Enhanced Thermoelectric Performance and Room-Temperature Spin

Article pubs.acs.org/JPCC

Enhanced Thermoelectric Performance and Room-Temperature Spin-State Transition of Co4+ Ions in the Ca3Co4−xRhxO9 System

Yanan Huang,† Bangchuan Zhao,*,† Ran Ang,† Shuai Lin,† Zhonghao Huang,† Shugang Tan,† Yu Liu,† Wenhai Song,† and Yuping Sun*,†,‡ †

Key Laboratory of Materials Physics, Institute of Solid State Physics, and ‡High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China ABSTRACT: The effects of Rh doping on the structural, magnetic, electrical, and thermal transport properties of Ca3Co4−xRhxO9 (0 ≤ x ≤ 0.4) samples have been investigated systematically. XRD and XPS results show that the doped Rh ions are in the form of Rh3+. Only a metal−insulator transition (MIT) and an anomaly of the slope related to the transition from a Fermi liquid to an incoherent metal at low temperatures were observed in the resistivity curve for the undoped sample. As Rh ions were doped into the samples, an additional anomaly and MIT occurred in the resistivity curve near room temperature, which are suggested to originate from the spin-state transition (SST) of Co ions. The low-temperature MIT temperature increased with increasing Rh-doping content, indicating that the spin-density-wave state became stable as a result of the enhanced random Coulomb potential in CoO2 octahedral block layers induced by Rh substitution. Based on an analysis of the thermopower and XPS data, Rh3+ ions are suggested to substitute at the Co3+ sites of CoO2 layers. The substitution induced a partial SST of Co4+ ions from the low-spin to the high-spin state, leading to the formation of a spin-state polaron. The evolution of the electrical and magnetic properties with Rh doping is summarized in a single phase diagram for Ca3Co4−xRhxO9. It should be noted that the thermopower of the system did not change obviously with Rh doping, but the thermal conductivity decreased significantly. As a result, the ZT value increased markedly with increasing Rh-doping content. The ZT value at room temperature for Ca3Co3.6Rh0.4O9 reached 0.014, which is about 2.4 times larger than that of Ca3Co4O9. The results show that Rh doping might be an effective route to improving the thermoelectric performance of the Ca3Co4O9 system.

1. INTRODUCTION It is well-known that thermoelectric (TE) materials can be utilized to interconvert thermal energy and electric energy directly in a single TE generation system.1,2 The efficiency and performance of TE energy conversion are characterized by the dimensionless TE figure of merit ZT.3 In general, a good TE material needs high thermopower S, low resistivity ρ, and low thermal conductivity κ to achieve a high value of ZT.4,5 Since the discovery of good TE properties in NaxCo2O4 (NaCo2O4),6 Ca−Co−O,7,8 Bi2Sr2Co2Oy,9,10 and Tl−Sr−Co−O,11,12 these layered cobalt oxides with octahedral CoO2 layers as a common unit have been the objects of renewed attention, because of the soaring short-term demand for energy sources and the potential long-term need to create a sustainable energy future.13 Here, the CoO2 layer plays the most important role in realizing good TE properties.14 For these layered cobalt oxides, Fujii and Terasaki proposed a block-layer concept;15 that is, a twodimensional (2D) triangular lattice of Co ions is formed by a network of edge-sharing CoO2 octahedral lattice (as shown in the left inset of Figure 1a).11−13 By modifying such a block layer, the lattice misfit and the carrier concentration can be modulated,14 leading to variations in the magnetic and transport properties of these materials. In this cobaltite family, the so-called Ca3Co4O9 system occupies a special place,7,16 © XXXX American Chemical Society

where the coexistence of metallic transport behavior and high thermopower makes it suitable as a potential candidate for TE materials.5,17 Usually, Ca3Co4O9 can be embodied as [Ca2CoO3]0.62[CoO2], which comprises two subsystems stacked alternately along the c axis: rocksalt- (RS-) type CaO−CoO−CaO layers and CdI2-type CoO2 layers.7 The former is regarded as the charge reservoir to supply charge carriers to the CoO2 layers. The latter dominates the carrier transport and the electronic structure.7,11−13 Both subsystems have monoclinic crystal symmetry (C2/m) with identical a, c, and β parameters but different b parameters, resulting in a structural misfit along the b axis.5,9,10 Because of this misfitlayered structure, the magnetic and electrical transport properties of Ca3Co4O9 system are quite intricate, including the low-temperature ferrimagnetic (FIM) state, the incommensurate spin-density-wave (IC-SDW) ordering, the metal− insulator transition (MIT), and the spin-state transition (SST) of Co ions.6,13,15,18 Here, spin state is one of the most fundamental concepts in cobalt-based compounds/complexes, Received: January 5, 2013 Revised: May 9, 2013

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dx.doi.org/10.1021/jp400146y | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1. (a) XRD patterns of Ca3Co4−xRhxO9 (x = 0, 0.1, 0.2, 0.3, and 0.4). Left inset: Edge-sharing CoO2 octahedral lattice of the Ca3Co4O9 system. Right inset: Average c-axis structural parameter dc as a function of Rh-doping content x. (b) Deconvoluted Rh 3d5/2 and Rh 3d3/2 peaks arising from the XPS result for Rh ions. (c) Magnified (002) peaks for all samples.

especially layered cobaltite oxides.19 In the CoO2 layers of these cobaltite oxides, Co3+ and Co4+ ions exist in three types of spin states: low-spin (LS), intermediate-spin (IS), and high-spin (HS).6 The spin states of Co ions transform at about 400 K because of the comparable crystal-field and Hund coupling energies.20 The SST temperature can be tuned by appropriate element substitution.21 A heptamer polaron (spin-state polaron) can be formed by Co ions with different spin states as observed in La1−xSrxCoO3 (x < 0.01), where one LS Co4+ ion and six IS Co3+ ions form the spin-state polaron at low temperatures.22,23 Such a polaron phenomenon will induce a higher occupation of charge in the 3d orbital and disorder in the spin state, which might be the origin of the additional entropy that attaches the entropy per carrier to enhance the thermopower.24 For the sake of clarifying the unusual magnetic and transport properties and improving the TE performance of the Ca3Co4O9 system, the study of element substitution at Co sites is a very important route because of the multivalence and variable spin state of Co ions. Substitution can occur in both RS and CoO2 layers. Doping at Co sites in the RS layers can adjust only the carrier concentration of the system. However, the electronic structure and transport mechanism can be altered by the replacement of Co ions in the CoO2 layers. So far, for the Ca3Co4O9 system, the doping effect at Co sites has been extensively investigated. Among these investigations, the doped elements have mainly been 3d transitional metals, such as Ti,21,25 Cr,5,26 Mn,13 Fe,5,13,26 Ni,26 Cu,13,27 and Zn.26 Compared to these 3d transition-metal elements, the doping of 4d transition-metal elements, for instance, Rh doping, might be more beneficial to TE performance or magnetic/electrical transport properties in this system because of their extended orbitals. Layered rhodium oxides, with CdI2-type RhO2 layers, are also known to exhibit good TE performance similar to that of layered cobalt oxides,28 such as CuRhO2,29,30 LaRhO3,28,30 and Bi1.8Sr2Rh1.6Oy.14 Moreover, Rh doping can induce the SST of Co ions to form a spin-state polaron. In other systems, similar behavior has been observed. For example, in the Rhdoped La1−xSrxCo1−yRhyO3 (x < 0.01) system, LS Rh3+ seems

to replace LS Co3+ and stabilize the neighboring HS Co3+.24 Consequently, in the present work, we investigated the structure, magnetic, electrical, and thermal transport properties of the Ca3Co4−xRhxO9 (x = 0, 0.1, 0.2, 0.3, and 0.4) series of samples systematically with the aim of enhancing their TE performance.

