Open access peer-reviewed chapter

Layered Cobaltites and Natural Chalcogenides for Thermoelectrics

Written By

Ran Ang

Submitted: March 31st, 2016 Reviewed: September 8th, 2016 Published: December 21st, 2016

DOI: 10.5772/65676

Chapter metrics overview

1,954 Chapter Downloads

View Full Metrics

Abstract

We have systematically investigated thermoelectric properties by a series of doping in layered cobaltites Bi2Sr2Co2Oy, verifying the contribution of narrow band. In particular, Sommerfeld coefficient is dependent on charge carriers’ density and as function of density of states (DOS) at Fermi level, which is responsible for the persistent enhancement of large thermoelectric power. Especially for Bi2Sr1.9Ca0.1Co2Oy, it may provide an excellent platform to be a promising candidate of thermoelectric materials. On the other hand, high‐performance thermoelectric materials require elaborate doping and synthesis procedures, particularly the essential thermoelectric mechanism still remains extremely challenging to resolve. In this chapter, we show evidence that thermoelectricity can be directly generated by a natural chalcopyrite mineral Cu1+xFe1−xS2 from a deep‐sea hydrothermal vent, wherein the resistivity displays an excellent semiconducting character, while the large thermoelectric power and high power factor emerge in the low x region where the electron‐magnon scattering and large effective mass manifest, indicative of the strong coupling between doped carriers and localized antiferromagnetic spins, adding a new dimension to realizing the charge dynamics. The present findings advance our understanding of basic behaviors of exotic states and demonstrate that low‐cost thermoelectric energy generation and electron/hole carrier modulation in naturally abundant materials is feasible.

Keywords

  • layered cobalt oxides
  • narrow band contribution
  • natural chalcopyrite mineral
  • thermoelectricity generation
  • electron‐magnon scattering

1. Introduction

Layered cobaltites with CdI2‐type CoO2 block provide an excellent platform for investigating thermoelectric properties. A key to unveil mysterious thermoelectric properties lies in the two‐dimensional (2D) conducting CoO2 layer. For layered Bi‐A‐Co‐O (A= Ca, Sr, and Ba), it also contains analogous conducting CoO2 layer [1]. In particular, layered Bi2Sr2Co2Oy(BSC) exhibits a rather large thermoelectric power S(∼100 μV/K) at room temperature, which makes Bi2Sr2Co2Oyone of promising thermoelectric materials from the viewpoint of potential applications, analogous to other misfit‐layered cobaltites, such as NaCo2O4 and Ca3Co4O9 [25]. However, most studies of Bi2Sr2Co2Oysystem are mainly focused on the thermoelectric improvement [2, 3, 6]. The transport mechanism based on resistivity ρand thermoelectric power Shas not been clarified. Moreover, large Sis totally different from conventional value (<10 μV/K) based on a broad band model [7]. In this chapter, we will show evidence on a narrow band contribution in doped Bi2Sr2Co2Oy[8]. And what's more, exotic enhancement of large Sis related to local density of states (DOS) near Fermi level (EF) [9]. It could be effectively modulated thermoelectric performance by utilizing different doping. It is plausible to distinguish, which thermoelectric materials in doped Bi2Sr2Co2Oycould be regarded as potential candidates.

On the other hand, ternary chalcogenides serve as an ideal platform for investigating intricate physical and chemical characteristics controlling the efficiency of thermoelectric materials, and also are promising materials for potential applications in photovoltaics, luminescence, as well as thermoelectric and spintronic devices [1013]. Ternary chalcopyrite‐structured chalcogenides, such as CuFeS2, have attracted particular attention owing to their unique optical, electrical, magnetic, and thermal properties [1428]. Studies on chalcopyrite (CuFeS2) have primarily focused on its electronic states [14, 15, 2931]. However, the microscopic mechanism of electronic structure and thermoelectric character in CuFeS2, which presumably arises from some scenarios such as delocalization of the Fe 3delectrons, charge‐transfer‐driven hybridization between Fe 3dand S 3porbitals, or density of the conduction band electron states, still remains highly controversial [17, 30, 32]. The intrinsic mechanism of good thermoelectric properties is still a vital question which needs to be clarified. Another important issue is that the fabrication of artificial chalcopyrite itself requires expensively complex synthesis procedures and relatively high cost of constituent precursors, thereby potentially limiting the large‐scale applications in the thermoelectric field.

In this chapter, we confirm that an unexpected thermoelectricity can directly be generated in a natural chalcopyrite mineral Cu1+xFe1−xS2 from a deep‐sea hydrothermal vent, and demonstrate that doped carriers have strong coupling with localized antiferromagnetic (AFM) spins, which greatly enhance the thermoelectric power Sand power factor, revealing the significance of electron‐magnon scattering and large effective mass [33]. This will open up another useful avenue in manipulating low‐cost thermoelectricity or even electron/hole carriers via the natural energy materials abundantly deposited in the earth.

Advertisement

2. Thermoelectric properties and narrow band contribution of Bi2Sr1.9M0.1Co2Oyand Bi2Sr2Co1.9X0.1Oy

2.1. Crystal structure and valence states of Co ions

The crystal structure of Bi2Sr2Co2Oyis shown in the inset in Figure 1, where conducting CoO2 layer with triangular lattice and insulating rocksalt Bi2Sr2O4 block layer are alternatively stacked along c‐axis, similar to the case of high‐temperature superconductors like Bi2Sr2CaCu2Oy. Scanning electron microscopy (SEM) characterization of Bi2Sr2Co2Oyindicates surface morphology of plate‐like grains. Figure 1 shows X‐ray diffraction (XRD) patterns of selected samples Bi2Sr2Co2Oy, Bi2Sr1.9Ca0.1Co2Oy, and Bi2Sr2Co1.9Mo0.1Oywith single phase, in agreement with XRD result of Bi1.4Pb0.6Sr2Co2Oy[34]. The average Co valence was determined based on energy dispersive spectroscopy (EDS) measurement for all samples. For Bi2Sr2Co2Oy, average Co valence is +3.330. For Bi2Sr1.9M0.1Co2Oy(M= Ag, Ca, and Y), average Co valence is +3.380, +3.330, and +3.280, respectively. For Bi2Sr2Co1.9X0.1Oy(X= Zr, Al, and Mo), average Co valence is +3.295, +3.347, and +3.189, respectively. X‐ray photoemission spectroscopy (XPS) spectra (see Figure 4a) also show the valence states of Co 2p3/2 and 2p1/2 for selected Bi2Sr1.9Ca0.1Co2Oysample. Photon energy of Co 2p3/2 and 2p1/2 is 779.4 and 794.2 eV, respectively, demonstrating mixed Co valence between +3 and +4.

