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High temperature electrical conductivity and thermoelectric power of NaxCoO2
Solid State Ionics 179 (2008) 2308–2312
Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s s i
High temperature electrical conductivity and thermoelectric power of NaxCoO2 Pusheng Liu a, Gang Chen a,⁎, Ying Cui a, Hongjie Zhang a, Feng Xiao a, Lin Wang a, Hiromi Nakano b a b
Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, People's Republic of China Electron Microscope Laboratory and High-Tech Research Center, Ryukoku University, Seta, Otsu, 520-2194, Japan
a r t i c l e
i n f o
Article history: Received 24 January 2008 Received in revised form 6 August 2008 Accepted 14 August 2008 Keywords: Transport properties Sodium ions Structure refinement
a b s t r a c t Layered cobalt oxides, NaxCoO2, with the density of sodium ions, x, ranging from 0.65 to 0.85, were prepared by rapid heat-up method. The determination by X-ray diffraction pattern of crystal structure and the grain morphology studied by scanning electron microscopy were reported. The measurements of electrical conductivity and thermoelectric power were carried out between 300 K and 1100 K. It has been found that the concentration of sodium ions sandwiched between two neighboring CoO2 layers plays a crucial role in transport properties. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Layered cobalt oxides, NaxCoO2, display several interesting properties with the variation of the concentration of sodium ions x. These properties include a superconducting phase of Na0.35CoO2 when it is intercalated by water molecules around 5 K [1], a charge-ordered insulating phase of Na0.5CoO2 [2] and potential applications of the composition(x ~ 0.70)in thermoelectric energy conversion field [3]. Since the discovery of moderately large thermoelectric power (Seebeck coefficient) together with high electrical conductivity in NaxCoO2(x ~ 0.70), tremendous experiments were done to search new phases for thermoelectric conversion applications. Although they simultaneously possess large thermoelectric power, high electrical conductivity and low thermal conductivity, the figure of merit was found to be too small to utilize in the practical thermoelectric conversion devices in this x range(x ~ 0.7). While in the x range of 0.7– 0.85, high temperature electrical conductivity and thermoelectric power are poorly reported [4,5]. These layered oxides consist of two layers: CoO2 layer and Na ions layer. CoO2 layers acting as an electron reservoir are responsible for the electrical conductivity and large thermoelectric power, while Na ions layer sandwiched between two neighboring CoO2 layers working as a structural unit directly adjust the concentration of electron in CoO2 layers and decrease the thermal conductivity along the stacking
⁎ Corresponding author. Tel./fax: +86 451 86 413753. E-mail address: [email protected] (G. Chen). 0167-2738/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2008.08.010
direction c [6,7]. Recently, neutron diffraction and electron diffraction measurements have shown that the crystal structure of the oxides is strongly dependent on sodium content [8–10]. Here we use X-ray diffraction pattern supported by Rietveld refinement to study the crystal structure at ambient temperature, the electrical conductivity and thermoelectric power have been systematically studied for the composition (x = 0.65–0.85) between 300 K and 1100 K. 2. Experimental method Polycrystalline samples of NaxCoO2 with various sodium content were prepared from Na2CO3 (99.99%) and Co3O4 (99.98%) by rapid heat-up method. The detailed procedure can be found elsewhere [11]. Due to the volatility of sodium element at high temperature, an extra amount of Na2CO3 was added. Taking the preparation of Na0.78CoO2 as a sample, Na2CO3 and Co3O4 in amounts according to Na0.80CoO2 were mixed together by ball milling for 24 h, and then the samples were directly put into the furnace at 1023 K for 12 h in air. After grinding, the powders were pressed into the pellet with the diameter of 15 mm and fired again in the furnace at 1023 K for another 12 h in the O2 flow. X-ray diffraction data were collected at ambient temperature from 5° to 100° with a step of 0.05° on Rigaku D/max diffractometer working with Cu Kα radiation. The sodium content was determined by means of inductively coupled plasma optical emission spectrometers (ICP-OES, Perkin Elmer, Optima 5300DV). Energy dispersive X-ray (EDX) spectroscopy implemented in a transmission electron microscope (TEM, JEM2000EX) was used to analyze the chemical composition of some of the prepared samples. The particles morphology was studied by scanning electron microscopy (SEM, Hitachi S4800). The electrical conductivity and thermoelectric power were measured using a laboratory-designed apparatus in the temperature range of 300 K to 1100 K in air. The