2. EXPERIMENTAL DETAILS Polycrystalline samples of Ca3Co4−xRhxO9 (x = 0, 0.1, 0.2, 0.3, and 0.4) were prepared by the conventional solid-state reaction method. High-purity CaCO3, Co3O4, and Rh2O3 powders were mixed thoroughly in the desired stoichiometric ratio and prefired at 1173 K for 24 h twice with an intermediate grinding. Then, the obtained mixtures were reground, pressed into dishshaped pellets, and sintered at 1173 K for 24 h in air to obtain homogeneous samples. The structure and phase purity of the samples were examined by powder X-ray diffraction (XRD) using a Philips X’pert PRO X-ray diffractometer with Cu Kα radiation at room temperature. X-ray photoelectron spectroscopy (XPS) was performed using Al Kα radiation at room temperature. The electrical and thermal transport properties were measured on a physical property measurement system (PPMS-9T) with the conventional four- or five- (Hall measurement) probe method. The magnetic properties were measured on a superconducting quantum interference device (SQUID) measurement system (MPMS-5T). 3. RESULTS AND DISCUSSION 3.1. Structure. The room-temperature XRD patterns of Ca3Co4−xRhxO9 (x = 0, 0.1, 0.2, 0.3, and 0.4) samples are shown in the main panel of Figure 1a. The XRD patterns of all studied samples agree well with standard Joint Committee for Power Diffraction Standards (JCPDS) card 21-139 and the previously reported data for the Ca3Co4O9 structure,7 indicating the formation of single-phase compounds. The results show that Rh doping did not change the lattice structure of the system. All peaks in the figure can be well indexed to a monoclinic lattice structure with space group C2/m.1,5,17 To B



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Figure 2. (a) Temperature dependence of the resistivity ρ(T) for Ca3Co4−xRhxO9 (x = 0, 0.1, 0.2, 0.3, and 0.4) measured at zero magnetic field (2− 360 K). Inset: Magnified ρ(T) curve for Ca3Co4O9. (b) Magnified ρ(T) curve for the x = 0.3 sample between 140 and 260 K, where S and M denote semiconductor and metal, respectively.

range, another two anomalies of the slope in ρ(T) curve appeared in the vicinity of Ta and T**, as shown in Figure 2b, where the magnified ρ(T) curve for the x = 0.3 sample is shown as an example. For both single-crystal18 and polycrystalline7 Ca3Co4O9 samples, similar anomaly around Ta in ρ(T) curve was considered to be induced by SST of Co ions and was observed around 400 K. Therefore, we can assume that the SST in our undoped sample is located at temperatures higher than 360 K. That is, Rh doping in the Ca3Co4O9 system might shift the SST temperature Ta and the high-temperature MIT temperature T** to a lower temperature range. The XPS results for Ca3Co4O9 revealed that Co 3d partial density of states (DOS) in Ca2CoO3 layers hardly reaches the Fermi level.34 In contrast, the Fermi level lies in the crystal-field gap of the d states of CoO2 layers.35 Therefore, the Co ions in the CoO2 layers dominate the electrical transport behavior of Ca3Co4O9 system.13 If the substitution takes place in the CoO2 layers, it not only produces strong disorder and distortion in the sample, but also changes the transport mechanism because the conduction path in the CoO2 layer is disturbed.36 To fully explore the reason for the distinct variations in electrical transport properties induced by Rh doping, we first examined the transport mechanisms of the studied samples in different temperature ranges. 3.2.1. Two-Dimensional VRH Behavior. For the lowtemperature ρ(T) data below ∼15 K, the Mott’s twodimensional variable-range hopping (2D VRH) model provides the best fitting of the ρ(T) curves, as shown in Figure 3a. According to the VRH theory,37 the relationship between resistivity and temperature in a 2D system can be expressed as

show the effect of Rh doping on the lattice clearly, the enlarged (002) peaks are shown in Figure 1c as an example for all studied samples. This figure shows that, as the Rh-doping content x increased, the peak moved to a lower angle position. As a result, the average c-axis structural parameter dc, calculated from all (00l) peaks according to the Bragg formula (2dc sin θ = nλ), increases monotonically with increasing x, which is plotted in the right inset of Figure 1a. Figure 1b shows the XPS result for Rh ions. It can be seen that the binding energy (BE) of the deconvoluted Rh 3d5/2 peak is ∼309.3 eV, corresponding to Rh3+, indicating that, in the present samples, Rh ions enter into Ca3Co4O9 system in the form of Rh3+. Considering the physical properties as discussed in later sections and the standard ionic radius of Rh3+ (0.665 Å), which is larger than those of Co3+ (0.545 Å) and Co4+ (0.53 Å) in CoO2 layers and is closer to that of Co3+, we speculate that Rh ions might mostly enter into Co3+ sites in CoO2 layers and thus increase dc, in agreement with the XRD results. 3.2. Electrical Transport Properties. We measured the temperature dependence of the resistivity, ρ(T), under zero magnetic field between 2 and 360 K for Ca3Co4−xRhxO9 (x = 0, 0.1, 0.2, 0.3, and 0.4) samples, and the results are shown in the main panel of Figure 2a. For a single-crystal Ca3Co4O9 sample, the in-plane resistivity ρ(T) in a wide temperature range (2− 600 K) can be distinguished as reflecting four transport regimes.18,31,32 In the low-temperature range, the sample shows semiconducting transport behavior (dρ/dT < 0). With increas ing temperature, a broad minimum around TMIN (defined as the low-temperature MIT temperature) appears in the ρ(T) curve, implying the existence of the IC-SDW state.33 The charge carriers are localized by the emergence of IC-SDW ordering, giving rise to the MIT.13 As temperature increases further, the sample exhibits metallic behavior (dρ/dT > 0) with two regimes that can be identified as a strongly correlated Fermiliquid regime up to T* and then an incoherent metal regime with an anomaly of slope around Ta. Finally, Ca3Co4O9 behaves as a semiconductor at elevated temperatures above T** (defined as the high-temperature MIT temperature). For the present undoped sample, only two characteristic temperatures, TMIN and T*, can be observed because of the limited temperature measurement range (2−360 K), as displayed in the inset of Figure 2a. However, for the Rh-doped samples, as the temperature increased from T* in the same temperature

