth and metallic atoms. Toward the uniform treatment of these misfit cobalt oxides, we propose to describe the compounds as ‘‘Mijk’’, where the integers i, j and k respectively express the numbers of inner M layers, outer A layers and CoO2 sheets in the unit stacking sequence along the c-axis. In Fig. 5, we schematically show the concept of our structure classification method. According to the method, three-layered [Ca2CoO3]pCoO2 and [Sr2Tl1 xO3]pCoO2 can be simply denoted as the Co-121 and Tl-121 phases, respectively. Similarly, four-layered [Sr2Bi2 xO4]pCoO2 can be expressed as the Bi-221 phase. If necessary, the p value should be attached next to the notation as Co-121 ( p = 0.62), and if the compound has a certain amount of the substituted element, the notation would be Bi(Pb)-221 and so on. For a more complicated substituted case, it would be helpful to use the complete description. Based on the method, the newly discovered [Ca2(Co0.65Cu0.35)2O4]pCoO2 can be denoted as the Co(Cu)-221 phase [7].
Fig. 5. Schematic drawings of our structure classification method. Using the ‘‘M-ijk’’ notation, the three-layered (left) and four-layered (right) RS-type compounds can be denoted as M-121 and M-221, respectively.
Other families, e.g., M-321 or M-021 phases, might be discovered by appropriately selecting the charge, size and electronic configuration of the ions. The former phase would be advantageous to reduce jph. To maintain the n+ value around + 3.2, the RS-type [A2M3O5] subsystem must have a valence around + 1.3 in the case of p f 0.6. A mixture of divalent and trivalent ions for the A and M sites would be possible for such a BL but usually results in the formation of perovskite phases. To search for a new phase in the missing link would be intriguing for further understanding the crystal chemistry of the misfit-layered compounds as well as discovering excellent TE materials. 6. Summary We have reviewed the crystal chemistry and thermoelectric properties of [Ca2CoO3]pCoO2 (Co-121) and related misfit-layered cobalt oxides. The misfit structure can be regarded as a natural superlattice, in which two subsystems have completely different roles. The CoO2 subsystem is responsible for realising large thermoelectric power factor while the disordered rock salt-type subsystem significantly lowers the lattice thermal conductivity. By a suitable chemical modification, we believe that the thermoelectric performance of the compounds can be further improved. Next our target would be a search for a new n-type misfit compound which has comparable thermoelectric performance with the p-type misfit cobaltites. Acknowledgements The author appreciates M. Onoda, A. Yamamoto, T. Kajitani, Y. Koike, M. Akoshima, K. Kudo, T. Oku, M. Kikuchi, Y. Ishii, Y. Shimojo, T. Miura, Y. Suzuki, Y. Ono and S. Kawano for their help. This work was supported, in part, by the Research Foundation for Materials Science and by CREST, JST. References [1] I. Terasaki, Y. Sasago, K. Uchinokura, Phys. Rev., B 56 (1997) R12685.
Y. Miyazaki / Solid State Ionics 172 (2004) 463–467 [2] Y. Miyazaki, K. Kudo, M. Akoshima, Y. Ono, Y. Koike, T. Kajitani, Jpn. J. Appl. Phys. 39 (2000) L531. [3] T. Itoh, T. Kawata, T. Kitajima, I. Terasaki, Proc. 17th Int. Conf. Thermoelectrics (ICT ’98), IEEE, Piscataway, 1999, p. 595. [4] H. Leligny, D. Grebille, O. Pe´rez, A.C. Masset, M. Hervieu, C. Michel, B. Raveau, C. R. Acad. Sci., Paris IIc. 2 (1999) 409. [5] S. He´bert, S. Lambert, D. Pelloquin, A. Maignan, Phys. Rev., B 64 (2001) 172101. [6] D. Pelloquin, A. Maignan, S. He´bert, C. Martin, M. Hervieu, C. Michel, L.B. Wang, B. Raveau, Chem. Mater. 14 (2002) 3100. [7] Y. Miyazaki, T. Miura, Y. Ono, T. Kajitani, Jpn. J. Appl. Phys. 41 (2002) L849. [8] A.C. Masset, C. Michel, A. Maignan, M. Hervieu, O. Toulemomde, F. Atuder, B. Raveau, J. Hejtmanek, Phys. Rev., B 62 (2000) 166. [9] S. Li, R. Funahashi, I. Matsubara, K. Ueno, H. Yamada, J. Mater. Chem. 9 (1999) 1659. [10] S. Li, R. Funahashi, I. Matsubara, K. Ueno, S. Sodeoka, H. Yamada, Chem. Mater. 12 (2000) 2424.