P. Liu et al. / Solid State Ionics 179 (2008) 2308–2312
electrical conductivity of the bar-shaped sample (20 × 4.5 × 2 mm3) was measured by the four probe method. For the measurement of the thermoelectric power, Thermo-electromotive force measured as a function of temperature gives a straight line and its slope is thermoelectric power. The detailed procedure is described below. The sample with a pair of Pt/Rh thermocouples attached to two ends (bar-like samples) was first heated to a certain temperature, and then one of the ends was heated by an extra source to produce the temperature gradient. The temperature and voltage signals were collected by a commercial data acquisition system (Keithley 2700, Keithley Instruments Inc., America). 3. Results and discussion Fig. 1 shows X-ray diffraction patterns of the as-synthesized NaxCoO2 (x = 0.65–0.85). All the patterns are in good agreement with hexagonal γ-NaxCoO2 phase when x ranging from 0.65 to 0.80. For x = 0.85 composition, a slight trace of the unreacted Na2CO3 was detected in X-ray diffraction pattern (marked by the small up-triangle symbol in Fig. 1). The refined crystallographic cell parameters for each sample are calculated on the basis of the X-ray diffraction patterns, as listed in Table 1. The lattice parameters a and c are found to exhibit a reverse behavior with increasing sodium content, while the cell volumes almost stay the same. The increase of crystallographic a, varying in a manner consistent with sodium content, may be caused by gradually increasing coulomb repulsion of sodium ions in the same layer. The shrinkage of lattice parameter c may result from an increasing coulomb attraction between CoO2 layers and sandwiched Na ions layer. X-ray diffraction patterns were fitted by least square method employing the software GSAS&EXPGUI [12,13]. Fig. 2 gives a fitting pattern for Na0.78CoO2 sample. Since the grain is easily oriented for this synthesis method, some of the reflections, such as (002), are refined in the process of the fitting. To demonstrate the preferred orientation of the grains, SEM was used to study the micromorphology of the composition (x = 0.78), as illustrated in Fig. 2. It can be seen from the panel on the left hand that the grains show hexagonal architecture and the panel on the right hand shows the sheet-like transect configuration. For the refinement of sodium
Fig. 1. Powder X-ray diffraction patterns of the as-synthesized NaxCoO2 (x = 0.65–0.85). The small up-triangle symbol denotes the unreacted Na2CO3. We have used rapid heatup method to prepare x = 0.60 or below, but single phase cannot be obtained.
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Table 1 Crystallographic cell parameters of NaxCoO2 (x = 0.65–0.85) x
a (Å)
b (Å)
0.65 0.70 0.75 0.78 0.80 0.85
2.828 (1) 2.830 (3) 2.831 (1) 2.831 (2) 2.833 (3) 2.835 (5)
2.828 2.830 2.831 2.831 2.833 2.835
(1) (3) (1) (2) (3) (5)
c (Å)
V (Å3)
10.953 (3) 10.950 (2) 10.946 (4) 10.944 (1) 10.935 (1) 10.933 (2)
75.88 (4) 75.96 (2) 76.00 (1) 76.00 (5) 76.01 (6) 76.03 (3)
fractional occupancy (occ.), the summed occupancy value should be made consistent with the known composition of Na ions, i.e. occ.Na(1) + occ.Na(2) = x, and then the occupancy for each position was allow to refine. The crystal parameters of Na0.78CoO2 at ambient temperature are given in Table 2. The sodium content was determined using ICP-OES method by several batches of samples. It is found that the volatility of sodium element cannot be ignored when samples fired at 1023 K. In the present study, an extra amount of Na2CoO3 was added to complement the sodium loss. The measured results of ICP-OES method are shown in Table 3. Fig. 3 presents curves of the electrical