Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Thermoelectric Properties of Variants of Cu4Mn2Te4 with SpinelRelated Structure Quansheng Guo,† Jean-Baptiste Vaney,† Raymond Virtudazo,† Ryunosuke Minami,‡ Yuichi Michiue,† Yoko Yamabe-Mitarai,‡ and Takao Mori*,†,§ †
WPI International Center for Materials Nanoarchitechtonics (WPI-MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba 305-0044, Ibaraki, Japan ‡ National Institute for Materials Science (NIMS), Sengen 1-2-1, Tsukuba 305-0047, Ibaraki, Japan § Graduate School of Pure and Applied Sciences, University of Tsukuba, Tennoudai 1-1-1, Tsukuba 305-8671, Japan S Supporting Information *
ABSTRACT: Thermoelectric properties of Cu4Mn2Te4, which is antiferromagnetic with a Néel temperature TN = 50 K and crystallizes in a spinel-related structure, have been investigated comprehensively here. The phase transition occurring at temperatures 463 and 723 K is studied by high-temperature X-ray diffraction (XRD) and differential scanning calorimetry (DSC), and its effect on thermoelectric properties is examined. Hypothetically Cu4Mn2Te4 is semiconducting according to the formula (Cu+)4(Mn2+)2(Te2−)4, while experimentally it shows p-type metallic conduction behavior, exhibiting electrical conductivity σ = 2500 Ω−1 cm−1 and Seebeck coefficient α = 20 μV K−1 at 325 K. Herein, we show that the carrier concentration and thus the thermoelectric transport properties could be further optimized through adding electron donors such as excess Mn. Discussions are made on the physical parameters contributing to the low thermal conductivity, including Debye temperature, speed of sound, and the Grüneisen parameter. As a result of simultaneously boosted power factor and reduced thermal conductivity, a moderately high zT = 0.65 at 680 K is obtained in an excess Mn\In co-added sample, amounting to 5 times that of the pristine Cu4Mn2Te4. This value (zT = 0.65) is the best result ever reported for spinel and spinel-related chalcogenides.
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phase n-type Mg3.2Sb1.5Bi0.49Te0.01 with zT = 1.51 at 716 K,11 and p-type Yb14Al0.8Mn0.2Sb11 with zT = 1.3 at 1223 K.12 More recently, Cu based chalcogenides have also been extensively investigated as novel and environmentally friendly thermoelectric materials.13−15 In particular, low thermal conductivity smaller than 1 W m−1 K−1 was exhibited by the phonon liquid electrical crystal ionic conductor β−Cu2Se, enabling a zT as high as 1.5 at 1000 K.16 However, the application of this material is still limited by Cu ionic mobility. Meanwhile, high zT around unity in synthetic tetrahedrite Cu12Sb4S13 was achieved through substituting transition metals (Mn, Fe, Co, Ni, Zn)17,18 or crystallogens (Ge, Sn) for Cu.19 In addition, chalcopyrite CuFeS2 displays power factor as high as 10 μW cm−1 K−2 in the temperature range of 300−400 K, which could be attributed to the high Seebeck coefficient originating from magnetic interactions and electron-magnon interaction.20−22 Other types of chalcopyrites like CuInTe2 (zT = 1.18 at 850 K)23 and CuGaTe2 (zT = 1.40 at 950 K)24 also demonstrate figures of merit comparable to those of the stateof-the-art thermoelectric materials.