467
[11] J.M. Tarascon, R. Ramesh, P. Barboux, M.S. Hedge, G.W. Hull, L.H. Greene, M. Giroud, Y. LePage, W.R. McKinnon, J.V. Waszczak, L.F. Schneemeyer, Solid State Commun. 71 (1989) 663. [12] R. Funahashi, I. Matsubara, S. Sodeoka, Appl. Phys. Lett. 76 (2000) 2385. [13] Y. Ono, R. Ishikawa, Y. Miyazaki, T. Kajitani, J. Phys. Soc. Jpn. 70 (Suppl. A) (2001) 235. [14] D.J. Singh, Phys. Rev., B 61 (2000) 13397. [15] Y. Miyazaki, M. Onoda, T. Oku, M. Kikuchi, Y. Ishii, Y. Ono, Y. Morii, T. Kajitani, J. Phys. Soc. Jpn. 71 (2002) 491. [16] A. Yamamoto, Acta Crystallogr. A49 (1993) 831. [17] Y. Miyazaki, T. Miura, Y. Ono, T. Kajitani, J. Jpn. Soc. Powder Poder Metall. 50 (2003) 475 (in japanese). [18] R. Ishikawa, Master thesis, Tohoku University, 2002. [19] J. Sugiyama, H. Itahara, T. Tani, J.H. Brewer, E.J. Ansaldo, Phys. Rev. B 66 (2002) 134413.
Crystal structure and thermoelectric properties of the misfit-layered cobalt oxides Yuzuru Miyazaki * Department of Applied Physics, Graduate School of Engineering, Tohoku University, Aramaki Aoba, Aoba, Sendai 980-8579, Japan Received 9 November 2003; received in revised form 20 January 2004; accepted 24 January 2004
Abstract On the basis of the crystal chemistry and electronic structure of g-NaxCoO2, we explored many cobalt-based oxide systems and encountered the compound ‘‘Ca3Co4O9’’. Through the detailed structure analyses, the cobalt oxide has been revealed to be a misfit-layered compound, which consists of two interpenetrating monoclinic subsystems. The first subsystem [CoO2] has a CdI2-type triangular lattice while the second subsystem [Ca2CoO3] is built up from an ordered three-layered NaCl-type (RS) block. The structure formula can be expressed as [Ca2CoO3]pCoO2, where p stands for the b-axis ratio, bCoO2/bRS f 0.62. A typical polycrystalline sample of the compound exhibits reasonable thermoelectric (TE) performance with thermopower S = 130 AV K 1, resistivity = 15 mV cm and thermal conductivity j = 1 W m 1 K 1 at 300 K. The resulting figure-of-merit Z (S2/qj) = 1.1 10 4 K 1 is comparable to that of polycrystalline samples of the g-phase, indicating that the present misfit cobalt oxide is a potential candidate for a thermoelectric material. D 2004 Elsevier B.V. All rights reserved. PACS: 61.44.Fw; 72.15.Jf Keywords: Thermoelectrics; Misfit-layered compounds; Superspace group; Crystal structure; Modulated structure
1. Introduction Layered cobalt oxides have attracted much attention as candidates for thermoelectric (TE) materials since the discovery of high TE performance in g-NaxCoO2 by Terasaki et al. [1]. In order to determine the essential structural components for the production of superb TE performance, we studied the Ca – Co –O system and discovered that the compound ‘‘Ca3Co4O9’’ exhibits large thermopower S, as well as low electric resistivity q and low thermal conductivity j [2], which are necessary for a good TE material. Through the conjoint structure analyses using electron, Xray and neutron diffraction techniques, we unexpectedly found that the compound has a misfit-layered structure. After this discovery, a number of misfit-layered cobalt oxides have been found and most of them show reasonable TE properties [3 – 12]. Since all the misfit compounds possess a CdI2-type CoO2 triangular lattice in their structures, the structure unit should play an important role for realising large TE power factor, S2/q. In this article, we will briefly review the crystal chemistry and TE properties of the * Tel.: +81-22-217-7971; fax: +81-22-217-7982. E-mail address: [email protected] (Y. Miyazaki). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.01.046