Figure 1.

Powder XRD patterns for Bi2Sr2Co2Oy, Bi2Sr1.9Ca0.1Co2Oy, and Bi2Sr2Co1.9Mo0.1Oysamples at room temperature. Inset: crystal structure and SEM image of Bi2Sr2Co2Oy.

2.2. Resistivity and transport mechanism

Figure 2a and d shows temperature dependence of resistivity ρ(T) of all samples. For parent Bi2Sr2Co2Oysample, an upturning point at Tp (∼75 K) is observed. Metallic behavior above Tp appears, demonstrating existence of itinerant charge carriers. Compared with Bi2Sr2Co2Oy, ρ(T) of all doped samples (except Bi2Sr1.9Ca0.1Co2Oy) display total increase in view of the disorder effect. Furthermore, enhanced random Coulomb potential because of the doping induces the obvious shift of Tp toward higher temperature. On the other hand, ρ(T) of Bi2Sr1.9Ca0.1Co2Oypresents an overall decrease due to introduction of hole–type charge carriers into conducting CoO2 layers.

Figure 2.

(a) Temperature dependence of resistivityρ(T) and inset: magnification plot ofρ(T) for Bi2Sr2Co2Oy(BSC) and Bi2Sr1.9M0.1Co2Oy(M= Ag, Ca, and Y) samples. (b) Plot of lnρagainstT−1 for Bi2Sr2Co2Oyand Bi2Sr1.9M0.1Co2Oysamples. Solid lines stand for TAC fitting. Dashed curves express VRH fitting. (c) Bi2Sr2Co2Oyand Bi2Sr1.9M0.1Co2Oydependence of activation energyΔE, onset temperatureTp of TAC, and onset temperatureThopping of VRH. The shadow in bold is guide to the eyes. (d)–(f) are similar to (a)–(c) but for Bi2Sr2Co1.9X0.1Oy(X= Zr, Al, and Mo) samples.

To get insight into the conduction mechanism below Tp, dependences of ln ρon T−1 are plotted in Figure 2b and e. At the beginning, it is found that thermally activated conduction (TAC) law matches ρ(T) data well below Tp, namely [35], ρ(T)=ρ0exp(ΔE/kBT), where ΔEis activation energy. Interestingly, ρ(T) apparently deviates from the TAC behavior with decreasing temperature further, and it follows Mott's variable‐range-hopping (VRH) model described by equation [35]: ρ(T)=ρ0exp[(T0/T)n]. As seen from Figure 2c and f, obtained values of ΔEand onset temperature Thopping of Bi2Sr1.9Ca0.1Co2Oy(0.66 meV and 15.3 K) are the respective minimum, even smaller, than those of parent Bi2Sr2Co2Oy(0.70 meV and 16.2 K), while ΔEand Thopping of Bi2Sr2Co1.9Zr0.1Oy(2.75 meV and 63.6 K) are both maximum among all samples.

2.3. Thermoelectric power and narrow band model

Figure 3a and b shows temperature dependence of thermoelectric power S(T) for all samples. Positive values of Sreflect electrical transport feature dominated by holes. Values of Sat room temperature for all doped samples produce a substantial increase, especially for Bi2Sr2Co1.9Mo0.1Oy(∼117 μV/K), compared with pristine Bi2Sr2Co2Oy(∼92 μV/K). Particularly, with decreasing the temperature until below Thopping, S(T) behavior follows with VRH model [36]: SVRH(T) ∼ aT1/2, where ais factor determined by density of localized states at Fermi level N(EF). The inset in Figure 3b reveals Anderson localization of Bi2Sr2Co1.9Mo0.1Oy, in correspondence with low‐temperature resistivity.

Figure 3.

Temperature dependence of thermoelectric powerS(T) for Bi2Sr2Co2Oy, (a) Bi2Sr1.9M0.1Co2Oy(M= Ag, Ca, and Y), and (b) Bi2Sr2Co1.9X0.1Oy(X= Zr, Al, and Mo) samples. Inset: calculated and fitted results of (a) Boltzmann formula and (b) VRH model for Bi2Sr2Co1.9Mo0.1Oysample, respectively. Schematic diagram of density of states in a narrow band with Anderson localization at (c) high temperatures (metallic or TAC region) and (d) low temperatures (VRH region).

In general, Sis extremely small (<10 μV/K) and presents a metallic behavior in a broad band [7]. Taking into account the huge difference, large Sat high temperatures (above 200 K) in a narrow band matches Heikes model [37]: S=kB/e{ln[d/(1d)]}, where dis concentration of Co4+. The enhanced Sat high temperatures is attributed to the competition between dand spin entropy. It is noted that S(T) is also described by narrow band model at intermediate temperatures. S(T) follows with Boltzmann formula [38]: S(T)=1/eT{(EEF)E2dE/[e(EEF)/2kBT+e(EEF)/2kBT]2}/{E2dE/[e(EEF)/2kBT+e(EEF)/2kBT]2}. Calculated S(T) indicates monotonous increase with increasing T, as well as experimental result as plotted in the inset in Figure 3a, revealing the validity of narrow band model.

Actually, activation energy ΔEis equal to EFEC, where EC is the upper mobility edge. As kBT/2 >ΔE, conduction mainly determined by contribution of excited holes in itinerant states as specified in Figure 3c. At high temperatures, the majority of acceptor‐like states are fully ionized, that is, occurs complete excitation of holes, that resulting in metallic behavior of ρ(T)and diffused S(T) (Heikes formula). As kBT/2 is near to ΔE, TAC conduction forms (Boltzmann dispersion). As kBT/2 <ΔE, VRH conduction dominates the transport mechanism as shown in Figure 3d.

2.4. X‐ray photoemission spectroscopy and thermal conductivity

In order to further verify the narrow band model, we carried out XPS spectra for Bi2Sr1.9Ca0.1Co2Oy. As shown in Figure 4b, XPS spectra present an intense peak at ∼ 0.95 eV, in line with large Sand metallic behavior. Between EF and ∼2.0 eV, Co 3dand O 2porbitals play an important role, similar to pristine Bi2Sr2Co2Oy[39]. Moreover, strong hybridization between Co 3dand O 2pforms [39, 40]. Namely, antibonding t2g narrow bands contribute to intense peak at ∼0.95 eV, while bonding eg broad bands are responsible to peak within 3–8 eV. In addition, calculated S(T) is also consistent with experimental value based on magnitude and temperature dependence [39]. Therefore, the narrow band model is very suitable for explaining all experimental and theoretical results.