INTRODUCTION
Thermoelectric conversion technology has the potential to improve global energy management through converting waste heat directly into electricity or vice versa. The efficiency of this compelling technology is determined by the thermoelectric materials’ figure-of-merit, zT = σα2T/κ, where σ is the electrical conductivity, α is the Seebeck coefficient, κ is the thermal conductivity, and T is the absolute temperature, respectively.1−3 A high zT is required to obtain a high efficiency, and zT above unity are usually considered as suitable for practical applications.4 However, interdependence among the transport parameters (σ, α, and κ) renders the development of high performance thermoelectric materials challenging.5 Advanced thermoelectric materials are generally narrow gap degenerate semiconductors containing heavy elements with a carrier concentration in the range of 1019−1021 cm−3.6 The past several decades have witnessed the rapid development of bulk high performance thermoelectrics, such as Bi0.5Sb1.5Te3 with zT = 1.22 at 340 K,7 PbTe−4 mol % SrTe−2% Na with zT = 2.2 at 915 K,8 the n-type triple-filled skutterudite Sr0.09Ba0.11Yb0.05Co4Sb12 with zT = 1.9 at 835 K,9 nanomicroporous rare-earth filler-free skutterudite CoSb2.75Si0.075Te0.175 with zT = 1.6 at 795 K,10 the Zintl © XXXX American Chemical Society
Received: February 2, 2018
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DOI: 10.1021/acs.inorgchem.8b00301 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
simultaneous electrical conductivity and Seebeck coefficient transport measurements via a commercial instrument (ZEM-2, ULVAC ShinkuRiko) with a standard four-probe configuration under a helium atmosphere. The microstructure and element distribution of Cu4−yInyMn2Te4 (y = 0.2) were characterized by field-emission scanning electron microscopy (FESEM, Hitachi S-4800) equipped with an energy-dispersive spectrometer (EDS). The Hall effect for Cu4Mn2+xTe4 (x = 0.1, 0.2) was measured on the Quantum Design physical properties measurement system (PPMS) with the ac transport option, with the magnetic field sweeping from −5 to +5 T. For Cu4Mn2.1Te4, the heat capacity Cp above room temperature was also measured by differential scanning calorimetry (DSC8231, Rigaku Thermo plus EVO 2 series) with a heating rate of 5 K/min under argon flow, using α-Al2O3 (SRM720 SYNTHETIC SAPPHIRE, NIST) as the reference. In the low-temperature range 2 − 300 K, Cp was measured on Quantum Design PPMS. The thermal diffusivity was also measured with the laser flash method (ULVAC Thermal Analysis TC7000) under vacuum from room temperature up to 750 K. The density d was obtained according to the ratio of mass to volume. The thermal conductivity was then calculated using the formula κ = λCpd. The thermal conductivity values from ULVAC TC7000 and DSC8231 Rigaku are quite close to those obtained from LFA467 except the values around the phase transition temperature range, probably due to the different heating rate in DSC and TC7000.
In contrast, Cu based chalcogenides CuCr2X4 (X = S, Se) with the spinel structure had been considered relatively low performance thermoelectric materials, despite their typically large unit cells and complex coordination configurations, both of which contribute to relatively low lattice thermal conductivity.25 By Sb doping, a respectably high zT = 0.43 at ∼923 K was achieved in the sulfide CuCr1.7Sb0.3S 4.26 CuCr1.7Sb0.3S4 is a magnetic semiconductor with ferromagnetic ground state and with an estimated high effective mass of 5.9m0 (m0 = free electron mass).26 In this contribution, we evaluate various derivatives of Cu4Mn2Te4, which belongs to a spinel-related structure.27 Compared with sulfur or selenium, tellurium is of higher atomic mass and larger atomic/ionic radius. Therefore, low thermal conductivity and possibly appealing thermoelectric properties might be anticipated. Until recently, only room-temperature electrical properties for Cu4Mn2Te4 were reported,27 and few systematic experimental investigations have been carried out.28 In what follows, the thermoelectric transport properties of Cu4Mn2Te4 will be presented and realization of further enhancement illustrated.