misfit-layered cobalt oxides, mainly focused on those of [Ca2CoO3]pCoO2. A structure classification method is also demonstrated for the better understanding of such complicated incommensurate compounds. 2. Crystal structure of ;-NaxCoO2 In Fig. 1, we show the reexamined crystal structure of gNaxCoO2 [13]. The compound possesses a CdI2-type CoO2 triangular sheet, in which the CoO6 octahedra are rhombohedrally distorted. The most remarkable finding is the splitting of the Na site into Na(1) and Na(2). Each Na site has three equivalent positions and both the Na atoms are statistically distributed over the sites with the occupancies of gNa(1) = 0.21(1) and gNa(2) = 0.49(1), respectively. Numbers in parentheses represent estimated standard deviations of the last significant digits. Then, the structure formula can be expressed as g-Na0.70(1)CoO2 based on the refined Na occupancies. Both the Na atoms form trigonal NaO6 prisms but the Na(1) site is sandwiched by the Co sites with the cation– ˚ , which is much cation (Na(1) – Co) distance of ca. 2.74 A ˚ . In addition, closer than the Na(2) – Co distance of ca. 3.04 A ˚ apart from the Na(2) site. It the Na(1) site is only ca. 1.24 A
464
Y. Miyazaki / Solid State Ionics 172 (2004) 463–467
large thermopower can be also realised near the compositional range because thermopower is roughly expressed as S f (B lnr(E)/BE)E = EF, where r(E) is proportional to the area of DOS at EF. Hence, large thermopower accompanied by low resistivity can be achieved for the compositional region where n+ is between 3.1 and 3.3. In fact, single crystalline g-NaxCoO2 (x = 0.70) exhibits a metallic conduction with low resistivitiy of q f 200 AV cm as well as large thermopower of S f 100 AVK 1 at room temperature, yielding excellent TE power factor of S2/q f 5 10 3 WK 1 m 2 [1].
4. Misfit-layered cobalt oxide [Ca2CoO3]pCoO2 4.1. Crystal structure
Fig. 1. Reexamined crystal structure of g-NaxCoO2 [13].
would be reasonable that such an energetically less-stable Na(1) site has lower occupancy than that of the Na(2) site. Then, the Na vacancies are statistically distributed in the Nalayers. Such disordering of Na and vacancies should be responsible for low n, being necessary for a TE material.
3. Electronic structure Let us consider the electronic states of the CoO2 sheet. As is well known, the 3d orbital of an octahedrally coordinated Co ion splits into doubly degenerated upper eg and triply degenerated lower t2g levels as shown in Fig. 2(a). The t2g levels further split into another doubly degenerated egV levels and a non-degenerated a1g level due to the rhombohedral distortion of the octahedron (Fig. 2(b)). From the charge neutrality, the Co ions are either Co3 + or Co4 + in g-NaxCoO2 (x = 0.70) and their nominal ratio is Co3 +: Co4 + = 3:7. Since the magnetic susceptibility measurement confirmed that both the Co ions are in the low-spin (LS) state, the electronic configuration of each ion is Co3 +(egV)4 (a1g)2 and Co4 +(egV)4(a1g)1, respectively. According to a recent band calculation of g-NaxCoO2 [14], the density of states (DOS) near the Fermi level (EF) consists of the narrow (localised) a1g band and the broad (itinerant) egV band as shown in (Fig. 2(c)). The height of EF depends on the nominal valence state of the Co ions, Con +, which can be controlled by x. For the case of x = 1 (n+ = 3.0), EF is located at the upper edge of the a1g band, and therefore, poor electric conduction is expected. With decreasing x (equivalent to the hole-doping), the n+ parameter gradually increases towards Co4 + and EF crosses the bands where egV and a1g are hybridised. Good electronic conduction is then expected for such a mixed valent compound. In addition,
