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Doping-Induced Metal-Insulator Transition and the Thermal Transport Properties in Ca3-xYxCo4O9 Yang Wang,†,| Yu Sui,*,†,‡ Jinguang Cheng,† Xianjie Wang,† Wenhui Su,† Xiaoyang Liu,§ and Hong Jin Fan*,| Center for Condensed Matter Science and Technology (CCMST), Department of Physics, Harbin Institute of Technology, Harbin 150001, People’s Republic of China, International Center for Materials Physics, Academia Sinica, Shenyang 110015, People’s Republic of China, State Key Laboratory of Inorganic Synthesis and PreparatiVe Chemistry, College of Chemistry, Jilin UniVersity, 2699 Qianjin Street, Changchun 130012, People’s Republic of China, and DiVision of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological UniVersity, 21 Nanyang Link, 637371, Singapore ReceiVed: NoVember 21, 2009; ReVised Manuscript ReceiVed: February 8, 2010
We report the electrical, thermal, magnetic, and thermoelectric properties of Y-doped Ca3Co4O9 from 300 down to 5 K. The results indicate that with Y doping, the increase of resistivity originates from the decreases of carrier concentration and mobility, while the increase of Seebeck coefficient is caused by the reduction of carrier concentration together with the enhanced electronic correlation. Point-defect scattering is the dominant thermal transport mechanism in this system. Due to the considerable difference in mass between Y3+ and Ca2+, thermal conductivity is observably suppressed by doping. The substitution of Y also disturbs the interlayer ferrimagnetic coupling. The ground state of this system converts from ferrimagnetism to paramagnetism gradually. The alteration of transport properties of Ca3-xYxCo4O9 reveals two crossovers: the transition from Fermi-liquid-like metal to thermally activated semiconductor occurring at x ≈ 0.25, and the transition from thermally activated semiconductor to two-dimensional variable range hopping semiconductor occurring at x ≈ 0.5. The optimal thermoelectric response in Ca3-xYxCo4O9 is found to exist only at the critical state after which the doping-induced metal-insulator transition takes place. On the basis of these experimental results, a possible phase diagram for Ca3-xYxCo4O9 is proposed. 1. Introduction Transition-metal oxides have been widely studied during the past decades because of their rich physical properties such as giant magnetoresistance (GMR) effect, colossal magnetoresistance (CMR) effect, superconductivity, and thermoelectric effect.1-5 Among these transition metal oxides, layered cobaltites NaxCoO2 and Ca3Co4O9 have attracted much attention in recent years due to their unusual thermoelectric properties: extraordinarily large Seebeck coefficient (∼100-125 µV K-1 at 300 K) coexisting with metallic-like electrical conductivity,4,5 and thus their potential thermoelectric applications. Compared with NaxCoO2, the Ca3Co4O9 system is more suitable for thermoelectric application on account of its thermal and chemical stabilities at high temperature in air. The composition of Ca3Co4O9 can be expressed by [Ca2CoO3][CoO2]b1/b2, with misfit-layered structure featuring the different periodicities along the b axis with b1 referring to the b-axis length of rocksalt-type Ca2CoO3 sublayer and b2 referring to the b-axis length of CdI2type CoO2 sublayer.5 The two sublayers alternatively stack along the c axis and they have the same a-axis and c-axis lengths.5 The origin of the unusual thermoelectric characteristics in these layered cobaltites is currently not fully understood, but the strong electronic correlations may be an important source.6,7 * To whom correspondence should be addressed. Y.S.: phone: +86451-86418403, e-mail:
[email protected]. H.J.F.: phone: +65-65137408, e-mail:
[email protected]. † Harbin Institute of Technology. | Nanyang Technological University. ‡ International Center for Materials Physics. § Jilin University.
NaxCoO2 shares the same structural component of Ca3Co4O9, CoO2 planes, in which a two-dimensional triangular lattice of Co ions is formed by a network of edge-sharing CoO6 octahedra, while the other subsystem in NaxCoO2 is the single disordered Na planes.4 The CoO2 subsystem dominates the carrier transport and is important in the electronic structure.4,5,8 However, the rocksalt-type Ca2CoO3 subsystem of Ca3Co4O9 is also likely to play a significant role in determining the transport and magnetic properties.9-11 Several differences in physical properties between Ca3Co4O9 and NaxCoO2 have been observed. For instance, the electrical conductivity of NaxCoO2 remains metallic-like from room temperature down to 5 K, whereas Ca3Co4O9 exhibits a metal-insulator (MI) transition at TMI ≈ 80 K and below TMI the electrical conductivity becomes semiconducting-like;4,5 Ca3Co4O9 shows a ferrimagnetic transition around 19 K, but NaxCoO2 does not show any magnetic transition below 30 K;9-11 magnetic susceptibility measurements reveal an anomaly around 380 K in Ca3Co4O9 that is a probable spin-state transition, whereas there are no clear magnetic anomalies in NaxCoO2.5,12,13 These differences may arise from their distinct crystal structure, viz., the distinction between rocksalt-type Ca2CoO3 subsystem and disordered Na subsystem. Therefore, ion doping in the Ca2CoO3 layer or Na layer may observably influence the properties of these systems. For the Ca3Co4O9 system, many investigations on the effects of substitution for Ca on the transport, magnetic, and thermoelectric properties have been conducted,9,14-22 but most only focus on the improvement of thermoelectric performance. It was reported that muon spin rotation-relaxation measurements indicate there exists an incommensurate spin-density-wave (IC-
10.1021/jp911078h 2010 American Chemical Society Published on Web 03/01/2010
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Figure 1. XRD patterns of Ca3-xYxCo4O9 (x ) 0, 0.5, and 0.75) samples. All the diffraction peaks of Ca3Co4O9 phase are indexed.