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EXPERIMENTAL PROCEDURES
RESULTS AND DISCUSSIONS Cu4Mn2Te4 adopts a spinel-related structure. The parent spinel is the mineral MgAl2O4 and generally spinels have the chemical formula AM2X4.30 Cu4Mn2Te4 could be considered as a derivative of the spinel CuMn2Te4. In a hypothetical normal spinel CuMn2Te4, the Te ions form a cubic closest-packing (ccp), with Cu occupying one-eighth of the tetrahedral interstices and Mn occupying half of the octahedral positions, as shown in Figure 1(left). Each unit cell contains eight formula
Synthesis and Phase Purity Analysis. Samples with the nominal composition listed as follows were prepared: Cu4Mn2+xTe4 (x = 0, 0.1, 0.2, 0.3), Cu4−yInyMn2Te4 (y = 0, 0.1, 0.2), and Cu4−nInnMn2+mTe4 (m = 0.1, n = 0.1; m = 0.1, n = 0.2; m = 0.2, n = 0.1; m = 0.2, n = 0.2). All of them were prepared directly from the elements by a combination of melting, quenching followed by spark plasma sintering (SPS). The starting materials are commercial high purity elements, including copper powder (min. 99.85%, Kanto Chemical), manganese powder (≥99%, −325 mesh, Sigma Aldrich), tellurium powder (99.8%, −200 mesh, Sigma Aldrich) and indium beads (≥99.9%, diameter 2−5 mm, Sigma Aldrich). Stoichiometric quantities of the elements were weighed, transferred into silica tubes coated with carbon and sealed under vacuum. The ampoules were gradually heated to 1223 K in a programmable furnace, held at this temperature for 6 h and eventually quenched to room temperature in a water bath. The phase compositions were characterized by powder X-ray diffraction (Rigaku, Ultima III) with Cu Kα radiation. High-temperature XRD was performed using a RINT TTR-III with Cu Kα radiation to acquire further understandings about the phase transition. Exploratory reactions to prepare the homologous Cu4Mn2S4 and Cu4Mn2Se4 instead resulted in mixtures of sulfides or selenides. It was also reported that neither could ball-milling prepare Cu4Mn2S4 single phase.29 The actual chemical composition for Cu4Mn2Te4 and Cu4Mn2.1Te4 was analyzed by means of inductively coupled plasma atomic emission spectroscopy (ICP-AES, HITACHI SPS3520UV-DD) and the elements ratio was determined as 40.4Cu/19.5Mn/40.1Te and 39.9Cu/20.1Mn/40.0Te atom %, respectively. These results compare reasonably well with the nominal composition. Consolidation and Physical Properties Measurement. The acquired ingots were pulverized into fine powder in an agate mortar by hand, loaded into a graphite die (Φ 10) and sintered by spark plasma sintering (SPS-1080 System, SPS SYNTEX Inc.) under a uniaxial pressure of 46 MPa. The sintering was performed in an argon atmosphere at 803 K for 5 min in average, slightly varying with the sample composition. Samples with x = 0.2 and 0.3 could not be consolidated into pellets of good mechanical quality; hence, the thermal conductivity values for x = 0.2 and 0.3 could not be collected. The thermal diffusivity λ and heat capacity Cp were measured on LFA467 Hyperflash (Netzch Co. Ltd.) under nitrogen flow. Pyroceram was used as the reference and the measurement was carried out between 300 and 750 K At each temperature point, five measurements were carried out, and the averaged results are presented herein. Thereafter the disks were cut into rectangular bars for
Figure 1. Crystal structure of the hypothetical spinel CuMn2Te4 (left) and the actual cubic Cu4Mn2Te4 in the temperature range of ∼463 to ∼723 K (right).Cu, black; Mn, cyan; and Te, red. For clarity, only the front four octets are shown. Smaller black spheres denote the Cu positions with 50% fractional occupancy.
units (Z = 8) and Cu, Mn, and Te occupy the Wyckoff positions 8b, 16c, and 32e, respectively, corresponding to Cu8Mn16Te32. When more Cu is randomly introduced onto half of the Wyckoff positions 48f, the formula unit becomes Cu 32 Mn 16 Te 32 Cu 4 Mn 2 Te 4 . The crystal structure of Cu4Mn2Te4 is shown in Figure 1(right). Besides, this modification only exists in the temperature range of ∼463 to ∼723 K. At lower temperatures, Cu4Mn2Te4 experiences very slight rhombohedral distortions with excessive Cu sitting along one of the four body diagonals.31 It is also interesting to note that the rhombohedral CuTi2S4 will transform into cubic spinel at temperatures over 723 K.32−34 The filling of the tetrahedral 48f sites by Cu ions is also observed in Cu excess Cu1+xCr2Te4 spinel phases, although in that case the amount of excess Cu B
DOI: 10.1021/acs.inorgchem.8b00301 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 2. Temperature dependence XRD patterns (a) and DSC results (b) for Cu4Mn2Te4. In the XRD patterns, reflections (311) and (511)\(333) gradually decrease as the temperature increases and disappear at 743 K.