⎛ T ⎞1/3 ρ(T ) = ρ(0) exp⎜ 0 ⎟ ⎝T ⎠

(1)

where ρ(0) is a constant and T0 = 8/[πkBN(εF)lν2] is the VRH characteristic temperature associated with the density of localized states at Fermi level N(εF), with lν as the localization length and kB as the Boltzmann constant.36 As shown in Figure 3a, the linear relationship of ln ρ(T) versus T−1/3 indicates that the data below ∼15 K can be well fitted, and the R2 values for the fitting parameters (denote the fitting quality) are all more than 0.999. The obtained fitting parameter T0 is listed in Table 1. From Table 1, one can see that the value of T0 increased C



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⎛ E ⎞ 1 = μ(T ) exp⎜ − 0 ⎟ ρ ⎝ kBT ⎠

(2)

This can be rewritten as ln ρ =

E 0 −1 T − ln μ = BT −1 + C kB

(3)

where μ(T) is the mobility of carriers, E0 is the energy gap resulting from the SDW at the Fermi surface, and B (= E0/kB) and C (= −ln μ) are the fitting parameters. We tried to fit the low-temperature ρ(T) data using eq 3 . The results are shown in Figure 3b, and the R2 values for the fitting parameters are all greater than 0.999. The obtained activation energy E 0 (= BkB) increases gradually with increasing x, as reported in Table 1, indicating that Rh doping has a positive effect on the formation of the SDW state. Spin density waves are known to propagate in the CoO2 plane. If the substitution for Co ions occurs in Ca2CoO3 layers,38 it should have little influence on the transport mechanism, in which case E0 should remain unchanged. 13 The obviously enhanced E0 value confirms that Rh doping occurs mainly in CoO2 layers, which is consistent with the results of XRD analysis. Actually, for the Ca3Co4O9 system, the Anderson model is usually introduced to describe the transport properties.39 In such a model, the activation energy E0 is suggested to equal EF − EC, where EF is the Fermi energy level and EC is the upper mobility edge. In the low-temperature range, as kBT/2 < E0, most of the holes are localized, and VRH conduction dominates the transport mechanism. With increasing temperature, when kBT/2 is in the vicinity of E0, the number of excited holes rapidly increases at intermediate temperatures, giving rise to TAC. As the temperature increases further, many more holes are excited, resulting in metallic ρ(T) behavior, and the system enters into the Fermi-liquid regime. 3.2.3. Strongly Correlated Behavior. In general, the ρ(T) curve of a strongly correlated Fermi-liquid system can be described by the equation

Figure 3. Plots of (a) ln ρ versus T−1/3 between 2 and 40 K, (b) ln ρ versus T−1 between 12 and 360 K, and (c) ρ versus T2 between 4 and 209 K with the fitted lines for Ca3Co4−xRhxO9 (x = 0, 0.1, 0.2, 0.3, and 0.4). The inset in panel c shows the fitting parameters T* and A as functions of Rh-doping content x. Here, T* is the transition temperature from Fermi liquid to incoherent metal.

monotonically with increasing x, implying the decrease of lν, which leads to enhanced carrier localization. In the lower-temperature range, the hole hopping tends toward farther low-energy sites rather than neighboring highenergy sites. Consequently, the VRH mechanism is located in the lower-temperature range when the thermally activated energy is not high enough to make holes hop to near-neighbor high-energy sites. This is just the case for the Rh-doped samples below ∼15 K, as discussed previously. Moreover, because of the anisotropic layered structure, the VRH transport in the present samples is two-dimensional. As the temperature increases, the carriers can gain sufficient energy, and the transport mechanism gradually changes from VRH to thermally activated conduction (TAC).36 3.2.2. TAC Behavior. In the temperature range above ∼15 K, the 2D VRH model cannot describe the ρ(T) data well. As studied previously,13 the low-temperature MIT is caused by the emergence of the IC-SDW state. Generally, in the SDW state, the variation of resistivity with temperature exhibits TAC behavior,9,15,19 which can be described by the expression

ρ = ρ0 + AT 2

(4)

where ρ0 is the residual resistivity owing to the domain boundaries and other temperature-independent scattering mechanisms40 and A is the Fermi-liquid transport coefficient.13 AT2 reflects the electron−electron scattering mechanism of carriers. To obtain the effect of Rh doping on the electronic correlation of the Ca3Co4O9 system, we attempted to fit the ρ(T) curve in the metallic range above TMIN using eq 4. The

Table 1. Magnetic and Electrical/Thermal Transport Parameters for Ca3Co4−xRhxO9 (x = 0, 0.1, 0.2, 0.3, and 0.4) x=0

x = 0.1

x = 0.2

x = 0.3

x = 0.4

dc (Å) T0 (K) E0 (meV) TMIN (K) Ta (K) ρ300 K(mΩ cm) ρTMIN (mΩ cm)

10.68 (±0.003) 18 (±0.1) 0.6 (±0.04) 61 (±1) ∼400 (±10) 17.6 (±0.06) 9.9 (±0.04)

10.69 (±0.002) 544 (±0.3) 1.5 (±0.05) 84 (±1) ∼345 (±5) 17.2 (±0.03) 11.2 (±0.01)

10.70 (±0.005) 2581 (±0.2) 1.9 (±0.10) 97 (±1) 263 (±2) 16.2 (±0.02) 12.2 (±0.03)

10.72 (±0.003) 9560 (±0.5) 3.2 (±0.10) 162 (±1) 199 (±1) 16.4 (±0.01) 16.8 (±0.03)

10.73 (±0.001) 13054 (±0.2) 3.5 (±0.10) 164 (±1) 196 (±1) 16.4 (±0.03) 16.7 (±0.04)

μ300 K (cm2 V−1 s−1) θ (K) s κch 300 K (W K−1 m−1)

0.88 (±0.003) −296 (±1.2) 0.314 (±0.004) 0.06 (±0.002)

0.61 (±0.004) −317 (±1.4) 0.316 (±0.004) 0.04 (±0.006)

0.59 (±0.002) −318 (±2.3) 0.325 (±0.003) 0.02 (±0.002)

0.52 (±0.002) −346 (±0.9) 0.326 (±0.001) 0.03 (±0.002)