conductivity vs. temperature for the as-synthesized composition (x = 0.65–0.85). We note that all the samples show metallic conduction behavior in the range measured. The magnitude of the electrical conductivity at ambient temperature is between 300–650 S/cm. It is found that the electrical conductivity is strongly dependent on the concentration of sodium ions x. In the x region of 0.65–0.78, the electrical conductivity undergoes a monotonic enhancement with increasing sodium content, when x reaches 0.78, the sample displays the best electrical conductive behavior. But for the samples with high sodium content, i.e. x ranging from 0.80 to 0.85, the electrical conductivity shows a reverse behavior, dropping to 300 S/cm for the composition (x = 0.85). The electrical conductivity initially increases as the amount of sodium increase up to 0.78. The same behaviors have been reported for the composition (0.55–0.75) by Motohashi et al. in the low temperature range [11]. As discussed in the introduction part, the density of electron in the CoO2 layer, which is critical to the electrical conductivity, can be altered by the sandwiched sodium content. It can be concluded that the ratio of Co3+ against Co4+ increase with increasing sodium content, via the simple calculation of chemical valence. Meanwhile the distortion of CoO2 layer is severe with the shrinkage of c axis. So the density of electron in the reservoir layer together with the distortion of CoO2 plays a crucial role in electrical conductivity. Fig. 4 gives transmission electron microscope (TEM) and X-ray spectroscopy (EDS) of the composition (x = 0.80). For the samples with the high concentration of the sodium content, TEM and EDS were performed to analyze the morphology and chemical composition of Na0.80CoO2. A lot of fiberlike grains were found in Fig. 4 and they were assigned to the nonmagnetic insulating Na-rich phase H3 [4,5,14,15]. Meanwhile with increasing sodium, a slight trace of unreacted Na2 CoO3 was detected in the X-ray diffraction pattern, as shown in Fig. 1. So the decrease of the electrical conductivity of the compositions (x N 0.78) may be attributed to the introduction of secondary phase, H3 or unreacted Na2CoO3. Fig. 5 displays the temperature dependence of the electrical conductivity for Na0.75CoO2 sample fired in different atmospheres. The response of electrical conductivity on atmosphere is prominent. It can be observed distinctly from Fig. 5 that the electrical conductivity increases with increasing PO2 partial pressure. For the sample fired under hydrogen atmosphere, the electrical conductivity differs much from the samples fired in pure oxygen and air. The introduction of reductive atmosphere obviously affects the transport properties of the sample. The hall coefficient measurement reveals that this kind of
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P. Liu et al. / Solid State Ionics 179 (2008) 2308–2312
Fig. 2. Fitting pattern and SEM images of the sample Na0.78CoO2 Fitting pattern and SEM images of the sample Na0.78CoO2. In the fitting pattern: the black crosses are the raw data; the red line is the calculated pattern; the green line below is the difference and the bars are reflections. The below panel on the left hand (a) is the particle morphology and the panel on the right hand (b) is the transect configuration. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
oxide is a hole-type, and the reductive atmosphere decreases the density of carriers [3,16–18]. Fig. 6 plots the temperature dependence of the thermoelectric power of NaxCoO2 (x = 0.65–0.85) samples. The thermoelectric power observed increase with rising temperature in the whole x range, compared with the electrical conductivity. Koshibe et al. have pro-
posed that the large thermoelectric power observed in NaxCoO2 can be originated from both large degeneracy in spin states of cobalt oxides and strong correlation of the 3 d electrons [19]. By generalizing the
Table 2 Structure parameters of Na0.78CoO2 at ambient temperature Atom Co Na (1) Na (2) O
Site 2a 2b 2d 4f
Atomic coordinates x
y
z
0 0 2/3 1/3
0 0 1/3 2/3
0 1/4 1/4 0.09052 (3)
Uiso × 100
Occupation
1.609 6.129 6.872 1.819
1 0.17 (1) 0.61 (2) 1
(2) (3) (5) (7)
Space group: P63/mmc; a = 2.8323 (2) Å, c = 10.943 Å (3), Rwp = 10.48%, Rp = 6.80%.