Figure 4.

(a) Co 2pXPS spectra and (b) XPS spectra in wide binding‐energy range for selected Bi2Sr1.9Ca0.1Co2Oysample at room temperature.

Temperature dependence of total thermal conductivity κ(T) for all samples are shown in Figure 5a and d. κ(T) can be expressed by the sum of phononic component κph(T) and mobile charge carriers’ component κe(T) as κ(T) = κph(T) + κe(T). Value of κe(T) can be estimated from the Wiedemann‐Franz law, κe(T) = L0T/ρ, where L0 ∼ 2.44 × 10−8 V2/K2 stands for Lorenz number. In Figure 5b and e, κph(T) dominates the thermal conductivity because CoO2 layer and Bi‐Sr‐O block layer induces the interface scattering. Dimension less figure of merit ZT= S2T/ρκreflects total thermoelectric performance (see Figure 5c and f). For pristine Bi2Sr2Co2Oy, ZTvalue reaches ∼ 0.007 at 300 K, while ZTvalue reaches 0.19 at 973 K, indicative of promising thermoelectric material for Bi2Sr2Co2Oyat high temperatures [2]. Especially for Bi2Sr1.9Ca0.1 Co2Oy, ZTvalue reaches maximum ∼ 0.012 at 137 K. Therefore, it is reasonable to predict that Bi2Sr1.9Ca0.1Co2Oycould be considered as one of potential ultra‐high temperature thermoelectric materials, as well as pristine Bi2Sr2Co2Oy.

Figure 5.

Temperature dependence of (a) total thermal conductivityκ(T), (b) phononic componentκph(T), and (c) dimensionless figure of meritZTfor BSC and Bi2Sr1.9M0.1Co2Oy(M= Ag, Ca, and Y) samples. (d)–(f) are similar to (a)–(c), but for Bi2Sr2Co1.9X0.1Oy(X= Zr, Al, and Mo) samples.

Advertisement

3. Exotic reinforcement of thermoelectric power in layered Bi2Sr2−xCaxCo2Oy

3.1. XRD patterns and electrical transport properties

The crystal structure of Bi2Sr2Co2Oyis shown in Figure 6a. Figure 6b shows XRD patterns of all Ca-doping samples with single phase in Bi2Sr2−xCaxCo2Oy(0.0 ≤ x≤ 2.0). With increasing Ca content, diffraction peak along [003] direction distinctly shifts to higher angle as shown in the inset in Figure 6b, confirming the smaller ionic radius of Ca2+, than that of Sr2+. SEM characterization indicates surface morphology of plate‐like grains and regular grain sizes for selected samples with x= 0.0 and 1.0, respectively.

Figure 6.

(a) Crystal structure of Bi2Sr2Co2Oy. (b) Powder XRD patterns for Bi2Sr2‐xCaxCo2Oy(0.0 ≤x≤ 2.0) samples at room temperature. Insets: magnified powder's XRD patterns along [003] direction for all samples and SEM images for selected samples withx= 0.0 and 1.0, respectively.

Figure 7a and b shows resistivity ρ(T) of all samples in Bi2Sr2−xCaxCo2Oy. For the present x= 0.0 polycrystalline sample, upturning point at Tp (∼150 K) appears. Metallic behavior above Tp is observed, demonstrating the existence of itinerant charge carriers. In comparison, for x= 0.0 single crystal [41], in‐plane resistivity ρabalso shows metallic behavior around room temperature, while it arises minimum near 80 K and diverges with further decreasing the temperature. Resistivity ρabvalue of single crystal for x= 0.0 at room temperature is ∼4 mOhm×cm and is smaller than that of our polycrystalline sample (∼15 mOhm×cm). On the other hand, compared with x= 0.0, ρ(T) of all Ca‐doped samples produce total increase due to disorder effect. For the samples with x≤ 0.5, enhanced random Coulomb potential because of Ca doping induces the shift of Tp toward higher temperature. Interestingly, for the samples with x≥ 1.0, the signature of transition at Tp completely vanishes and ρ(T) only presents an insulating‐like behavior.

Figure 7.

(a) and (b) Temperature dependence of resistivityρ(T). Insets: magnification plot ofρ(T) for Bi2Sr2‐xCaxCo2Oysamples. (c) and (d) Plot of lnρagainst 1/T. Solid lines present TAC fitting. Dashed curves stand for VRH fitting. (e) Ca concentrationxdependence of activation energyΔE, onset temperatureTp of TAC, and onset temperatureThopping of VRH. (f) Ca concentrationxdependence of resistivityρ300 K at room temperature.

To discern conduction mechanism below Tp, relationship of lnρagainst 1/Tis plotted in Figure 7c and d. As for x≤ 0.5, at the beginning, it is found that TAC law matches ρ(T) data well below Tp, namely [35], ρ(T)=ρ0exp(ΔE/kBT), where ΔEis activation energy. But ρ(T) apparently deviates from TAC behavior with decreasing the temperature further, and it follows Mott's VRH model described by equation [32]: ρ(T)=ρ0exp[(T0/T)n]. However, as for x≥ 1.0, ρ(T) meets VRH model only, in agreement with the insulating feature of x= 2.0 single crystal [1, 42, 43]. Obtained values of ΔEand onset temperature Thopping are plotted in Figure 7e. Basically, ΔEincreases with Ca content, as well as Tp for x≤ 0.5. In comparison, the present value of ΔEbased on sintering temperature 800°C is larger than the previous one of x= 0.0 at 900°C [8], revealing the difference of grain size effect. It is worth noting that values of Thopping and ρ300K at room temperature first increase and then decrease in whole Ca‐doped range (see Figure 7e and f).

3.2. Enhancement of thermoelectric power driven by Ca doping

Figure 8a shows thermoelectric power S(T) for all samples. Positive values of Sdemonstrate that majority of charge carriers are hole type. In addition, Sexhibits a nearly T‐independent behavior above 200 K, while Sstrongly depends on Tpeculiarly below 150 K. Ca doping obviously boosts S300K at room temperature especially for heavy Ca contents (see Figure 8b). Large S300K value monotonously increases from 105 μV/K(x= 0.0) to 157 μV/K (x= 2.0). In general, the change of Sshould be related to variation of n. For x= 0 single crystal [38], Hall coefficient (RH) is positive and strongly dependent on the temperature in the range from 300 to 0 K. Increase of RH toward the lowest temperature is not simple due to the decrease of n, but rather due to anomalous Hall effect. It is noted that variation of RH with Pb doping is also similar to that of ρab. Pb doping slightly reduces the magnitude of RH, but the increase in number of charge carriers is much smaller than expected from chemical composition [41, 44].