0.39 (±0.003) −373 (±1.6) 0.327 (±0.003) 0.03 (±0.005)

parameter

D



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fitting results are shown in the main panel of Figure 3c, and the fitting discrepancy of experimental data is less than 3%. From the figure, one can see that the temperature range in which the ρ(T) curve can be well fitted shifts to higher temperature and becomes narrower with increasing x. To clarify, the value of TMIN is listed in Table 1, and the fitting parameters T* and A are plotted against the Rh-doping content x in the inset of Figure 3c. Here, T* is defined as the end temperature of the linear dependence for ρ ∼ T2. From the inset of Figure 3c, one can see that A decreases and T* increases with increasing x. According to the dynamical mean field theory (DMFT),13,41 a key role of the effective mass m* of a Fermi liquid is predicted as A ∼ (m*)2 and T* ∼ 1/m*. The decrease of A and increase of T* both reflect a decrease of m*. The notable decrease of m* implies an increased bandwidth and a weakened electronic correlation in these compounds.13 3.2.4. SST Behavior. As the temperature increases from T*, the quasiparticle resonance becomes strongly temperaturedependent, and the resistivity increases with T. The higher resistivity value (exceeding the Mott limit) is associated with a bad or incoherent metal, and the system enters into the incoherent metal regime with a slope anomaly in the ρ(T) curve around Ta, as shown in Figure 2b.18 The slope anomaly at Ta is usually suggested to be induced by the SST of Co ions. As the Rh-doping content x increases, the SST temperature Ta shifts to lower temperatures, as listed in Table 1, implying that Rh doping in the Ca3Co4O9 system could induce the SST of Co ions at lower temperatures. The reason for such a phenomenon is discussed below. The SST suppresses electron hopping through a spin blockade and gives rise to the high-temperature MIT around T**.42 In the high-temperature range above T**, the quasiparticle resonance disappears altogether, leaving a pseudogap and forming a semiconductor at elevated temperatures.18 3.2.5. Electronic Phase Diagram. Based on the analysis of electrical transport mechanisms for the Ca3Co4−xRhxO9 (x = 0, 0.1, 0.2, 0.3, and 0.4) samples, the variations of the four characterized temperatures with temperature T and Rh-doping content x are plotted in Figure 4a. From the figure, several prominent features are worthy of special note: (1) TMIN shifts to higher temperatures with increasing x, indicating that the ICSDW state becomes more stable in these Rh-doped samples, which is suggested to be induced by the enhanced random Coulomb potential resulting from the introduction of Rh ions in the 2D triangular lattice of the conducting CoO2 block layers. (2) T* increases monotonically with increasing x, implying that the electronic correlation weakened in these Rhdoped samples. (3) More striking is the observation that Ta decreases monotonically from 400 K for Ca3Co4O9 to 197 K for the x = 0.4 sample. These results show that Rh doping in the Ca3Co4O9 system can induce the SST of Co ions at lower temperatures. As x increases, T** decreases, and the metal regime becomes obviously narrower. According to the variations of the four characteristic temperatures, we obtained the electronic phase diagram, which is exhibited in Figure 4b. For the electrical transport properties, the Ca3Co4−xRhxO9 system undergoes a gradual evolution with increasing temperature: from a low-temperature insulator/semiconductor to a strongly correlated Fermi-liquid metal (FLM) by way of the low-temperature MIT, then to an incoherent metal (ICM) followed by an SST, and finally to a semiconductor at elevated temperatures.15,17

Figure 4. (a) Variations of the four characterized temperatures TMIN, T*, Ta, and T** with temperature T and Rh-doping content x. (b) Electronic phase diagram for Ca3Co4−xRhxO9 (x = 0, 0.1, 0.2, 0.3, and 0.4), where S denotes semiconductor.

3.2.6. Physical Origin of Resistivity Variation. In Figure 2, aside from the evolution of the electrical transport mechanism, one can also observe the variation of the resistivity in the whole measured temperature region. As Rh ions are doped into the system, the room-temperature resistivity, ρ300 K, remains almost unchanged, whereas the low-temperature resistivity increases considerably. To explore the physical origin of the variation of the resistivity with temperature T and Rh-doping content x, Hall measurements at a series of temperatures were performed as a function of field for all samples. Figure 5a shows the fielddependent Hall resistivity (transverse resistivity), ρxy(H), for

Figure 5. (a) Field-dependent Hall resistivity ρ xy (H) for Ca3Co3.9Rh0.1O9 at four temperatures. (b) Carrier concentration n at four temperatures for Ca3Co4−xRhxO9 (x = 0, 0.1, 0.2, 0.3, and 0.4). Inset: Hall coefficient RH in the full investigated temperature range for all samples. E



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Figure 6. (a−c) ZFC and FC magnetic susceptibilities χ(T) as functions of temperature for samples with x = (a) 0, (b) 0.2, and (c) 0.4. Inset: Freezing temperature Tf as a function of Rh-doping content x. (d) Relationship between M and H at 5 K for x = 0 and 0.2. Inset: FC susceptibility at 5 K, χ5 K, as a function of Rh-doping content x.

induced by Rh doping in the conducting CoO2 block layers, resulting in a decrease of μ300 K, as listed in Table 1. The competition between the increasing n300 K and the decreasing μ300 K in the system leads to the invariance of ρ300 K. As temperature decreases, the electronic correlation between carriers enhances, and more carriers are localized. The carrier concentration plays a more important role in determining the low-temperature resistivity than the carrier mobility at low temperatures. As the temperature decreases to ∼200 K, the samples with x = 0−0.2 are still in the incoherent metal regime, and the variation of the carrier concentration is influenced mainly by the net Rh-doping content x. Thus, n200 K increases with increasing x from 0 to 0.2 as shown in the main panel of Figure 5b. However, for the x = 0.3 and 0.4 samples, the temperature of ∼200 K is close to their T* values, and the electronic correlation might need to be considered. With increasing x, the electronic correlation weakens because of the decreasing effective electron (that is, carrier) mass m*, leading to a decreasing effective carrier concentration n200 K in the x = 0.3 and 0.4 samples. Thus, n200 K first increases and then decreases, in coherence with the variation of 1/ρ200 K. As the temperature decreases to ∼100 K, n100 K decreases monotonically first for the lightly doped samples because of the weakened electronic correlation. The samples with x > 0.2 are close to the semiconducting regime dominated by the TAC mechanism. The increased E0 value enhances the carrier localization and decreases the effective carrier concentration. Thus, n100 K decreases monotonically with increasing x for all samples, resulting in a monotonic increase in ρ100 K. For the lowertemperature resistivity ρ10 K, because of the influence of the enhanced carrier localization, n10 K decreases by orders of magnitude with increasing x, as shown in the main panel of Figure 5b. Combined with the influence of the disorder effect on the carrier mobility, the low-temperature resistivity ρ10 K increases exponentially.