Table 3 Content of sodium by the determination of ICP-OES Sample
Adding content (x)
Detecting content (x)
0.65 0.70 0.75 0.78 0.80 0.85
0.670 0.720 0.770 0.800 0.820 0.870
0.651 0.698 0.747 0.781 0.804 0.848
(2) (1) (3) (1) (2) (4)
Fig. 3. Temperature dependence of the electrical conductivity for the as-synthesized NaxCoO2 (x = 0.65–0.85).
P. Liu et al. / Solid State Ionics 179 (2008) 2308–2312
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Fig. 4. High (a) and low (b) magnification TEM images and EDS ((c) and (d)) of the composition (x = 0.80). EDS analysis (c) and (d) was carried out to determine the chemical composition of Na0.80CoO2. The fiber-like phase is Na-rich phase (H3). With increasing sodium content, the non-magnetic insulating phase H3 is gradually introduced and this would have a prominent effect on the transport properties.
Heikes formula, the thermoelectric power in high temperature limit is given below, kB g3 n SðTY∞Þ ¼ − ln ; e g4 1−n where kB, e, g3 and g4, n are Boltzmann constant, the electron charges of the carrier, the multiplicity of the electron configurations of Co3+
Fig. 5. Temperature dependence of the electrical conductivity for sample x = 0.78 fired under various atmospheres.
and Co4+ and the concentration of Co4+ ions in the CoO2 layer, respectively. In the framework of the above-mentioned formula, the increasing Na ions concentration would contribute to the decreasing concentration of Co4+ ions in the CoO2 layer. As a result, the ascending trend could be expected from the thermoelectric power measurement, as shown in Fig. 6.
Fig. 6. Temperature dependence of the thermoelectric power for the as-synthesized NaxCoO2 (x = 0.65–0.85). In the whole range of sodium content, the thermoelectric power undergoes a monotonic enhancement.
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4. Conclusion A series of sodium cobalt oxides were prepared by rapid heat-up method. Their high temperature electrical conductivity and thermoelectric power were measured between 300 K and 1100 K. For x = 0.65–0.78, the increasing sodium content can contribute to the enhancement of both electrical conductivity and thermoelectric power. As x equals or exceeds 0.80, the measured result of electrical conductivity may result from the introduction of secondary phase, H3 or unreacted Na2CO3. Acknowledgment This work was supported by the National Nature Science Foundation of China (Project No. 20571019). References [1] K. Takada, H. Sakurai, E. Takayama-Muromachi, F. Izumi1, R.A. Dilanian, T. Sasaki, Nature 422 (2003) 53. [2] M.L. Foo, W. Yayu, S. Watauchi, H.W. Zandbergen, T. He, R.J. Cava, N.P. Ong, Phys. Rev. Lett. 92 (2004) 247001.
[3] I. Terasaki, Y. Sasago, K. Uchinokura, Phys. Rev. B 56 (1997) R12685. [4] M. Lee, L. Viciu, L. Li, Y. Wang, M.L. Foo, S. Watauchi, R.A. Pascal Jr., R.J. Cava, N.P. Ong, Nat. Mater. 5 (2006) 537. [5] M. Lee, L. Viciu, L. Li, Y. Wang, M.L. Foo, S. Watauchi, R.A. Pascal Jr., R.J. Cava, N.P. Ong, Physica B 403 (2008) 1564. [6] M. Roger, D.J.P. Morris, D.A. Tennant, M.J. Gutmann, J.P. Goff, J.U. Hoffmann, R. Feyerherm, E. Dudzik, D. Prabhakaran, A.T. Boothroyd, N. Shannon, B. Lake, P.P. Deen, Nature 445 (2007) 631. [7] H. Nakatsugawa, K. Nagasawa, J. Solid State Chem. 177 (2004) 1137. [8] Q. Huang, M.L. Foo, R.A. Pascal Jr., J.W. Lynm, B.H. Toby, T. He, H.W. Zandbergen, R.J. Cava, Phys. Rev. B 70 (2004) 184110. [9] R.J. Balsys, R.L. Davis, Solid State Ionics 93 (1996) 279. [10] H.W. Zandbergen, M. Foo, Q. Xu, V. Kumar, R.J. Cava, Phys. Rev. B 70 (2004) 024101. [11] T. Motohashi, M. Karppinen, H. Yamauchi, Research Signpost, India, (2002) 73. [12] A.C. Larson, R.B. Von Dreele, Los Alamos National Laboratory Report LAUR, 1994, p. 86. [13] B.H. Toby, J. Appl. Crystallogr. 34 (2001) 210. [14] G. Lang, J. Bobroff, H. Alloul, P. Mendels, N. Blanchard, G. Collin, Phys. Rev. B 72 (2005) 094404. [15] C. de Vaulx, M.H. Julien, C. Berthier, M. Horvatic, P. Bordet, V. Simonet, D.P. Chen, C.T. Lin, Phys. Rev. Lett. 95 (2005) 186405. [16] A.A. Taskin, A.N. Lavrov, Y. Ando, Phys. Rev. B 71 (2005) 134414. [17] Y. Wang. N.S. Rogado, R.J. Cava, N.P. Ong, arXiv:cond-mat/0305455 unpublished. [18] W. Koshibae, A. Oguri, S. Maekawa, Phys. Rev. B 75 (2007) 205115. [19] W. Koshibae, K. Tsutsui, S. Maekawa, Phys. Rev. B 62 (2000) 6869.
Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s s i
High temperature electrical conductivity and thermoelectric power of NaxCoO2 Pusheng Liu a, Gang Chen a,⁎, Ying Cui a, Hongjie Zhang a, Feng Xiao a, Lin Wang a, Hiromi Nakano b a b
Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, People's Republic of China Electron Microscope Laboratory and High-Tech Research Center, Ryukoku University, Seta, Otsu, 520-2194, Japan
a r t i c l e
i n f o
Article history: Received 24 January 2008 Received in revised form 6 August 2008 Accepted 14 August 2008 Keywords: Transport properties Sodium ions Structure refinement
a b s t r a c t Layered cobalt oxides, NaxCoO2, with the density of sodium ions, x, ranging from 0.65 to 0.85, were prepared by rapid heat-up method. The determination by X-ray diffraction pattern of crystal structure and the grain morphology studied by scanning electron microscopy were reported. The measurements of electrical conductivity and thermoelectric power were carried out between 300 K and 1100 K. It has been found that the concentration of sodium ions sandwiched between two neighboring CoO2 layers plays a crucial role in transport properties. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Layered cobalt oxides, NaxCoO2, display several interesting properties with the variation of the concentration of sodium ions x. These properties include a superconducting phase of Na0.35CoO2 when it is intercalated by water molecules around 5 K [1], a charge-ordered insulating phase of Na0.5CoO2 [2] and potential applications of the composition(x ~ 0.70)in thermoelectric energy conversion field [3]. Since the discovery of moderately large thermoelectric power (Seebeck coefficient) together with high electrical conductivity in NaxCoO2(x ~ 0.70), tremendous experiments were done to search new phases for thermoelectric conversion applications. Although they simultaneously possess large thermoelectric power, high electrical conductivity and low thermal conductivity, the figure of merit was found to be too small to utilize in the practical thermoelectric conversion devices in this x range(x ~ 0.7). While in the x range of 0.7– 0.85, high temperature electrical conductivity and thermoelectric power are poorly reported [4,5]. These layered oxides consist of two layers: CoO2 layer and Na ions layer. CoO2 layers acting as an electron reservoir are responsible for the electrical conductivity and large thermoelectric power, while Na ions layer sandwiched between two neighboring CoO2 layers working as a structural unit directly adjust the concentration of electron in CoO2 layers and decrease the thermal conductivity along the stacking
⁎ Corresponding author. Tel./fax: +86 451 86 413753. E-mail address: [email protected] (G. Chen). 0167-2738/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2008.08.010
direction c [6,7]. Recently, neutron diffraction and electron diffraction measurements have shown that the crystal structure of the oxides is strongly dependent on sodium content [8–10]. Here we use X-ray diffraction pattern supported by Rietveld refinement to study the crystal structure at ambient temperature, the electrical conductivity and thermoelectric power have been systematically studied for the composition (x = 0.65–0.85) between 300 K and 1100 K. 2. Experimental method Polycrystalline samples of NaxCoO2 with various sodium content were prepared from Na2CO3 (99.99%) and Co3O4 (99.98%) by rapid heat-up method. The detailed