SDW) order below ∼100 K, and the IC-SDW is considered to be responsible for the localization of carriers and the MI transition.9-11 Interestingly, a slight substitution of trivalent cations for Ca increases the IC-SDW transition temperature TSDW while the substitution of divalent cations does not affect TSDW,9 suggesting the IC-SDW transition strongly depends on the average valence of Co ions. Also, the ferrimagnetism is caused by the interlayer coupling between the Ca2CoO3 and CoO2 subsystems, and the interaction of these two subsystems via Ca-O bonds can lead to a displacive modulation.10 That is to say, the substitution of high-valent cations for Ca in a large doping range may gradually change the transport mechanism and induce the variation of the magnetic ground state. In the present study, we investigate the transport mechanism, magnetic properties, and the thermoelectric characteristics of Y3+-doped Ca3Co4O9 in order to further understand the unusual thermoelectric response and strongly correlated properties in this system. Moreover, thermal conductivity, which is as important as resistivity and Seebeck coefficient to a thermoelectric material, has not been paid enough attention in this system, so we also conduct a detailed investigation on the thermal transport behavior of the Ca3-xYxCo4O9 series in this study. 2. Experimental Section Ca3-xYxCo4O9 (x ) 0-0.75) polycrystalline samples were synthesized by conventional solid-state reaction. Reagent grade CaCO3, Co2O3, and Y2O3 powders in a stoichiometric ratio were mixed and annealed in air at 1173 K for 12 h. Then the mixture was thoroughly reground, pressed into disk-shaped pellets, and sintered at 1223 K for 36 h under O2 flow with intermediate grindings. Finally, the pellets were slowly cooled to room temperature in the furnace. X-ray diffraction (XRD) data were collected by using a XRD diffractometer (D8 Advanced) with Cu KR (λ ) 0.15406 nm) radiation. Temperature dependence of resistivity, Seebeck coefficient, thermal conductivity, Hall coefficient, and magnetic measurements were all carried out by using the Quantum Design physical property measurement system (PPMS). Detailed measurement methods were reported elsewhere.19 3. Results and Discussion The XRD patterns for Ca3-xYxCo4O9 (x ) 0, 0.5, and 0.75) are shown in Figure 1. Other samples have similar patterns as the above specimens. The XRD patterns of all the samples agree well with the standard JCPDS card (21-139) and reported data for the Ca3Co4O9 structure.5 All the diffraction peaks in the patterns can be indexed by the Ca3Co4O9 phase, and there is not any detectable impurity peak, which indicates the formation of single-phase compounds. As described in Figure 2, the
Figure 2. Schematic structural view of Ca3-xYxCo4O9, where Ca2+ sites are partially occupied by Y3+ ions.
Figure 3. Temperature dependence of resistivity F for all the samples. The inset shows the MI transition temperatures. The broken line denotes a possible extrapolation of TMI versus x.