Ca3Co3.9Rh0.1O9 at four temperatures as an example, and the deduced Hall coefficient RH for all samples is shown in the inset of Figure 5b. One can observe a positive RH value throughout the investigated temperature range, implying that most of the charge carriers in these samples are holes. Moreover, RH exhibits globally a rather strong temperature dependence from 10 to 300 K along with a similarly changed tendency to the resistivity curve ρ(T), which may be related to the variation of transport mechanism following x and T. The obtained carrier concentration n is shown in the main panel of Figure 5b at four selected temperatures for all samples. It should be noted that the room-temperature carrier concentration, n300 K, increases monotonically with increasing x. These results further confirm that the valence of the doped Rh ions is indeed 3+ and that the doped Rh3+ ions introduce hole carriers into the system. The usual resistivity (longitudinal resistivity) ρ is the reciprocal of electrical conductivity σ and can be written as

ρ−1 = σ = neμ

(5)

where n and μ are the energy-correlated carrier concentration and mobility, respectively. In the present samples, the variation of the resistivity induced by Rh doping can be related to the changes in the concentration (n) and the mobility (μ) of the carriers. For the invariance of the room-temperature resistivity ρ300 K as listed in Table 1, on one hand, when the change in oxygen content is slight, the valence of Rh3+ is lower than the average valence (between 3+ and 4+) of Co ions in CoO2 layers.38 The substitution of Rh ions for Co in CoO2 layers can be considered as “hole doping”, as occurs naturally.13 Consequently, the room-temperature carrier concentration, n300 K, increases monotonically with increasing x; this result is consistent with that obtained from the Hall coefficient measurement as shown in the main panel of Figure 5b. On the other hand, the average distance between hole carriers decreases, and the scattering among the carriers enhances because of the increased n300 K value and the disorder effect F



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3.3. Magnetic Properties. Magnetic measurements of the Ca3Co4−xRhxO9 (x = 0, 0.1, 0.2, 0.3, and 0.4) samples were performed under zero-field-cooling (ZFC) and field-cooling (FC) modes with an applied magnetic field 0.1 T (5−350 K). The ZFC and FC magnetic susceptibilities χ(T) as functions of temperature for the samples with x = 0, 0.2, and 0.4 below 100 K are shown in the panels a−c, respectively, of Figure 6. For all samples, χ(T) exhibits Curie−Weiss paramagnetic (PM) behavior in the high-temperature range. As the temperature decreases to ∼19 K, the FC susceptibility increases sharply, suggesting that it originates from the FIM transition.43,44 Compared to the FC curves, the ZFC χ(T) curves are more complicated. As temperature decreases, the ZFC χ(T) increases continuously and reaches a maximum at a certain temperature, which is defined as the freezing temperature Tf, and then decreases rapidly in the limited temperature range of Tδ < T < Tf. As the temperature decreases further, the ZFC χ(T) increases again. That is, there exists a cusp at Tf and a valley at Tδ in the ZFC χ(T) curve for all samples. Moreover, there exists a distinctive separation between the FC and ZFC χ(T) curves in the low-temperature range. Similar magnetic behavior was also observed in Ca3Co4−xTixO921 and Ca3Co4−xCuxO927 single crystals, which is usually considered to originate from the appearance of the spin-glass (SG) or cluster-glass (CG) state because of the competition between ferromagnetic (FM) and antiferromagnetic (AFM) exchange interactions. Indeed, in the Ca3Co4O9 system, the FIM interaction is the interlayer coupling interaction between the Ca2CoO3 and CoO2 layers, whereas both FM and AFM interactions are found according to the Co−Co bonds within CoO2 layers at low temperatures.16 To further explore the variation of the magnetic properties, Tf and the low-temperature FC susceptibility at 5K, χ5 K, are plotted in the insets of panels c and d, respectively, of Figure 6. As shown in the inset of Figure 6c, the value of Tf increases with increasing x, implying the enhancement of competition between the FM and AFM exchange interactions in CoO2 layers induced by Rh doping. From the inset of Figure 6d, one can see that χ5 K decreases monotonically with increasing x, implying that the low-temperature FIM state can be well suppressed by Rh doping, which can also be further confirmed by the M(H) curves shown in the main panel of Figure 6d. The M(H) curve of the undoped sample exhibits a clear hysteresis loop, whereas the hysteresis phenomenon is obviously weakened for the Rh-doped samples. Considering the alternating stacking structure of the Ca3Co4O9 system along the c axis, the ferrimagnetism is considered to be caused by the interlayer coupling interaction of Co ion spins between the Ca2CoO3 and CoO2 layers. The Ca2CoO3 layers consist of two Ca−O planes and one Co−O plane, with the Co−O plane sandwiched by the two Ca−O planes. If element substitution occurs in the Co−O plane of the Ca2CoO3 layers and the doping content is low, the FIM interaction between different layers should not be affected considerably. A previous study also showed that the ferrimagnetism is not sensitive to slight doping at the Co sites in Ca2CoO3 layers.13 The fact that Rh doping can give rise to an obvious suppression of the FIM interaction in the Ca3Co4O9 system confirms that Rh doping probably occurs mainly in the CoO2 layers, which is consistent with the XRD results and electrical transport analysis mentioned previously. In the main panel of Figure 7a, the temperature dependence of FC, dχ(T)/dT, for Ca3Co3.8Rh0.2O9 is plotted. It can be seen that there exists an obvious anomaly in the dχ/dT versus T

Figure 7. (a) Temperature dependence of FC, dχ(T)/dT, for Ca3Co3.8Rh0.2O9. Inset: FIM transition temperature TFIM and effective magnetic moment μeff as functions of Rh-doping content x. (b) Temperature dependence of dχ−1(T)/dT for Ca3Co4−xRhxO9 (x = 0, 0.1, 0.2, 0.3, and 0.4) below 100 K. Inset: Completion temperature of the long-rang IC-SDW ordering, Tend SDW, and FIM transition temperature, T1, as functions of Rh-doping content x.

curve in the vicinity of TFIM. The anomaly near TFIM might be related to the FIM transition. As x increases, the value of TFIM decreases monotonically, as shown in the inset of Figure 7a. These results show that Rh doping has a negative effect on the FIM transition in the Ca3Co4O9 system, as already mentioned. To study the magnetic structure of the present samples in depth, we plot the low-temperature values of dχ−1(T)/dT [i.e., the slope of the inverse FC susceptibility χ, χ−1(T)] versus T for all samples below 100 K in the main panel of Figure 7b. Interestingly, there exists a small shoulder in the dχ−1(T)/dT versus T curves around Tend SDW for all samples, as shown by the circle in the main panel of Figure 7b. Such a magnetic shoulder has also been observed in both single-crystal and polycrystalline Ca3Co4O9 samples,45 for which it was suggested to originate from the completion of the long-rang IC-SDW ordering. This behavior has already been detected by muon spin rotation and relaxation (μSR) measurements.33,43,45,46 Therefore, we can ascribe the anomaly in the dχ−1(T)/dT versus T curve near Tend SDW in the present samples to the natural completion of the long-rang IC-SDW ordering. Moreover, with increasing x, Tend SDW increases monotonically, as shown in the inset of Figure 7b. These results show that Rh doping can stabilize the SDW state in these layered cobaltite materials, which is consistent with the results of the electrical transport analysis already discussed. In addition to the anomaly near Tend SDW, a cusp near T1 is also observed in the dχ/dT versus T curve that is close to TFIM in the dχ(T)/dT curve. The anomaly near T1 might also be related to the FIM transition. As x increases, the value of T1 decreases monotonically similarly to TFIM, as shown in the inset of Figure 7b. To analyze quantitatively the effect of Rh doping on the magnetic properties, we tried to fit the χ−1(T) curves in the high-temperature range according to the Curie−Weiss law χ −1 (T ) = 3kB(T − θ )N0−1μeff −2 G

(6)