procedure can be found elsewhere [11]. Due to the volatility of sodium element at high temperature, an extra amount of Na2CO3 was added. Taking the preparation of Na0.78CoO2 as a sample, Na2CO3 and Co3O4 in amounts according to Na0.80CoO2 were mixed together by ball milling for 24 h, and then the samples were directly put into the furnace at 1023 K for 12 h in air. After grinding, the powders were pressed into the pellet with the diameter of 15 mm and fired again in the furnace at 1023 K for another 12 h in the O2 flow. X-ray diffraction data were collected at ambient temperature from 5° to 100° with a step of 0.05° on Rigaku D/max diffractometer working with Cu Kα radiation. The sodium content was determined by means of inductively coupled plasma optical emission spectrometers (ICP-OES, Perkin Elmer, Optima 5300DV). Energy dispersive X-ray (EDX) spectroscopy implemented in a transmission electron microscope (TEM, JEM2000EX) was used to analyze the chemical composition of some of the prepared samples. The particles morphology was studied by scanning electron microscopy (SEM, Hitachi S4800). The electrical conductivity and thermoelectric power were measured using a laboratory-designed apparatus in the temperature range of 300 K to 1100 K in air. The
P. Liu et al. / Solid State Ionics 179 (2008) 2308–2312
electrical conductivity of the bar-shaped sample (20 × 4.5 × 2 mm3) was measured by the four probe method. For the measurement of the thermoelectric power, Thermo-electromotive force measured as a function of temperature gives a straight line and its slope is thermoelectric power. The detailed procedure is described below. The sample with a pair of Pt/Rh thermocouples attached to two ends (bar-like samples) was first heated to a certain temperature, and then one of the ends was heated by an extra source to produce the temperature gradient. The temperature and voltage signals were collected by a commercial data acquisition system (Keithley 2700, Keithley Instruments Inc., America). 3. Results and discussion Fig. 1 shows X-ray diffraction patterns of the as-synthesized NaxCoO2 (x = 0.65–0.85). All the patterns are in good agreement with hexagonal γ-NaxCoO2 phase when x ranging from 0.65 to 0.80. For x = 0.85 composition, a slight trace of the unreacted Na2CO3 was detected in X-ray diffraction pattern (marked by the small up-triangle symbol in Fig. 1). The refined crystallographic cell parameters for each sample are calculated on the basis of the X-ray diffraction patterns, as listed in Table 1. The lattice parameters a and c are found to exhibit a reverse behavior with increasing sodium content, while the cell volumes almost stay the same. The increase of crystallographic a, varying in a manner consistent with sodium content, may be caused by gradually increasing coulomb repulsion of sodium ions in the same layer. The shrinkage of lattice parameter c may result from an increasing coulomb attraction between CoO2 layers and sandwiched Na ions layer. X-ray diffraction patterns were fitted by least square method employing the software GSAS&EXPGUI [12,13]. Fig. 2 gives a fitting pattern for Na0.78CoO2 sample. Since the grain is easily oriented for this synthesis method, some of the reflections, such as (002), are refined in the process of the fitting. To demonstrate the preferred orientation of the grains, SEM was used to study the micromorphology of the composition (x = 0.78), as illustrated in Fig. 2. It can be seen from the panel on the left hand that the grains show hexagonal architecture and the panel on the right hand shows the sheet-like transect configuration. For the refinement of sodium
Fig. 1. Powder X-ray diffraction patterns of the as-synthesized NaxCoO2 (x = 0.65–0.85). The small up-triangle symbol denotes the unreacted Na2CO3. We have used rapid heatup method to prepare x = 0.60 or below, but single phase cannot be obtained.