Since g-NaxCoO2 has a serious problem of the Na+ ion’s diffusion to deteriorate TE performance, we tried to search a new potential cobalt oxide and encountered ‘‘Ca3Co4O9’’. At that time, neither precise structure analyses nor physical properties of the compound were reported. After careful sample preparation, we found that the single-phase metallic composition is Ca/Co = 3.00:3.92(1) when the samples were prepared under 1 atm of O2. The determined oxygen content of 9.34 (2) is also different from the proposed formula, implying that the compound has a rather complicated structure. The structure model was constructed on the basis of our preliminary structure analyses [15] and we assigned the 2.82 ˚ cell (b1) as subsystem 1 and the 4.55 cell (b2) as A subsystem 2. Neutron and X-ray diffraction data were then simultaneously refined using a Rietveld program PREMOS [16], adopting a superspace group of C2/m(1p0)s0, where the first subsystem has C2/m symmetry while the latter has C21/m symmetry. The p parameter, being a b-axis ratio b1/b2 ( u bCoO2/bRS), was refined to be 0.6196(1). In Fig. 3, we show the refined fundamental structure of ‘‘Ca3Co4O9’’. The structure consists of a CdI2-type CoO2 triangular lattice (subsystem 1) and a three-layered rock-salt (RS)-type [Ca2CoO3] block (subsystem 2), stacked along the c-axis.
Fig. 2. Splitting of the 3d energy levels of the Co ion under octahedral coordination of the O ions (a) and that under rhombohedrally distorted CoO6 octahedron (b). The density of states (DOS) near the Fermi surface is schematically shown in (c).
Y. Miyazaki / Solid State Ionics 172 (2004) 463–467
465
Fig. 3. The refined fundamental structure of [Ca2CoO3]0.62CoO2 projected in perspective from the b-axis (left) and parallel to the a-axis (right) [17].
Using the p parameter, the structure formula can be represented as [Ca2CoO3]pCoO2. In the real structure, displacive modulation of the atomic sites occurs through mutual potential interaction between the subsystems. This is not shown in the figure but such a modulation is most significant in the Co – O layer of the RS-type subsystem. Small modulation is also observed in the Ca– O layers and the CoO2 conduction sheets [17].
contribution from the electric term je, the phonon term jph of [Ca2CoO3]0.62CoO2 should be smaller because both the subsystems are incommensurate to each other,
4.2. Thermoelectric properties Since the compound also possesses a CoO2 conduction sheet and the nominal Co valence of Co+ 3.23, good TE properties are expected. In Fig. 4, we show the temperature dependence of q (a), S (b) and j (c) below 300 K. As a reference, corresponding data of a polycrystalline sample of g-Na0.70CoO2 are plotted in each panel [18]. Both the samples show metallic q(T) behaviour but the magnitude of q(300 K) = 15 mV cm, of [Ca2CoO3]0.62CoO2 is five times larger than that of the g-phase, implying a lower carrier concentration. Below 60 K, q(T) of the Ca-phase turns to increase due to the possible occurrence of a spin density wave (SDW) transition [19]. A small kink around 18 K is attributed to a ferrimagnetic transition, which is originated from the inequivalent (modulated) magnetic sublattices of Co ions in the RS-type subsystem. Despite its metallic behaviour, a relatively large positive ( p-type) value of S f 130 AV K 1 is observed at 300 K. This performance surpasses that of the g-phase, more than 1.6 times larger at 300 K and almost twice as large at 100 K. The observed S(T) behaviour, i.e., both magnitude and T-dependence, closely resembles that of a slightly holedoped cuprate system, suggesting that magnetically correlated electrons play an important role for producing large thermopower. We observed qualitatively similar j(T) behaviour to that of the g-phase but the magnitude of the Ca-phase is considerably smaller. For example, j(290 K) of f 1 W m 1 K 1 is approximately 30% smaller than that of the g-phase. Although, we are not yet able to determine the
Fig. 4. Temperature dependence of the resistivity q (a), thermopower S (b) and thermal conductivity j (c) of [Ca2CoO3]0.62CoO2 and those of its reference compound g-Na0.70CoO2.