substitution of Y for Ca does not affect the crystalline structure, but from the following analyses, we will see that Y doping strongly influences the transport and magnetic properties as well as the thermoelectric characteristics in this system. Transport Mechanism and Doping-Induced MI Transition. Figure 3 displays the temperature dependence of resistivity F for all the samples. With Y doping, the magnitude of F increases evidently, and the shape of F-T curves alters gradually. Undoped Ca3Co4O9 exhibits an MI transition at TMI ) 79 K, below which it is semiconducting-like (i.e., dF/dT < 0) while above TMI it becomes metallic-like (i.e., dF/dT > 0). As doping level x increases, TMI rises from 79 K for the Ca3Co4O9 parent to 165 K for the x ) 0.2 sample (see the inset of Figure 3). When the relative Y content exceeds 0.25, the samples remain semiconducting-like in the whole temperature range. For the x ) 0.25 sample, it shows a quite wide temperature range between 150 and 300 K in which F remains almost unchanged. Although it does not exhibit a distinct MI transition, the extrapolation of TMI versus x plot suggests that a MI transition may exist around ∼180-220 K. The negative temperature dependence of carrier mobility between 150 and 300 K (see below and Figure 4b) also suggests a possible metallic-like state in this temperature range. As for the x ) 0.3 sample, its semiconducting-like character is clear even at room temperature. Therefore, doping level x ) 0.25 may be the critical point of the doping-induced MI transition in the Ca3-xYxCo4O9 system. The increase in F with doping should be attributed to the decrease in carrier concentration along with the reduction of mobility. Figure 4 presents the temperature dependence of carrier concentration n and carrier mobility µ. n lessens considerably
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Wang et al. hanced localization of carriers, so with doping (i.e., with gradually enhanced localization) if the carriers cannot become itinerant at room temperature or even high temperature, the system will remain semiconducting-like in the whole temperature range. Actually, in such a strongly correlated Fermi-liquid system, the increase of average distance between carriers will enhance electronic correlations and cause the decrease of bandwidth. Accordingly, when the bandwidth narrows to approach a narrow-band limit for itinerant electron, dopinginduced MI transition occurs. In contrast to the case of trivalent Y3+ doping, the substitution of univalent Ag+ for Ca2+ introduces hole carriers and decreases the average distance between carriers, which results in the decrease of electronic correlations and the enhancement of metallicity.19 For another, as the localization of carrier increases, the semiconductivity becomes more stable, so the MI transition will happen at a higher temperature and thus TMI increases consequently. Such evolution also corresponds to the variation of IC-SDW by doping. Since the IC-SDW order is dependent on the average valence of Co ions as confirmed by the positive muon spin rotation and relaxation (µ+SR) experiments, similar to other cases of substitutions of high-valent ions for Ca,9 Y3+ doping will also increase IC-SDW transition temperature. Because the formation of IC-SDW induces the low temperature semiconducting-like behavior, gradual TMI increase with doping can be anticipated. Since F-T behavior gradually varies with doping, next we pay attention to the transport mechanism. It is well-known that in Ca3Co4O9 with temperature decreasing, the metallic-like Fermi-liquid transport behavior is followed by a thermally activated behavior,23,24 viz., the variation of F with T in the low temperature semiconducting-like range obeys
F ) F0 exp(Ea /kBT)
Figure 4. Temperature dependences of (a) carrier concentration n and (b) mobility µ for the samples. (c) Room temperature n and µ as a function of x.
with increasing doping level. On the basis of valence equilibrium, the substitution of trivalent Y3+ for divalent Ca2+ will induce Co3+ and reduce the number of holes. Since the dominate carrier of the Ca3Co4O9 system is the hole, evidenced by the positive Hall coefficient and Seebeck coefficient,5 n of the system will decrease by doping as a result. Due to the reduction of n, the average distance between hole carriers increases, which can lead to the increase in the localization of holes. Consequently, µ of the system decreases with doping. It should be noted that the change in µ is very slight when the relative Y content is less than 0.25 (see Figure 4c), which implies that the enhancement of F in these lightly doped samples mainly arises from the reduction of n. In contrast, the decreases of not only n but also µ both contribute to the increase in F for the samples with higher doping level. In addition, n shows globally an obvious temperature dependence, consistent with the strongly correlated characteristic of this system. In Ca3Co4O9, there exists Fermi-liquid transport behavior in the metallic-like range, and the appearance of IC-SDW around 100 K gives rise to the localization of carriers at low temperature.7,9-11 As mentioned above, Y doping causes en-
(1)
where F0 is a constant factor and Ea is the activation energy. By the fitting of experimental data using eq 1, one can see from Figures 5 and 6 that lightly substituted samples retain such a transport mechanism in the semiconducting-like range, but the activation energy Ea increases gradually. For the samples with doping content larger than 0.25 but not more than 0.5, they exhibit thermally activated conduction in the whole measured temperature range, and Ea observably increases. However, for the samples with doping content larger than 0.5, although they remain semiconducting-like, their F-T behavior deviates obviously from the exponential law (see the inset of Figure 5d) but can be well fitted by Mott’s variable range hopping (VRH) model. According to Mott’s VRH theory,25 the relationship between electric conductivity σ and temperature T is expressed by
ln σ(T) ) -(T0 /T)R + η
(2)
T0 ≈ 1/[kBN(εF)lvd]
(3)
with
where T0 is the VRH characteristic temperature associated with the density of localized states at the Fermi energy N(εF), lv is localization length, η is a constant, and R ) 1/(d + 1) with the dimension d of a system. By the best fit of the data using eq 2, one gets R ) 0.31-0.33, i.e., d ≈ 2 for the samples. This result
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Figure 5. (a-c) Fitting plots of F-T curves according to the thermally activated model for the samples. (d) Fitting plot of F-T curves according to the 2D-VRH model for the x ) 0.75 sample. The inset of panel c presents the 2D-VRH fitting for the x ) 0.5 sample while the inset of panel d presents the thermally activated fitting for the x ) 0.75 sample.