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Table 1 Crystallographic cell parameters of NaxCoO2 (x = 0.65–0.85) x
a (Å)
b (Å)
0.65 0.70 0.75 0.78 0.80 0.85
2.828 (1) 2.830 (3) 2.831 (1) 2.831 (2) 2.833 (3) 2.835 (5)
2.828 2.830 2.831 2.831 2.833 2.835
(1) (3) (1) (2) (3) (5)
c (Å)
V (Å3)
10.953 (3) 10.950 (2) 10.946 (4) 10.944 (1) 10.935 (1) 10.933 (2)
75.88 (4) 75.96 (2) 76.00 (1) 76.00 (5) 76.01 (6) 76.03 (3)
fractional occupancy (occ.), the summed occupancy value should be made consistent with the known composition of Na ions, i.e. occ.Na(1) + occ.Na(2) = x, and then the occupancy for each position was allow to refine. The crystal parameters of Na0.78CoO2 at ambient temperature are given in Table 2. The sodium content was determined using ICP-OES method by several batches of samples. It is found that the volatility of sodium element cannot be ignored when samples fired at 1023 K. In the present study, an extra amount of Na2CoO3 was added to complement the sodium loss. The measured results of ICP-OES method are shown in Table 3. Fig. 3 presents curves of the electrical conductivity vs. temperature for the as-synthesized composition (x = 0.65–0.85). We note that all the samples show metallic conduction behavior in the range measured. The magnitude of the electrical conductivity at ambient temperature is between 300–650 S/cm. It is found that the electrical conductivity is strongly dependent on the concentration of sodium ions x. In the x region of 0.65–0.78, the electrical conductivity undergoes a monotonic enhancement with increasing sodium content, when x reaches 0.78, the sample displays the best electrical conductive behavior. But for the samples with high sodium content, i.e. x ranging from 0.80 to 0.85, the electrical conductivity shows a reverse behavior, dropping to 300 S/cm for the composition (x = 0.85). The electrical conductivity initially increases as the amount of sodium increase up to 0.78. The same behaviors have been reported for the composition (0.55–0.75) by Motohashi et al. in the low temperature range [11]. As discussed in the introduction part, the density of electron in the CoO2 layer, which is critical to the electrical conductivity, can be altered by the sandwiched sodium content. It can be concluded that the ratio of Co3+ against Co4+ increase with increasing sodium content, via the simple calculation of chemical valence. Meanwhile the distortion of CoO2 layer is severe with the shrinkage of c axis. So the density of electron in the reservoir layer together with the distortion of CoO2 plays a crucial role in electrical conductivity. Fig. 4 gives transmission electron microscope (TEM) and X-ray spectroscopy (EDS) of the composition (x = 0.80). For the samples with the high concentration of the sodium content, TEM and EDS were performed to analyze the morphology and chemical composition of Na0.80CoO2. A lot of fiberlike grains were found in Fig. 4 and they were assigned to the nonmagnetic insulating Na-rich phase H3 [4,5,14,15]. Meanwhile with increasing sodium, a slight trace of unreacted Na2 CoO3 was detected in the X-ray diffraction pattern, as shown in Fig. 1. So the decrease of the electrical conductivity of the compositions (x N 0.78) may be attributed to the introduction of secondary phase, H3 or unreacted Na2CoO3. Fig. 5 displays the temperature dependence of the electrical conductivity for Na0.75CoO2 sample fired in different atmospheres. The response of electrical conductivity on atmosphere is prominent. It can be observed distinctly from Fig. 5 that the electrical conductivity increases with increasing PO2 partial pressure. For the sample fired under hydrogen atmosphere, the electrical conductivity differs much from the samples fired in pure oxygen and air. The introduction of reductive atmosphere obviously affects the transport properties of the sample. The hall coefficient measurement reveals that this kind of
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P. Liu et al. / Solid State Ionics 179 (2008) 2308–2312