Figure 8.

(a) Temperature dependence of thermoelectric powerS(T) for Bi2Sr2‐xCaxCo2Oysamples. (b) Ca concentrationxdependence ofSand charge carriers’ densitynat room temperature, respectively. (c) Ca concentrationxdependence of Co4+ ion (deduced from charge carriers’ densityn) and correspondingSHeikes (originating from Heikes formula) at room temperature, respectively. (d) Ca concentrationxdependence of electronic coefficientγderiving from specific heatC(T). Inset: temperature dependence ofC(T) plotted asC/TversusT2 based on fitting lines forx= 0.0, 0.5, 1.5, and 2.0, respectively.

As we know, Sis rather low (<10 μV/K) with a metallic behavior in a broad band [7]. Taking into account the tremendous discrepancy, large Sof Bi2Sr2−xCaxCo2Oywith a nearly T-independence at high temperatures in a narrow band should follow the so‐called Heikes formula [37]: S=kB/e{ln[(g3/g4)d/(1d)]}, where dis concentration of Co4+, and g3and g4are spin orbital degeneracies for Co3+ and Co4+ ions, respectively. Concentration dat room temperature can be deduced from charge carriers’ density n. As visible in Figure 8c, as for x< 1.5, ddecreases, while SHeikes (deriving from Heikes formula) increases, which is consistent with the trend of S300K. But for x≥ 1.5, reduced SHeikes is reverse to persistent enhancement of S300K. Thus, we have to consider other possible reason of enhanced Sfor heavily doped samples.

3.3. Specific heat and Sommerfeld coefficient

Next we will check whether the enhanced Soriginates from the increased effective masses through electronic correlation. To test this point, we performed measurement of specific heat C(T), which is plotted as C/Tversus T2 (see the inset in Figure 8d) for selected samples with x= 0.0, 0.5, 1.5, and 2.0. C(T) at low temperatures can be described as C(T) = γT+ βT3 [45], where γTand βT3 denote electronic and lattice contribution to C(T), respectively. We can get electronic coefficient γby the linear fitting according to C/T= γ+ βT2 [45]. Here, we need to explicitly interpret Sommerfeld coefficient γ. For the present system, unit formula should involve two cobalt atoms. For our polycrystalline sample with x= 0.0, a conventional way to get γby extrapolating high‐temperature linear part of C/Tversus T= 0 gives very large value of ∼ 135 mJ mol−1 K−2 (see Figure 8d), comparable with that of x= 0.0 single crystal (∼140 mJ mol−1 K−2) [41]. However, it is observed that γrapidly decreases with increasing Ca doping. For our sample with x= 2.0, value of γis ∼85 mJ mol−1 K−2. Differently, it is noted that value of γis only 50 mJ mol−1 K−2 for Bi‐Ca‐Co‐O system, while such a unit formula merely includes one cobalt atom [45].

Now we discuss the underlying implications of enhanced Swith Ca doping. As mentioned above, as for x< 1.5, decreased dbased on Heikes formula should be responsible for the enhanced S. But for x≥ 1.5, local modification of DOS and band structure near EF could play crucial role. S(T) can be defined by Mott formula [39]: S(T)=(π2kBT)/(3e)[dlnσ(E)/dE]E=EF, where σ(E) is electrical conductivity with σ(E) = n(E)(E), υ(E) is mobility, n(E) is charge carriers’ density with n(E) = D(E)f(E), D(E) is DOS, and f(E) is Fermi function. Apparently, in terms of Mott formula, the enhancement of Sfor x≥ 1.5 should be attributed to the increase of local DOS near EF. In details, with decreasing A‐site ionic radius (i.e., with increasing Ca content), tolerance factor decreases (not shown here), which leads to changes of lattice distortion in CoO2 layer and local band structure near EF, reminiscent of layered perovskite cobaltite SrLnCoO4 (Ln stands for different rare earth elements) [46]. Ultimately, value of Sfor x≥ 1.5 would be enhanced. Based on all of above results, one should emphasize that Sommerfeld coefficient γis dependent on n, and also as function of DOS at EF, which leads to continuous enhancement of large S.

Advertisement

4. Thermoelectricity generation and electron‐magnon scattering in a natural chalcopyrite mineral

4.1. Crystal structure and SEM characterization

A series of natural chalcopyrite minerals, Cu1+xFe1−xS2 (x= 0.17, 0.08, and 0.02), were obtained from a hydrothermal vent site named Snow Chimney in the Mariner field of Lau Basin [47]. Basically, mineral composition obtained from intact natural sulfide chimneys has no variation. Subsamples with x= 0.02 and 0.08 were obtained from the most interior chimney part, whereas subsample with x= 0.17 was obtained from the middle chimney wall region. The highly fluctuated and variable physicochemical conditions lead to obvious differences in mineral composition [48]. Figure 9 shows sketches of its crystal structure and atomic planes, in which chalcopyrite crystallizes in a tetragonal lattice with space group of I‐42dand produces honeycomb structure characteristic [49]. Each Fe and Cu atom is encircled by tetrahedron of S atom. The highlighted planes indicateatomic zig‐zag pattern, which is likely responsible to phonon scattering. XRD Rietveld refinement of power pattern indicates that three natural samples are single phase with standard chalcopyrite structure. For x= 0.08, refined lattice parameters aand care 5.278 and 10.402 Å, respectively (see Figure 10).

Figure 9.

Crystal structure of Cu1+xFe1‐xS2. Ball‐and‐stick model of the crystal structure (left) viewed alonga‐axis with black lines indicating unit cell. Stick model (right) showing characteristic honeycomb structure of chalcopyrite. Identical atomic arrangement is highlighted in gray in both structures, but projection is along different axes.

Figure 10.

Powder XRD patterns with Rietveld refinement for natural sample of Cu1+xFe1‐xS2 (x= 0.08). Red line indicates experimentally observed data, and black line overlapping them refers to calculated data. Vertical tick is related to the Bragg angles positions in space groupI‐42d. The lowest profile shows the difference between observed and calculated patterns. Rietveld refinement indicates that it is standard chalcopyrite structure.