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where kB, T, θ, N0, and μeff are the Boltzmann constant, the absolute temperature, the Weiss temperature, Avogadro’s number, and the effective magnetic moment, respectively.21,25 The R2 values for the fitting parameters are all greater than 0.999, and the values of θ and μeff were obtained. As reported in Table 1, the Weiss constants θ of all studied samples are negative, and the absolute value of θ increases monotonically with increasing x. The negative values of θ indicate the presence of AFM interactions at low temperatures in the CoO2 layers for all studied samples. In addition, the change in the absolute value of θ reflects the enhancement of the AFM interaction in the samples.13 The values of the effective magnetic moment μeff are 1.2837, 1.2908, 1.3141, 1.3147, and 1.3182 μB/Co for the samples with x = 0, 0.1, 0.2, 0.3, and 0.4, respectively, as shown in the inset of Figure 7a. The value of μeff for the undoped sample is consistent with those reported previously, namely, 1.24 μB/Co.46,47 Based on this analysis for the Ca3Co4−xRhxO9 (x = 0, 0.1, 0.2, 0.3, and 0.4) samples, the successive evolution of the magnetic properties with T and x can be summarized in the simple magnetic diagram presented in Figure 8a. It shows that the

S(T), for Ca3Co4−xRhxO9 (x = 0, 0.1, 0.2, 0.3, and 0.4) samples. The positive values of S reflect the p-type conductive feature. For the undoped sample Ca3Co4O9, the thermopower increases monotonically with increasing T up to ∼200 K. However, for the Rh-doped samples, the S(T) curve shows a distinct minimum at TSmin, and the thermopower is strongly temperature-dependent in the range between TSmin and 200 K.48 The presence of the minimum in the S(T) curve might be related to the presence of the renormalized band in the vicinity of the Fermi level induced by the substitution of Rh ions with extended 4d electron orbitals. In the high-temperature range above 200 K, the thermopower, S(T), of all samples exhibits a nearly temperature-independent behavior. It should be noted that the S300 K value of Ca3Co4O9 is about 122.6 μV/K, which is close to that reported earlier for a single-crystal sample.25 In addition, S300 K remains keeps constant for all samples, as shown in the inset of Figure 9a. In general, the thermopower, S, of the Ca3Co4O9 system can be expressed by the Mott formula13,21,25 S=

Ce π 2kB 2T ⎡ ∂ ln μ(ε) ⎤ + ⎢ ⎥ n 3e ⎣ ∂ε ⎦ε = ε

F

(7)

where Ce, n, kB, and μ(ε) are the electronic specific heat, carrier concentration, Boltzmann constant, and energy-correlated carrier mobility, respectively. In a general material, thermopower is dominated by the first term of eq 7.13,21 However, in our studied samples, the variation of thermopower induced by Rh doping cannot be explained by the first term of eq 7 alone, because both the increased carrier concentration and the weakened electronic correlation can induce a decreased thermopower. Another factor must be considered to clarify this issue, such as the carrier mobility, μ(ε), in the second term of eq 7. As discussed previously, with increasing x, the roomtemperature carrier mobility, μ300 K, decreases because of the disorder caused by Rh doping, and its rate of change increases obviously, as reported in Table 1. This is why S300 K remains almost constant with increasing x in the present studied samples. In the following analysis, we consider the effect of Rh doping on the thermal transport properties of the Ca3Co4−xRhxO9 (x = 0, 0.1, 0.2, 0.3, and 0.4) system. The main panel of Figure 9b shows the temperature dependence of the thermal conductivity κ(T). In general, the thermal conductivity in a material can be expressed as the sum of the phonon thermal conductivity component κph and the carrier thermal conductivity component κch4

Figure 8. (a) Simple magnetic phase diagram for Ca3Co4−xRhxO9 (x = 0, 0.1, 0.2, 0.3, and 0.4), along with variations of the three on characterized temperatures TFIM, Tend SDW, and TSDW, with temperature T and Rh-doping content x. (b) Electronic/magnetic phase diagram for Ca3Co4−xRhxO9 (x = 0, 0.1, 0.2, 0.3, and 0.4).

κ = κ ph + κch

Ca3Co4−xRhxO9 system undergoes continuous evolution with decreasing temperature: It changes from a PM phase to an FIM phase by way of the low-temperature FIM transition. In the PM state, short-range IC-SDW ordering appears below Ton SDW, and long-range IC-SDW ordering is completed around Tend SDW. 3.4. Electronic/Magnetic Phase Diagram. To show the correlation between the electrical and magnetic properties, we present the magnetic phase diagram overlaid on the electronic phase diagram in Figure 8b. This figure clearly shows the electronic/magnetic evolution with temperature T and Rhdoping content x for the Ca3Co4−xRhxO9 (x = 0, 0.1, 0.2, 0.3, and 0.4) system. 3.5. Thermoelectric Properties. The main panel of Figure 9a shows the temperature dependence of the thermopower,

(8)

where the value of the κch can be calculated from the Wiedemann−Franz (WF) law, which relates κch to ρ as κch = LT/ρ, where L is the Lorentz number (2.45 × 10−8 V2 K−2 for free electrons). From the calculation results, κch in the present samples is less than 1.5% of the κ value as listed in Table 1.36 Therefore, the contribution of κch to the total κ value is quite small, and the phonon thermal conductivity is the main source of the total thermal conductivity in the present studied samples.13 Considering that the ionic radius of Rh ions is larger than those of Co3+ and Co4+, Rh doping can cause more disorder and structural distortion in the conducting CoO2 block layers. The induced lattice disharmony will scatter phonons, resulting in the suppression of phonon transport. As the RhH



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Figure 9. Temperature dependence of (a) thermopower S(T), (b) thermal conductivity κ(T), and (c) dimensionless figure of merit ZT(T) for Ca3Co4−xRhxO9 (x = 0, 0.1, 0.2, 0.3,and 0.4). The insets in panels a−c show the room-temperature thermopower S300 K, thermal conductivity κ300 K, and figure of merit ZT300 K, respectively, as functions of Rh-doping level x.