Fig. 2. Fitting pattern and SEM images of the sample Na0.78CoO2 Fitting pattern and SEM images of the sample Na0.78CoO2. In the fitting pattern: the black crosses are the raw data; the red line is the calculated pattern; the green line below is the difference and the bars are reflections. The below panel on the left hand (a) is the particle morphology and the panel on the right hand (b) is the transect configuration. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
oxide is a hole-type, and the reductive atmosphere decreases the density of carriers [3,16–18]. Fig. 6 plots the temperature dependence of the thermoelectric power of NaxCoO2 (x = 0.65–0.85) samples. The thermoelectric power observed increase with rising temperature in the whole x range, compared with the electrical conductivity. Koshibe et al. have pro-
posed that the large thermoelectric power observed in NaxCoO2 can be originated from both large degeneracy in spin states of cobalt oxides and strong correlation of the 3 d electrons [19]. By generalizing the
Table 2 Structure parameters of Na0.78CoO2 at ambient temperature Atom Co Na (1) Na (2) O
Site 2a 2b 2d 4f
Atomic coordinates x
y
z
0 0 2/3 1/3
0 0 1/3 2/3
0 1/4 1/4 0.09052 (3)
Uiso × 100
Occupation
1.609 6.129 6.872 1.819
1 0.17 (1) 0.61 (2) 1
(2) (3) (5) (7)
Space group: P63/mmc; a = 2.8323 (2) Å, c = 10.943 Å (3), Rwp = 10.48%, Rp = 6.80%.
Table 3 Content of sodium by the determination of ICP-OES Sample
Adding content (x)
Detecting content (x)
0.65 0.70 0.75 0.78 0.80 0.85
0.670 0.720 0.770 0.800 0.820 0.870
0.651 0.698 0.747 0.781 0.804 0.848
(2) (1) (3) (1) (2) (4)
Fig. 3. Temperature dependence of the electrical conductivity for the as-synthesized NaxCoO2 (x = 0.65–0.85).
P. Liu et al. / Solid State Ionics 179 (2008) 2308–2312
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Fig. 4. High (a) and low (b) magnification TEM images and EDS ((c) and (d)) of the composition (x = 0.80). EDS analysis (c) and (d) was carried out to determine the chemical composition of Na0.80CoO2. The fiber-like phase is Na-rich phase (H3). With increasing sodium content, the non-magnetic insulating phase H3 is gradually introduced and this would have a prominent effect on the transport properties.
Heikes formula, the thermoelectric power in high temperature limit is given below, kB g3 n SðTY∞Þ ¼ − ln ; e g4 1−n where kB, e, g3 and g4, n are Boltzmann constant, the electron charges of the carrier, the multiplicity of the electron configurations of Co3+
Fig. 5. Temperature dependence of the electrical conductivity for sample x = 0.78 fired under various atmospheres.
and Co4+ and the concentration of Co4+ ions in the CoO2 layer, respectively. In the framework of the above-mentioned formula, the increasing Na ions concentration would contribute to the decreasing concentration of Co4+ ions in the CoO2 layer. As a result, the ascending trend could be expected from the thermoelectric power measurement, as shown in Fig. 6.
Fig. 6. Temperature dependence of the thermoelectric power for the as-synthesized NaxCoO2 (x = 0.65–0.85). In the whole range of sodium content, the thermoelectric power undergoes a monotonic enhancement.
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4. Conclusion A series of sodium cobalt oxides were prepared by rapid heat-up method. Their high temperature electrical conductivity and thermoelectric power were measured between 300 K and 1100 K. For x = 0.65–0.78, the increasing sodium content can contribute to the enhancement of both electrical conductivity and thermoelectric power. As x equals or exceeds 0.80, the measured result of electrical conductivity may result from the introduction of secondary phase, H3 or unreacted Na2CO3. Acknowledgment This work was supported by the National Nature Science Foundation of China (Project No. 20571019). References [1] K. Takada, H. Sakurai, E. Takayama-Muromachi, F. Izumi1, R.A. Dilanian, T. Sasaki, Nature 422 (2003) 53. [2] M.L. Foo, W. Yayu, S. Watauchi, H.W. Zandbergen, T. He, R.J. Cava, N.P. Ong, Phys. Rev. Lett. 92 (2004) 247001.
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