Figure 3. Temperature dependence of thermal diffusivity (a) and electrical transport properties (b) for Cu4Mn2Te4.
ions is limited to x = 1 and non-negligible Cu filling (0.41 Cu per formula unit) of the octahedral 96h positions was also evidenced by neutron diffraction.35 When temperature exceeds 723 K, Cu and Mn will statistically occupy half of the tetrahedral sites and half of the octahedral sites, respectively. Consequently, the cell edge becomes that of the Te cubic packing (Z = 1). Likewise, the argyrodites Ag8BIVS6 (B = Ge, Sn) transits from a high-temperature fcc γ-phase with the lattice parameter a = 1.1 nm (Z = 4) to a cubic β-phase with a doubled lattice parameter a′ = 2.1 nm (Z = 32) upon cooling.36,37 To facilitate understanding the phase transitions mentioned above, temperature dependent XRD was measured at room temperature, 473, 623, 680, 710, and 743 K. As illustrated in Figure 2a, the relative intensity of peaks (311) and (511)\(333) gradually decrease as the temperature increases and finally disappear at 743 K. A reversible exothermic reaction was observed upon heating at around 720 K in the DSC curve for Cu4Mn2Te4 (Figure 2b), which could be explained in terms of the phase transition mentioned above. Neither the temperature-dependent XRD patterns nor DSC results exhibit evidence of the 463 K phase transition, except for a slight change of slope at 430 K in the DSC profile during the cooling process. Physical properties measurement revealed that the thermal diffusivity decreases abruptly first as the temperature approaches ∼700 K and then increases (Figure 3a), perhaps owing to the phase transition taking place around that temperature point. Similar abnormalities in the thermoelectric
properties were also found for Cu2Se at the temperature of the transformation from the low temperature monoclinic phase to the high temperature antifluorite structure. By contrast, the electrical conductivity σ and Seebeck coefficient α of Cu4Mn2Te4 vary smoothly without sudden changes. At 325 K, Cu4Mn2Te4 displays σ = 2500 Ω−1 cm−1 and α = 20 μV K−1 (Figure 3b). The latter value is close to 16 μV K−1 reported by Lotgering et al.27 Room temperature (300 K) values σ = 1765 Ω−1 cm−1 and α = 27 μV K−1 for Cu4Mn2Te4 were also reported by Chen et al.28 The electrical conductivity declines monotonically with the increasing temperature because of stronger electron scattering by crystal vibrations, characteristic of metallic behavior. The positive and small Seebeck coefficient values imply that the concentration of the dominant carriers − holes − is rather high despite the semiconducting nature that could be expected from the ionic formula (Cu+)4(Mn2+)2(Te2−)4 by counting the electrons, further emphasizing the degenerate semiconducting nature of the compound. Targeting further enhancement of the thermoelectric properties of Cu4Mn2Te4, three groups of samples (1) Cu4Mn2+xTe4 (x = 0.1, 0.2) with extra Mn, (2) In added Cu4Mn2Te4 with the nominal composition Cu4−yInyMn2Te4 (y = 0.1, 0.2), and (3) simultaneous In and Mn added samples Cu4−nInnMn2+mTe4 (m = 0.1, 0.2; n = 0.1, 0.2) were examined, taking into account of that each extra Mn will theoretically provide two more valence electrons thereby optimizing the concentration of holes. As to the In containing samples, it was found that In mainly exists in the form of In2Te3 secondary phase, according to the XRD and C