466
Y. Miyazaki / Solid State Ionics 172 (2004) 463–467
and thus, very few lattice waves can propagate along the c-axis. Using q, S and j values, we can evaluate the TE figureof-merit, Z = S2/qj = 1.1 10 4 K 1 and the dimensionless figure-of-merit ZT = 0.035 at 300 K, which are approximately equal to the polycrystalline samples of g-Na0.70CoO2. Among the TE parameters, only the q value is unacceptably large for the present compound and it must be lowered for the practical application. In order to reduce q while maintaining S and j, we are currently studying the carrier-doping effect into the RS-type [Ca2CoO3] subsystem by a partial substitution of the Ca ions.
5. Structure classification method Up to now, similar misfit cobalt oxides have been discovered in the Bi –Sr – Co –O [3,4], Tl– Sr –Co – O [5], Pb – Sr – Co –O [6] and Ca – Co –Cu – O [7] systems and all the compounds show good TE properties. However, due to the lack of a global classification method, the hithertodiscovered misfit cobalt oxides have been expressed in different ways. For example, the compound [Ca2CoO3]p CoO2 is expressed as Ca3Co4O9 [8], Ca9Co12O28 [9], Ca2Co2O5 [10], etc. As for the Bi-based cobalt oxides, numerous formulas, such as Bi2Sr3Co2O9 [11] and Bi2Sr2 Co2Ox [12], were used to express the identical phase. These inconsistencies are derived from the difficulty in describing such incommensurate compounds by the rational chemical formulas. Inconveniently, these cobalt oxides change their composition by the possible partial substitution and/or their nonstoichiometry, because the chemical formula directly depends on the b-axis ratio, p = bCoO2/bRS. All the misfit cobalt oxides discovered so far contain alkali-earth elements and other metallic element(s) in the RS-type subsystem. For comprehensive understanding, it is more convenient to describe the compound as [A2MxO2 + x]pCoO2, where A and M stand for alkali-earth and metallic atoms. Toward the uniform treatment of these misfit cobalt oxides, we propose to describe the compounds as ‘‘Mijk’’, where the integers i, j and k respectively express the numbers of inner M layers, outer A layers and CoO2 sheets in the unit stacking sequence along the c-axis. In Fig. 5, we schematically show the concept of our structure classification method. According to the method, three-layered [Ca2CoO3]pCoO2 and [Sr2Tl1 xO3]pCoO2 can be simply denoted as the Co-121 and Tl-121 phases, respectively. Similarly, four-layered [Sr2Bi2 xO4]pCoO2 can be expressed as the Bi-221 phase. If necessary, the p value should be attached next to the notation as Co-121 ( p = 0.62), and if the compound has a certain amount of the substituted element, the notation would be Bi(Pb)-221 and so on. For a more complicated substituted case, it would be helpful to use the complete description. Based on the method, the newly discovered [Ca2(Co0.65Cu0.35)2O4]pCoO2 can be denoted as the Co(Cu)-221 phase [7].
Fig. 5. Schematic drawings of our structure classification method. Using the ‘‘M-ijk’’ notation, the three-layered (left) and four-layered (right) RS-type compounds can be denoted as M-121 and M-221, respectively.