To probe the microstructures of natural Cu1+xFe1−xS2, we performed SEM characterization (Figure 11). SEM analysis revealed that natural chalcopyrite with x= 0.08 had layered structure. Three examined natural samples were found to contain morphological diversity, which is characteristic of chalcopyrite minerals, and suggest different physical and chemical behaviors of various microstructures. The SEM observation may provide important insights of the relevance between physical and chemical functions and behaviors of chalcopyrite minerals.

Figure 11.

Surface morphology of natural sample of Cu1+xFe1‐xS2 (x= 0.08) showing characteristic layered structure. (a) Areas showing cracked layered structure in natural sample Cu1+xFe1−xS2 (x= 0.08), scale bar: 10 μm. (b) Densely layered structure, scale bar: 5 μm. (c) Triangular pattern surrounded by layered structure, scale bar: 1 μm.

4.2. Thermoelectricity generation and electronic states

To examine the functional properties of natural Cu1+xFe1−xS2 samples, we first measured resistivity (ρ) as function of temperature (T). Three examined natural samples exhibited excellent conductive behavior with semiconductive characteristics (Figure 12a). With the reduction of x, the overall resistivity decreased due to the emergence of doped charge carriers. Value of ρ300K for x= 0.17, 0.08, and 0.02 was 4.97, 0.11, and 1.01 Ohm×cm, respectively. Compared with x= 0.08, the increase of resistivity for x= 0.02 stems from the enhanced random Coulomb potential owing to the natural doping.

Figure 12.

Formation of thermoelectricity by Cu1+xFe1‐xS2. (a) Temperature dependence of resistivityρin three natural samples of Cu1+xFe1‐xS2. (b) Temperature dependence of thermoelectric powerSfor three samples.

In order to track the evolution of electronic states, we carried out thermoelectric power (S) measurement (Figure 12b), where the sign of Schanges. For x= 0.17, the sign of Sswitches from negative to positive around 235 K with decreasing temperature (Figure 12b). It is amazing to observe two unusual peaks: a broad peak (Tm; 32 μV/K, 186 K) and a sharper peak (Tp; 215 μV/K, 11 K), indicating the majority of hole carriers (p-type). It is of particular interest that, for x= 0.08 and 0.02, Tp peak utterly disappears, while Tm peak becomes wider and rapidly shifts to a lower temperature, where Spresents very large negative values, demonstrating the majority of electron carriers (n-type), in line with negative Hall coefficient RH (Figure 13). Large S300K reached a remarkable value of −713 and −457 μV/K for x= 0.08 and 0.02, respectively. Namely, more electrons are activated at room temperature with increasing Fe concentration. For x= 0.08, charge carriers’ mobility μ300K and density n300K are 1.8 cm2 V−1 s−1 and 3.5 × 1019 cm−3, obtained from RH = 1/neand μ= 1/neρ. In addition, Fe magnetic moment may also play an key role to induce large S, indicative of strong coupling between magnetic ions and doped charge carriers because synthetic CuFeS2 presents AFM ordering at 823 K [15].

Figure 13.

Hall effect of natural sample of Cu1+xFe1‐xS2 (x= 0.08). (a) Temperature dependence of Hall coefficientRH. (b) Temperature dependence of charge carriers’ densityn. Value ofRH (cm3 C−1) is determined byn(cm−3) and electron chargee, that is,RH = 1/ne, wheree= 1.602176 × 10−19 C. The shadow in bold is guide to the eyes.

4.3. Electron‐magnon scattering and large effective mass

The matter of imperative concern is how to understand the origin of Tm peak and conduction mechanism. According to Mott's formula, Scan be qualitatively expressed as S=π2kB2T/3e[σ’(EF)/σ(EF)], where kB is Boltzmann constant, σ(EF)is electrical conductivity at Fermi level EF, and σdenotes d[σ(E)]/dE[35]. If one assumes σis a constant accompanied by isotropic electrical transport properties, namely, σ−1= ρ, then ΔS/S0 Δρ/ρ0 can be derived. Plot of ΔS/S0 versus Δρ/ρ0 for x= 0.17 (Figure 14) shows that all experimental data near Tm at T0 from 155 to 300 K deviate from the theoretical calculation, the linearity. These results verify that exotic mechanism of S(T) in natural sample is beyond the framework of conventional thermoelectric picture [50].

Figure 14.

Correlation between thermoelectric powerS(T) and resistivityρ(T). Relative changes of ΔS/S0 versus Δρ/ρ0 in natural sample withx= 0.17 at various temperatures (T0 = 155, 185, 200, 215, 230, 240, 250, 270, and 300 K). The present experimental data substantially deviates from the linear relationship predicted by Mott's formula, which is indicated by dotted line.

To better discern intrinsic transport mechanism of Cu1+xFe1−xS2, we incorporate spin-wave theory to analyze temperature dependence of S. For x= 0.08 and 0.02, field‐cooling magnetization and loop hysteresis indicate the localized ferromagnetism (FM) at low temperatures because of additional Fe moments (Figure 15). However, strong AFM interaction at high temperatures dominates for three natural samples. Generally speaking, spin waves can scatter electrons for AFM or FM materials, resulting in magnon‐drag effect [12]. To check this issue, we developed magnon‐drag model, S=S0+S3/2T3/2+S4T4, where S0 is value of Sat T= 0, S3/2T3/2 term stems from electron‐magnon scattering, and S4T4 term is related to spin‐wave fluctuation in AFM phase. Using this model of magnon drag, the predicted values for three samples closely matched S(T) data (Figure 16a and b). As the absolute value of S3/2 is nearly six orders of magnitude larger than that of S4 (Table 1), electron‐magnon scattering dominates S(T) curve. Thus, Tm peak is predicted to originate from magnon drag due to the strong electron‐magnon interaction.

Figure 15.

Magnetic properties of natural Cu1+xFe1−xS2. (a,b) Temperature dependence of field‐cooling (FC) magnetization,M, in three natural samples of Cu1+xFe1−xS2, measured in applied magnetic field ofH= 0.1 T (a) andH= 1 T (b). (c) Magnetic field dependence of magnetization,M, for three samples, measured at 40 K.

Figure 16.

Temperature dependence ofSfor Cu1+xFe1−xS2 samples withx= 0.17 (a) andx= 0.08 and 0.02 (b). Symbols represent experimental data and solid lines correspond to theoretical simulation based on the model of magnon drag,S=S0 +S3/2T3/2 +S4T4. (c) Obtained parametersS3/2 and ΔEare plotted as function of Fe content, whereS3/2 represents the electron‐magnon scattering process and ΔEis activation energy. (d) Temperature dependence of power factor,S2/ρ, for three samples.