doping content x increases, κph and then κ decrease, as shown in the inset of Figure 9b. Using the measured ρ, S, and κ values, we calculated the dimensionless TE figure of merit ZT (defined as ZT = S2T/ρκ) for the samples. The ZT values as functions of temperature for the Ca3Co4−RhxO9 (x = 0, 0.1, 0.2, 0.3, and 0.4) samples are shown in the main panel of Figure 9c. Although a sharp thermopower upturn exists in the S(T) curves of all of the Rhdoped samples, no obvious anomaly can be seen in their ZT(T) curves in the same temperature range because of the exponential increase of the resistivity. For all samples, ZT decreases monotonically with decreasing temperature. As the Rh-doping content x increases, the value of ZT increases monotonically because of the rapid decrease of κ. The ZT value of Ca3Co3.6Rh0.4O9 at room temperature reaches 0.014, which is about 2.4 times larger than that of Ca3Co4O9, as shown in the inset of Figure 9c. These results show that Rh doping is an effective route to improving the TE performance of the Ca3Co4O9 system. 3.6. Discussion: Room-Temperature SST of Co4+ Ions. In a CoO2 octahedron, the 3d transition-metal Co ions are surrounded by oxygen anions. The 5-fold-degenerate 3d orbitals are split into the triply degenerate t2g orbitals and the doubly degenerate eg orbitals. The energy gap between the t2g and eg levels, called the crystal-field splitting energy Δ, competes with the Hund coupling energy K, forming three kinds spin states, namely, LS, IS, and HS for both Co3+ and Co4+ ions. The numbers of configurations g3 and g4 (defined as the spin−orbital degeneracies for Co3+ and Co4+ ions, respectively) are related to the parameters Δ, K, and T. When the crystal-field splitting is large, that is, kBT ≪ Δ and K ≪ Δ, the 3d electrons first occupy low-energy t2g states to minimize the total spin quantum number, forming the LS state.20,49 X-ray absorption and photoemission spectroscopy studies have already shown that, in undoped Ca3Co4O9, the Co−O triangular lattice have spin-1/2 Co4+ (LS, t2g5eg0) species in the nonmagnetic Co3+ (LS, t2g6eg0) background, and g3 and g4 are 1 and 6, respectively.42,44,46,50 For the doped 4d transition-metal Rh ions, on one hand, the volume of the electron shell of 4d electrons is larger than that of 3d electrons. Therefore, its bandwidth is wider than that of 3d electrons, implying a weakened electronic correlation. Thus, when 3d Co ions are substituted by 4d Rh ions, the bands become wider, leading to a narrower gap between the t2g and eg orbitals. On the other hand, the Rh ion has a larger ionic radius

than the Co3+ and Co4+ ions. The longer Rh−O distances would produce a weaker crystal field Δ.42 In this case, some electrons in t2g orbitals can more easily jump to the higherenergy eg orbitals to form a higher spin state. Thus, the energy to excite such a jump is lower than that in the undoped sample, so that the SST can occur in a lower temperature range. From the analysis of the resistivity, x = 0.2 Rh doping is sufficient to bring the SST to a temperature below 300 K. Moreover, the magnitude of Δ that gives a level crossing between HS and LS states in the d5 configurations for Co4+ (Δ4) is larger than that in the d6 configurations for Co3+ (Δ3).20 As Δ decreases, for kBT ≪ Δ, K, Δ ≈ Δ4, in the present samples, the electron jump induced by Rh doping might occur mostly in Co4+ ions. Consequently, the LS states of all Co3+ ions and the LS states of most Co4+ ions are stable, and the LS and HS states of some Co4+ ions are close in energy to form mixed states (LS + HS). In addition, the LS state is realized in Co3+ sites, and the thermopower has a larger value.20 Therefore, the spin states in the Rh-doped samples are LS states for Co3+ and LS states and mixed states (LS + HS) for Co4+, which has been added into the electronic and electronic/magnetic phase diagrams in Figures 4b and 8b, respectively. It is suggested that g3 = 1 for Co3+, and the average value of the degeneracy of Co4+, g4, increases. Here, the degeneracy of the mixed states (LS + HS) for Co4+ is 12, which is given by the total degeneracy of the LS and HS states of Co4+. To summarize, the doped Rh ions occupy the Co3+ sites, induce the SST of Co4+ ions in CoO2 layers, and the Co2+ ions remain mainly unchanged because of the invariant crystal structure in Ca2CoO3 layers. These results suggest that a LS Rh3+ ion preferentially substitutes for a LS Co3+ ion to induce the SST of a Co4+ ion and the excited state of the Rh-doped samples is described by the HS−LS model.51 In such an HS−LS model, it is proposed that the LS Rh3+ stabilizes the neighboring HS Co4+ to form a spin-state polaron.20 It is well-known that, in a strongly correlated system, the thermopower can also be expressed using the modified Heikes formula in the high-temperature limit52 S=−

k B ⎡ g 3 ⎛ y ⎞⎤ ln⎢ ⎜ ⎟⎥ e ⎢⎣ g4 ⎝ 1 − y ⎠⎥⎦

(9)

where y is the Co4+ concentration. Now, we discuss the roomtemperature SST of Co4+ ions quantitatively in this system according to eq 9. For the calculation, we consider several cases I



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as listed in Table 2:31 (i) For Ca3Co4O9, S300 K is about 122.6 μV/K, and g3 and g4 are 1 and 6.00, respectively. Thus, the

states and all [LS + (LS + HS)] states. Hence, the LS states of all Co3+ ions and 85.07% of the Co4+ in this sample are stable. Of the LS Co4+ ions, 7.46% can be excited to the HS state to form a mixed spin state (LS + HS), which is in favor of a larger degeneracy (∼12) in the sample. (iii) For Ca3Co3.8Rh0.2O9, g3 and g4 are 1 and 7.58 for Co3+ and Co4+, respectively, and 13.17% of the LS Co4+ ions are excited to HS state. (iv) For Ca3Co3.7Rh0.3O9, g3 and g4 are 1 and 8.93 for Co3+ and Co4+, respectively, and 24.37% of the LS Co4+ ions are excited to the HS state. (v) For Ca3Co3.6Rh0.4O9, g3 and g4 are 1 and 10.22 for Co3+ and Co4+, respectively, and 35.21% of the LS Co4+ ions are excited to the HS state. From the calculations, one can conclude that the ratio of HS Co4+ ions increases from 0 for Ca3Co4O9 to 35.21% for the x = 0.4 sample at room temperature. In other words, the SST of Co4+ appears at lower temperatures, and thus, the SST temperature shifts to lower values gradually in the Rh-doped samples compared to the undoped sample (∼400 K), which is consistent with the shift of the slope anomaly around Ta in the ρ(T) curve. The increased relative concentration and the SST from LS (s = 1/2, μeff = 1.73 μB) to HS (s = 5/2, μeff = 5.92 μB) for Co4+ ions induced by Rh doping can lead to a gradual evolution in the magnetic and transport properties. For example, the average values of the quantum number of spin s and then the effective magnetic moment μeff will increase as shown in Table 1 and the inset of Figure 7a, respectively. To test the interpretation for the Rh-doped site and content, room-temperature XPS measurements were performed on three selected samples with x = 0, 0.1, and 0.3, and the results are shown in Figure 10. From Figure 10a, it can be seen two peaks located at BEs of 309.3 and 314.1 eV, which can be attributed to Rh 3d5/2 and Rh 3d3/2, respectively. We tried to deconvolute the Rh 3d5/2 peak for the sake of exploring the Rhdoping content, as shown in panels c and d of Figure 10 for the samples with x = 0.1 and 0.3, respectively. No obvious peak shifting is observed for different samples, confirming that the doped Rh ions are indeed in the Rh3+ state corresponding to