Figure 6. x dependence of activation energy Ea from the thermally activated fitting and localization length lv from the 2D-VRH fitting. The broken lines suggest possible extrapolations for Ea and lv.
implies that the transport mechanism of heavily substituted samples (x > 0.5) has turned into a two-dimensional (2D) VRH mechanism. Therefore, the relationship between F and T can be expressed by
()
F ) F′0 exp
T0 T
1/3
(4)
where F′0 is a constant, and T0 ) 8/[πkBN(εF)lv2]. As shown in Figure 5d, the linear relationship of ln F versus T-1/3 for x ) 0.75 clearly demonstrates the 2D-VRH regime in the whole temperature range. Actually, for the x ) 0.5 sample, although it shows thermally activated conduction in the whole measured temperature range,
Figure 7. Temperature dependence of magnetic susceptibility χ of the samples. The inset shows 1/χ versus temperature curves with fitting straight lines.
it seems that 2D-VRH is also suitable below ∼100 K (see the inset of Figure 5c). This means doping level x ) 0.5 may be the critical point of the transport mechanism converting from thermal activation into 2D VRH in the Ca3-xYxCo4O9 series. Such a change in transport mechanism is understandable. Substitution can induce distortion as well as potential disorder and thus stronger localization, which causes the increase of activation energy; for heavily doped samples, due to severe distortion and disorder, the thermally activated energy may be hard to make holes transport by neighbor hopping, so the carriers hopping tends toward farther low-energy sites. As a result, the electrical transport is realized through VRH. The fitted values of T0 for x ) 0.5 and 0.75 samples are 8.98 × 104 K (between 5 and 100 K) and 2.76 × 105 K (between 5 and 300 K), respectively. Then one can obtain the estimated localization
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Figure 8. Magnetic field dependence of magnetization at 5 K for the samples.
Figure 9. Temperature dependence of thermal conductivity κ of the samples. The inset shows specific heat C versus T for Ca3Co4O9.
length lv ≈ 2.4 nm and lv ≈ 0.92 nm for x ) 0.5 and 0.75 samples, respectively (see Figure 6), indicating the reduction of localization length (i.e., stronger carrier localization) with increasing Y content. Besides, owing to the anisotropic layered structure characteristic of the Ca3Co4O9 system, the VRH mechanism is two-dimensional, rather than the usual threedimensional as found in isotropic semiconductors. The substitution of Y for Ca also influences the magnetic properties. Figure 7 presents the temperature dependence of magnetic susceptibility χ of the samples. χ monotonically increases with decreasing temperature. For undoped Ca3Co4O9, χ increases observably as temperature further decreases from ∼19 K, which is suggested to be a ferrimagnetic transition.10 With Y doping, the values of χ lessens, the sudden raise becomes
Wang et al. mild, and the temperature of raise decreases, which all mean that the ferrimagnetism is suppressed by doping. As shown in the inset of Figure 7, the x ) 0.5 sample only exhibits quite weak ferrimagnetism, whereas for the x ) 0.75 sample, it has turned into a paramagnetic (PM) phase entirely. The ferrimagnetism of Ca3Co4O9 originates from the interlayer coupling between CoO2 and Ca2CoO3 sublayers.9-11 On account of the structural feature the Ca2CoO3 layer that consists of two Ca-O planes and one Co-O plane, where the Co-O plane is sandwiched by the two Ca-O planes,5 and the Ca-O planes are located between the Co-O plane and CoO2 sublayers. Accordingly, the doping into the Ca-O plane disturbs the interlayer ferrimagnetic coupling, so the ferrimagnetism is gradually suppressed. The magnetic field dependence of magnetization M at 5 K (see Figure 8) also clearly reveals the evolution of magnetic ground state of the samples. Thermal Transport Properties. Thermal conductivity as one of the most fundamental transport properties in solids can provide important information on the various interplays. For a thermoelectric material, thermal conductivity κ is as crucial as resistivity F and Seebeck coefficient S since the thermoelectric figure of merit Z is defined by Z ) S2/Fκ. Figure 9 shows the temperature dependences of thermal conductivity κ of the samples. All the samples present similar thermal conduction behavior, but κ monotonously decreases with increasing Y content. κ can be expressed by the sum of the lattice component (κph), the carrier component (κcar), and the spin wave component (κm) as κ ) κph + κcar + κm. The calculated values of κcar from Wiedemann-Franz’s law κcar ) L0T/F, where L0 ) π2kB2/3e2 is the Lorentz constant, are quite small in the whole temperature range,