ParameterTm(K)S0(μVK−1)S3/2(μVK−5/2)S4 (μVK−5)ΔE(meV)
x= 0.17186−6.210.03−3.84×10−860.1
x= 0.0868−75.45−0.08−5.47×10−84.9
x= 0.0238−10.61−0.04−3.95×10−811.8

Table 1.

Obtained parameters based on theoretical simulation.

The parameter Tm represents the peak of magnon drag, which stems from the experimental S(T) curve. The parameters S0, S3/2, and S4 stem from the model of magnon drag, S= S0 + S3/2T3/2 + S4T4. The parameter ΔEis the activation energy, which stems from the TAC model, ρ(T) = ρ0 exp (ΔE/kBT).


To gain more insight into the correlation between magnon drag, doped carriers, and S, we plotted parameters S0, S3/2, and S4 as a function of x(Table 1). S0, S3/2, and S4 for x= 0.08 has largest absolute values among three natural samples, in agreement with the largest S, smallest ρ,and highest power factor. Unlike S0 and S4, dependence of S3/2 is quite unique (Figure 16c). The sign of S3/2 varies from positive to negative with increasing Fe concentration, suggesting the alternation of p‐type and n‐type charge carriers and orbital degree of freedom of Fe 3dband with AFM ordering. Additionally, electron‐magnon scattering occupies thermoelectric properties, indicating strong coupling between doped charge carriers and AFM spins. Furthermore, ρ(T) follows TAC model ρ(T)=ρ0exp(ΔE/kBT), where ΔEis activation energy [35]. Notably, the fitted energy gap of ΔE(60.1, 4.9, and 11.8 meV for x= 0.17, 0.08, and 0.02, respectively), which verifies the existence of localized Fe spins, is markedly smaller than that of artificial chalcopyrite [21, 2931]. It is noted that experimental S(T) result is well described by electron‐magnon scattering up to ∼200 K, while it deviates from theoretical lines for higher temperatures. In particular, power factor S2/ρshows an abrupt enhancement above 200 K for x= 0.08 (Figure 16d), in agreement with that of RH and n(Figure 13). Above 200 K, large effective mass (m*) leads to high power factor and large Sdue to low μand high n. For x= 0.08, it exhibits the largest m* value (1.6 m0) at room temperature, where m0 is free electron mass. Therefore, we can conclude that robust electron‐magnon scattering and large m* induce unexpected thermoelectricity generation in natural chalcopyrite mineral.

In terms of thermal conductivity κ, phononic component κph dominates for three natural samples owing to negligible electronic component κe (Figure 17). For the optimal sample with x= 0.08, value of ZTcan reach 0.03 at room temperature (Figure 17), thus indicating that natural chalcopyrite semiconductor is a promising candidate for thermoelectric energy materials. It is quite striking that the spontaneous doping process during deep‐sea hydrothermal vent mineral precipitations led to natural thermoelectric improvement, which is similar to natural mineral tetrahedrites [51].

Figure 17.

Thermal conductivity and phonon scattering of natural Cu1+xFe1−xS2. (a) Temperature dependence of total thermal conductivityκ. (b) Temperature dependence of electronic componentκe. (c) Temperature dependence of phononic componentκph. (d) Temperature dependence of dimensionless figure of meritZT.

Advertisement

5. Conclusions

Our results of layered cobaltites Bi2Sr2Co2Oysystem based on narrow band model are not only helpful to understand large Sand transport mechanism but also differentiate other systems based on a broad band model. In particular, we give the experimental evidence by Hall effect and C(T) measurements, demonstrating that Sommerfeld coefficient γis dependent on charge carriers’ density n, and also as a function of DOS at EF, which induces exotic enhancement of large Sin Bi2Sr2−xCaxCo2Oy. Especially for Bi2Sr1.9Ca0.1Co2Oy, it may provide an excellent platform to be regarded as potential candidates for thermoelectric materials.

In addition, we demonstrated direct thermoelectricity generation in natural chalcogenides, Cu1+xFe1−xS2, which was shown to have large Svalue and high power factor in the low xregion, in which electron‐magnon scattering and large m* values were detected. Since doped charge carriers exist in strong coupling with localized spins, the unusual alternation of p‐ and n‐type carriers should be of paramount importance in understanding charge dynamics arising from 3dorbital degrees of freedom. Such a finding of exotic thermoelectric properties in natural but not synthetic chalcopyrite opens a novel research field for manipulating low‐cost thermoelectricity or even electron/hole carriers, providing therefore a new perspective on technical feasibility for designing and pinpointing the surface‐morphology‐engineered devices via the naturally abundant materials.

Advertisement

Acknowledgments

The author gratefully thanks L. H. Yin, W. H. Song, Y. P. Sun, A. U. Khan, N. Tsujii, K. Takai, R. Nakamura, and T. Mori for their fruitful collaboration in the study of layered cobaltites and natural chalcogenides for thermoelectrics. This work was supported by the National Natural Science Foundation of China under Contract No. 10904151, the Fund of Chinese Academy of Sciences for Excellent Graduates, and the NIMS Open Innovation Center (NOIC) of Japan. The author thanks the Sichuan University Talent Introduction Research Funding (grant No. YJ201537) and Sichuan University Outstanding Young Scholars Research Funding (grant No. 2015SCU04A20) of China for financial support.