Table 2. g3/g4 Ratios and Ratios of the Mixed Spin States (LS + HS) z in Cases i−v for Ca3Co4−xRhxO9 (x = 0, 0.1, 0.2, 0.3, and 0.4) case I Rh3+/FU(x) Co/FU(1− x) Co2+/FU Co3+/FU Co4+/FU y g3/g4 LS Co3+/FU LS Co4+/FU HS Co4+/ FU HS Co4+ (%) (LS + HS) Co4+/FU z (%)

case ii

case iii

case iv

case v

0 4

0.1 3.9

0.2 3.8

0.3 3.7

0.4 3.6

1.58 1.00 1.42 0.59 (±0.003) 1/6.00 (±0.01) 1.00 1.42 (±0.000) 0 (±0.000)

1.58 0.90 1.42 0.61 (±0.002) 1/6.90 (±0.01) 0.90 1.31 (±0.004) 0.11 (±0.004) 7.46 (±0.29) 0.22 (±0.008) 14.93 (±0.57)

1.58 0.80 1.42 0.64 (±0.000) 1/7.58 (±0.00) 0.80 1.23 (±0.003) 0.19 (±0.003) 13.17 (±0.21) 0.38 (±0.006) 26.34 (±0.42)

1.58 0.70 1.42 0.67 (±0.000) 1/8.93 (±0.05) 0.70 1.07 (±0.004) 0.35 (±0.004) 24.37 (±0.27) 0.70 (±0.008) 48.73 (±0.55)

1.58 0.60 1.42 0.70 (±0.003) 1/10.22 (±0.04) 0.60 0.92 (±0.000) 0.50 (±0.000) 35.21 (±0.00) 1.00 (±0.000) 70.42 (±0.00)

0 (±0.000) 0 (±0.000) 0 (±0.00)

calculated Co4+ concentration y is about 0.59 according to the Heikes formula. (ii) For Ca3Co3.9Rh0.1O9, S300 K is about 125.7 μV/K, the LS states of Co3+ are stable, and g3 is 1. The calculated Co4+ concentration y is about 0.61 based on the nominal content listed in Table 2. Thus, according to the Heikes formula, the average value of g4 is calculated as 6.90 for Co4+. Then, based on the degeneracy balance for Co4+ ions, the concentrations of the mixed state z and the HS state of Co4+ ions are estimated as 14.93% and 7.46%, respectively. Here, z is defined as the ratio of Co4+ ions between mixed (LS + HS)

Figure 10. (a,b) XPS results for (a) Co and (b) Rh ions. (c,d) Deconvoluted results of Rh 3d5/2 for samples with x = (c) 0.1 and (d) 0.3. (e−g) Deconvoluted results of Co 2p3/2 for samples with x = (e) 0.0, (f) 0.1 and (g) 0.3. J



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BE ≈ 309.3 eV. Additionally, as x increases, the area of the deconvoluted peak for Rh3+ ions increases obviously between panels c and d of Figure 10. Here, the area of the deconvoluted peak represents the element content, indicating the increase in Rh-doping content. In Figure 10b, two peaks are located near ∼779 and 794 eV, which correspond to Co 2p3/2 and Co 2p1/2, respectively. To obtain the molar contents of Co2+, Co3+, and Co4+ ions, the Co 2p3/2 peak was also deconvoluted, and the results are shown in Figure 10e−g. It can be observed that the total peak area for Co 2p3/2 decreases obviously with increasing x, indicating that Rh ions are doped into the Co sites in this system. The Co 2p3/2 peak can be deconvoluted into three peaks, and the peaks corresponding to the higher BE, middle BE, and lower BE are attributed to Co2+, Co3+, and Co4+, respectively. Based on the peak area ratios among them, the deconvoluted molar contents for the three types of Co ions along with that of Rh3+ ions are listed in Table 3. From Table 3,

does not change obviously upon Rh doping, but the thermal conductivity decreases significantly. As a result, the ZT value increases markedly with increasing Rh-doping content. The ZT value at room temperature for Ca3Co3.6Rh0.4O9 reaches 0.014, which is about 2.4 times larger than that of Ca3Co4O9. The results show that Rh doping might be an effective route to improving the TE performance of the Ca3Co4O9 system.

■

Corresponding Author

*E-mail: [email protected] (B.Z.), [email protected] (Y.S.). Tel.: +86-551-6559-2757. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Basic Research under Contract 2011CBA00111; the National Nature Science Foundation of China under Contracts 11174293, U1232140, 11174288, and 10904151; and the Scientific Research Foundation for the Returned Overseas Chinese Scholar.

Table 3. True Stoichiometry of Ca3Co4−xRhxO9 (x = 0, 0.1, and 0.3) Arising from the Results of XPS Analysis Ca3Co4O9−δ Rh3+/FU(x) Co/FU Co2+/FU Co3+/FU Co4+/FU

0.00 4.00 1.58 1.00 1.42

(±0.05) (±0.02) (±0.03) (±0.00)

Ca3Co3.9Rh0.1O9−δ 0.13 3.87 1.58 0.92 1.37

(±0.01) (±0.06) (±0.04) (±0.01) (±0.01)

Ca3Co3.7Rh0.3O9−δ 0.35 3.65 1.59 0.68 1.38

AUTHOR INFORMATION

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(±0.01) (±0.03) (±0.01) (±0.01) (±0.01)

REFERENCES

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one can see that (i) the doped Rh-ion content is close to the nominal content, verifying that Rh ions are indeed doped into the system; (ii) the contents of Co2+ and Co4+ remain invariant as a whole, whereas the Co3+ content decreases monotonically, indicating that Rh ions are doped into the Co3+ sites in CoO2 layers, which is in agreement with the XRD analysis and confirms our interpretation for the Rh-doped site and content, as discussed earlier and listed in Table 2.

4. CONCLUSIONS Polycrystalline samples of Ca3Co4−xRhxO9 (x = 0, 0.1, 0.2, 0.3, and 0.4) have been prepared by the conventional solid-state reaction method. The effects of 4d Rh doping on the structural, magnetic, electrical, and thermal transport properties were investigated systematically. The XRD and XPS results show that the doped Rh ions are in the form of Rh3+. Only a metal− insulator transition (MIT) and an anomaly of slope related to the transition from the Fermi liquid to the incoherent metal at low temperatures are observed in the resistivity curve for the undoped sample. As Rh ions are doped into the samples, an additional anomaly and MIT occur in the resistivity curve near room temperature, which are suggested to originate from the spin-state transition of Co ions. The low-temperature MIT temperature increases with increasing Rh-doping content, indicating that the spin-density-wave state becomes more stable because of the enhanced random Coulomb potential in the CoO2 octahedral block layers induced by Rh substitution. Based on an analysis of the thermopower and XPS data, Rh3+ ions are suggested to substitute in Co3+ sites in CoO2 layers. The substitution induces the SST of some of the Co4+ ions from the low-spin to high-spin state, leading to the formation of the spin-state polaron, corresponding to the anomaly in the slope of the ρ(T) curves near Ta. The evolution of the electrical and magnetic properties with Rh doping is summarized in a single phase diagram for Ca3Co4−xRhxO9. The thermopower K



Article

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