Other families, e.g., M-321 or M-021 phases, might be discovered by appropriately selecting the charge, size and electronic configuration of the ions. The former phase would be advantageous to reduce jph. To maintain the n+ value around + 3.2, the RS-type [A2M3O5] subsystem must have a valence around + 1.3 in the case of p f 0.6. A mixture of divalent and trivalent ions for the A and M sites would be possible for such a BL but usually results in the formation of perovskite phases. To search for a new phase in the missing link would be intriguing for further understanding the crystal chemistry of the misfit-layered compounds as well as discovering excellent TE materials. 6. Summary We have reviewed the crystal chemistry and thermoelectric properties of [Ca2CoO3]pCoO2 (Co-121) and related misfit-layered cobalt oxides. The misfit structure can be regarded as a natural superlattice, in which two subsystems have completely different roles. The CoO2 subsystem is responsible for realising large thermoelectric power factor while the disordered rock salt-type subsystem significantly lowers the lattice thermal conductivity. By a suitable chemical modification, we believe that the thermoelectric performance of the compounds can be further improved. Next our target would be a search for a new n-type misfit compound which has comparable thermoelectric performance with the p-type misfit cobaltites. Acknowledgements The author appreciates M. Onoda, A. Yamamoto, T. Kajitani, Y. Koike, M. Akoshima, K. Kudo, T. Oku, M. Kikuchi, Y. Ishii, Y. Shimojo, T. Miura, Y. Suzuki, Y. Ono and S. Kawano for their help. This work was supported, in part, by the Research Foundation for Materials Science and by CREST, JST. References [1] I. Terasaki, Y. Sasago, K. Uchinokura, Phys. Rev., B 56 (1997) R12685.
Y. Miyazaki / Solid State Ionics 172 (2004) 463–467 [2] Y. Miyazaki, K. Kudo, M. Akoshima, Y. Ono, Y. Koike, T. Kajitani, Jpn. J. Appl. Phys. 39 (2000) L531. [3] T. Itoh, T. Kawata, T. Kitajima, I. Terasaki, Proc. 17th Int. Conf. Thermoelectrics (ICT ’98), IEEE, Piscataway, 1999, p. 595. [4] H. Leligny, D. Grebille, O. Pe´rez, A.C. Masset, M. Hervieu, C. Michel, B. Raveau, C. R. Acad. Sci., Paris IIc. 2 (1999) 409. [5] S. He´bert, S. Lambert, D. Pelloquin, A. Maignan, Phys. Rev., B 64 (2001) 172101. [6] D. Pelloquin, A. Maignan, S. He´bert, C. Martin, M. Hervieu, C. Michel, L.B. Wang, B. Raveau, Chem. Mater. 14 (2002) 3100. [7] Y. Miyazaki, T. Miura, Y. Ono, T. Kajitani, Jpn. J. Appl. Phys. 41 (2002) L849. [8] A.C. Masset, C. Michel, A. Maignan, M. Hervieu, O. Toulemomde, F. Atuder, B. Raveau, J. Hejtmanek, Phys. Rev., B 62 (2000) 166. [9] S. Li, R. Funahashi, I. Matsubara, K. Ueno, H. Yamada, J. Mater. Chem. 9 (1999) 1659. [10] S. Li, R. Funahashi, I. Matsubara, K. Ueno, S. Sodeoka, H. Yamada, Chem. Mater. 12 (2000) 2424.
467
[11] J.M. Tarascon, R. Ramesh, P. Barboux, M.S. Hedge, G.W. Hull, L.H. Greene, M. Giroud, Y. LePage, W.R. McKinnon, J.V. Waszczak, L.F. Schneemeyer, Solid State Commun. 71 (1989) 663. [12] R. Funahashi, I. Matsubara, S. Sodeoka, Appl. Phys. Lett. 76 (2000) 2385. [13] Y. Ono, R. Ishikawa, Y. Miyazaki, T. Kajitani, J. Phys. Soc. Jpn. 70 (Suppl. A) (2001) 235. [14] D.J. Singh, Phys. Rev., B 61 (2000) 13397. [15] Y. Miyazaki, M. Onoda, T. Oku, M. Kikuchi, Y. Ishii, Y. Ono, Y. Morii, T. Kajitani, J. Phys. Soc. Jpn. 71 (2002) 491. [16] A. Yamamoto, Acta Crystallogr. A49 (1993) 831. [17] Y. Miyazaki, T. Miura, Y. Ono, T. Kajitani, J. Jpn. Soc. Powder Poder Metall. 50 (2003) 475 (in japanese). [18] R. Ishikawa, Master thesis, Tohoku University, 2002. [19] J. Sugiyama, H. Itahara, T. Tani, J.H. Brewer, E.J. Ansaldo, Phys. Rev. B 66 (2002) 134413.