References

  1. 1. Tarascon J M, Ramesh R, Barboux P, Hedge M S, Hull G W, Greene L H, et al.: New non‐superconducting layered Bi‐oxide phases of formula Bi2M3Co2Oycontaining Co instead of Cu. Solid State Communications. 1989;71:663–668. DOI: 10.1016/0038‐1098(89)91813‐9
  2. 2. Funahashi R, Matsubara I, Sodeoka S.: Thermoelectric properties of Bi2Sr2Co2Oxpolycrystalline materials. Applied Physics Letters. 2000;76:2385–2387. DOI: 10.1063/1.126354
  3. 3. Koumoto K, Terasaki I, Funahashi R.:Complex oxide materials for potential thermoelectric applications. MRS Bulletin. 2006;31:206–210. DOI: 10.1557/mrs2006.46
  4. 4. Terasaki I, Sasago Y, Uchinokura K.: Large thermoelectric power in NaCo2O4 single crystals. Physical Review B. 1997;56:R12685–12687. DOI: 10.1103/PhysRevB.56.R12685
  5. 5. Masset A C, Michel C, Maignan A, Hervieu M, Toulemonde O, Studer F, et al.: Misfit‐layered cobaltite with an anisotropic giant magnetoresistance: Ca3Co4O9. Physical Review B. 2000;62:166–175. DOI: 10.1103/PhysRevB.62.166
  6. 6. Itoh T, Terasaki I.: Thermoelectric Properties of Bi2.3‐xPbxSr2.6Co2Oysingle crystals. Japanese Journal of Applied Physics. 2000;39:6658–6660. DOI: 10.1143/JJAP.39.6658
  7. 7. Heikes R R, Ure R W.: Thermoelectricity: Science and Engineering. New York: Interscience; 1961. 576 p.
  8. 8. Yin L H, Ang R, Huang Y N, Jiang H B, Zhao B C, Zhu X B, et al.: The contribution of narrow band and modulation of thermoelectric performance in doped layered cobaltites Bi2Sr2Co2Oy. Applied Physics Letters. 2012;100:173503. DOI: 10.1063/1.4705429
  9. 9. Yin L H, Ang R, Huang Z H, Liu Y, Tan S G, Huang Y N, et al.: Exotic reinforcement of thermoelectric power driven by Ca doping in layered Bi2Sr2‐xCaxCo2Oy. Applied Physics Letters. 2013;102:141907. DOI: 10.1063/1.4801644
  10. 10. Sootsman J R, Chung D Y, Kanatzidis M G.: New and old concepts in thermoelectric materials. Angewandte Chemie International Edition. 2009;48:8616–8639. DOI: 10.1002/anie.200900598
  11. 11. Chen Y L, Liu Z K, Analytis J G, Chu J H, Zhang H J, Yan B H, et al.: Single Dirac cone topological surface state and unusual thermoelectric property of compounds from a new topological insulator family. Physical Review Letters. 2010;105:266401. DOI: 10.1103/PhysRevLett.105.266401
  12. 12. Costache MV, Bridoux G, Neumann I, Valenzuela SO.: Magnon‐drag thermopile. Nature Materials. 2011;11:199–202. DOI: 10.1038/nmat3201
  13. 13. Ekwo P I, Okeke C E.: Thermoelectric properties of the PbS ZnS alloy semiconductor and its application to solar energy conversion. Energy Conversion and Management. 1992;33:159–164. DOI: 10.1016/0196‐8904(92)90121‐C
  14. 14. Donnay G, Corliss L M, Donnay J D H, Elliott N, Hastings J M.: Symmetry of magnetic structures: Magnetic structure of chalcopyrite. Physical Review. 1958;112:1917–1923. DOI: 10.1103/PhysRev.112.1917
  15. 15. Teranishi T.: Magnetic and electric properties of chalcopyrite. Journal of the Physical Society of Japan. 1961;16:1881–1887. DOI: 10.1143/JPSJ.16.1881
  16. 16. Tossell J A, Urch D S, Vaughan D J, Wiech G.: The electronic structure of CuFeS2, chalcopyrite, from x-ray emission and x-ray photoelectron spectroscopy and Xα calculations. The Journal of Chemical Physics. 1982;77:77–82. DOI: 10.1063/1.443603
  17. 17. Fujisawa M, Suga S, Mizokawa T, Fujimori A, Sato K.: Electronic structures of CuFeS2 and CuAl0.9Fe0.1S2 studied by electron and optical spectroscopies. Physical Review B. 1994;49:7155–7164. DOI: 10.1103/PhysRevB.49.7155
  18. 18. Nakamura R, Takashima T, Kato S, Takai K, Yamamoto M, Hashimoto K.: Electrical current generation across a Black Smoker Chimney. Angewandte Chemie International Edition. 2010;49:7692–7694. DOI: 10.1002/anie.201003311
  19. 19. Lovesey S W, Knight K S, Detlefs C, Huang S W, Scagnoli V, Staub U.: Acentric magnetic and optical properties of chalcopyrite (CuFeS2). Journal of Physics: Condensed Matter. 2012;24:216001. DOI: 10.1088/0953‐8984/24/21/216001
  20. 20. Lyubutin I S, Lin C R, Starchikov S S, Siao Y J, Shaikh M O, Funtov K O, et al.: Synthesis, structural and magnetic properties of self‐organized single‐crystalline nanobricks of chalcopyrite CuFeS2. Acta Materialia. 2013;61:3956–3962. DOI: 10.1016/j.actamat.2013.03.009
  21. 21. Tsujii N, Mori T.: High thermoelectric power factor in a carrier‐doped magnetic semiconductor CuFeS2. Applied Physics Express. 2013;6:043001. DOI: 10.7567/APEX.6.043001
  22. 22. Goodman C H L, Douglas R W.: New semiconducting compounds of diamond type structure. Physica. 1954;20:1107–1109. DOI: 10.1016/S0031‐8914(54)80247‐3
  23. 23. Austin I G, Goodman C H L, Pengelly A E.: New semiconductors with the chalcopyrite structure. Journal of The Electrochemical Society. 1956;103:609–610. DOI: 10.1149/1.2430171
  24. 24. Nikiforov K G.: Magnetically ordered multinary semiconductors. Progress in Crystal Growth and Characterization of Materials. 1999;39:1–104. DOI: 10.1016/S0960‐8974(99)00016‐9
  25. 25. Koschel W H, Sorger F, Baars J.: Optical phonons in I‐III‐VI2 compounds. Le Journal De Physique Colloques. 1975;36:C3:177–181. DOI: http://dx.doi.org/10.1051/jphyscol:1975332
  26. 26. Koschel W H, Bettini M.: Zone‐centered phonons in AIBIIIS2 chalcopyrites. Physica Status Solidi B. 1975;72:729–737. DOI: 10.1002/pssb.2220720233
  27. 27. Sato K, Harada Y, Taguchi M, Shin S, Fujimori A.: Characterization of Fe 3d states in CuFeS2 by resonant X‐ray emission spectroscopy. Physica Status Solidi A. 2009;206:1096–1100. DOI: 10.1002/pssa.200881196
  28. 28. Woolley J C, Lamarche A M, Lamarche G, Quintero M, Swainson I P, Holden T M.: Low temperature magnetic behaviour of CuFeS2 from neutron diffraction data. Journal of Magnetism and Magnetic Materials. 1996;162:347–354. DOI: 10.1016/S0304‐8853(96)00252‐1
  29. 29. Austin I G, Goodman C H L, Pengelly A E.: Semiconductors with chalcopyrite structure. Nature. 1956;178:433. DOI: 10.1038/178433a0
  30. 30. Hamajima T, Kambara T, Gondaira K I, Oguchi T.: Self‐consistent electronic structures of magnetic semiconductors by a discrete variational Xα calculation. III. Chalcopyrite CuFeS2. Physical Review B. 1981;24:3349–3353. DOI: 10.1103/PhysRevB.24.3349
  31. 31. Teranishi T, Sato K, Kondo K.:Optical properties of a magnetic semiconductor: Chalcopyrite CuFeS2.: I. Absorption spectra of CuFeS2 and Fe‐Doped CuAlS2 and CuGaS2. Journal of the Physical Society of Japan. 1974;36:1618–1624. DOI: 10.1143/JPSJ.36.1618
  32. 32. Tsujii N, Mori T, Isoda Y.: Phase stability and thermoelectric properties of CuFeS2‐based magnetic semiconductor. Journal of Electronic Materials. 2014;43:2371–2375. DOI: 10.1007/s11664‐014‐3072‐y
  33. 33. Ang R, Khan A U, Tsujii N, Takai K, Nakamura R, Mori T.: Thermoelectricity generation and electron‐magnon scattering in a natural chalcopyrite mineral from a deep‐sea hydrothermal vent. Angewandte Chemie International Edition. 2015;54:12909–12913. DOI: 10.1002/anie.201505517
  34. 34. Yamamoto T, Tsukada I, Uchinokura K, Takagi M, Tsubone T, Ichihara M, et al.: Structural phase transition and metallic behavior in misfit layered (Bi,Pb)‐Sr‐Co‐O System. Japanese Journal of Applied Physics. 2000;39:L747–750. DOI: 10.1143/JJAP.39.L747
  35. 35. Mott N F, Davis E A.: Electronic Processes in Non‐Crystalline Materials. Oxford: Clarendon; 1971. 437 p.
  36. 36. Zvyagin I P.: On the theory of hopping transport in disordered semiconductors. Physica Status Solidi B. 1973;58:443–449. DOI: 10.1002/pssb.2220580203
  37. 37. Kittel C.: Introduction to Solid State Physics. Singapore: Wiley; 2001.
  38. 38. MacDonald D K C.: Thermoelectricity: An Introduction to the Principles. New York: Wiley; 1962. 133 p.
  39. 39. Takeuchi T, Kondo T, Takami T, Takahashi H, Ikuta H, Mizutani U, et al.: Contribution of electronic structure to the large thermoelectric power in layered cobalt oxides. Physical Review B. 2004;69:125410. DOI: 10.1103/PhysRevB.69.125410
  40. 40. Asahi R, Sugiyama J, Tani T.: Electronic structure of misfit‐layered calcium cobaltite. Physical Review B. 2002;66:155103. DOI: 10.1103/PhysRevB.66.155103
  41. 41. Yamamoto T, Uchinokura K, Tsukada I.: Physical properties of the misfit‐layered (Bi,Pb)‐Sr‐Co‐O system: Effect of hole doping into a triangular lattice formed by low‐spin Co ions. Physical Review B. 2002;65:184434. DOI: 10.1103/PhysRevB.65.184434
  42. 42. Watanabe Y, Tsui D C, Birmingham J T, Ong N P, Tarascon J M.: Infrared reflectivity of single‐crystal Bi2Mm+1ComOy(M=Ca,Sr,Ba; m=1,2), Bi2Sr3Fe2O9.2, and Bi2Sr2MnO6.25, isomorphic to Bi‐Cu‐based high‐Tc oxides. Physical Review B. 1991;43:3026–3033. DOI: 10.1103/PhysRevB.43.3026
  43. 43. Terasaki I, Nakahashi T, Maeda A, Uchinokura K.: Optical reflectivity of single‐crystal Bi2M3Co2O9+δ (M=Ca, Sr, and Ba) from the infrared to the vacuum‐ultraviolet region. Physical Review B. 1993;47:451–456. DOI: 10.1103/PhysRevB.47.451
  44. 44. Yamamoto T, Tsukada I, Takagi M, Tsubone T, Uchinokura K.: Hall effect in a layered magnetoresistive cobalt oxide. Journal of Magnetism and Magnetic Materials. 2001;226–230:2031–2032. DOI: 10.1016/S0304‐8853(00)00670‐3
  45. 45. Limelette P, Hbert S, Hardy V, Frsard R, Simon Ch, Maignan A.: Scaling Behavior in thermoelectric misfit cobalt oxides. Physical Review Letters. 2006;97:046601. DOI: 10.1103/PhysRevLett.97.046601
  46. 46. Ang R, Sun Y P, Luo X, Hao C Y, Song W H.: Studies of structural, magnetic, electrical and thermal properties in layered perovskite cobaltite SrLnCoO4 (Ln = La, Ce, Pr, Nd, Eu, Gd and Tb). Journal of Physics D: Applied Physics. 2008;41:045404. DOI: 10.1088/0022‐3727/41/4/045404
  47. 47. Takai K, Nunoura T, Ishibashi J, Lupton J, Suzuki R, et al. Variability in the microbial communities and hydrothermal fluid chemistry at the newly discovered Mariner hydrothermal field, southern Lau Basin. Journal of Geophysical Research. 2008;113:G02031. DOI: 10.1029/2007JG000636
  48. 48. Tivey M K. The influence of hydrothermal fluid composition and advection rates on black smoker chimney mineralogy: Insights from modeling transport and reaction. Geochimica et Cosmochimica Acta. 1995;59:1933–1949. DOI: 10.1016/0016‐7037(95)00118‐2
  49. 49. Goodman C H L. A new group of compounds with diamond type (chalcopyrite) structure. Nature. 1957;179:828–829. DOI: 10.1038/179828b0
  50. 50. Asamitsu A, Moritomo Y, Tokura Y. Thermoelectric effect in La1‐xSrxMnO3. Physical Review B. 1996;53:R2952–2955. DOI: 10.1103/PhysRevB.53.R2952
  51. 51. Lu X, Morelli D T, Xia Y, Zhou F, Ozolins V, Chi H, et al. High performance thermoelectricity in earth‐abundant compounds based on natural mineral tetrahedrites. Advanced Energy Materials. 2013;3:342–348. DOI: 10.1002/aenm.201200650

Written By

Ran Ang

Submitted: March 31st, 2016 Reviewed: September 8th, 2016 Published: December 21st, 2016