DOI: 10.1021/acs.inorgchem.8b00301 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 4. Electrical conductivity, Seebeck coefficient, and power factor of Cu4Mn2+xTe4 (top) and Cu4−yInyMn2Te4 (bottom)
valence electrons and therefore behaves as electron donors. Similarly, for the indium-containing samples, σ decreases down to σ = 1007 Ω−1 cm−1 for y = 0.2 around 325 K, which is considerably lower than that of pristine Cu4Mn2Te4. For metals or degenerate semiconductor materials a decrease in electrical conductivity is generally accompanied by an increase in Seebeck coefficient since their respective dependences on carrier concentration are inversely proportional, as is indicated by the two following equations
EDS results (Figures S1 and S2). The addition of Mn and In slightly enlarges the lattice constants of the matrix, as the XRD peaks shift toward lower angle (Figure S1). The formation of the In2Te3 precipitates not only greatly facilitates the SPS sintering but also contributes to the enhancement of thermoelectric performance. This could be rationalized based on the reaction (4−y)Cu + yIn + 2Mn + 4Te → (8 − 3y)/8 Cu8(4−y)/(8−3y)Mn16/(8−3y)Te4 + y/2 In2Te3, resulting in an increased amount of Cu and Mn in the matrix in the In2Te3 precipitated samples and consequent optimization of carrier concentration, which is beneficial to the thermoelectric properties as discussed below. The electrical properties of Cu4Mn2+xTe4 (x = 0.1, 0.2) and Cu4−yInyMn2Te4 (y = 0.1, 0.2) are plotted in Figure 4. As shown in Figure 4a,b, in each case, the electrical conductivity exhibits a metallic temperature dependence, indicative of the degenerate semiconducting behavior. The electrical conductivity decreases with increasing Mn content, from 2510 Ω−1 cm−1 for x = 0 to 1230 Ω−1 cm−1 for x = 0.2 at 325 K. Hall effect measurements evidenced that the carrier concentration is high and decreases from 1.63 × 1021 cm−3 for x = 0.1 to 1.03 × 1021 cm−3 for x = 0.2 at 300 K, as shown in Table 1. This compositional dependence is consistent with the assumption that each extra Mn in the crystal lattice offers two additional
⎛ ⎞2/3 *T ⎜ π ⎟ m ⎝ 3n ⎠ 3eh2 σ = neμ
α=
where kB is the Boltzmann constant, h is the Planck constant, n is the carrier concentration, m* is the density of states effective mass, and μ is the carrier mobility.2 This principle is indeed applicable for the Cu 4 Mn 2+x Te 4 (x = 0.1, 0.2) and Cu4−yInyMn2Te4 (y = 0.1, 0.2) investigated here, with the samples that exhibit higher conductivity showing the smaller Seebeck coefficient, as depicted in Figure 4c,d. Specifically, increasing the amount of Mn which consequently reduces the number of holes gives rise to a steady increase of the Seebeck coefficient, from 22 μV K−1 for x = 0 to 40 μV K−1 for x = 0.1 and to 60 μV K−1 for x = 0.2 at 325 K. A similar trend was also observed in the In-containing series with y = 0.2 having a high α = 68 μV K−1 at 325 K since adding In increases Cu and Mn contents in the matrix, both of which are electron donors. The Seebeck coefficient increases with temperature in a quasi linear fashion for all samples and is suggestive of p-type conduction. The effective mass of the carriers for Cu4Mn2+xTe4 (x = 0.1, 0.2) are estimated to be around 1.93m0, which perhaps is responsible for the high Seebeck coefficient of the samples.
Table 1. Electronic Transport Parameters for Cu4Mn2+xTe4 (x = 0.1, 0.2) at Room Temperature
Cu4Mn2.1Te4 Cu4Mn2.2Te4
Hall coefficient (cm3/C)
carrier concentration (cm−3)
Hall mobility (cm2 V−1 s−1)
effective mass (m0)
0.00562 0.00934
1.11 × 1021 6.68 × 1020
9.5 12.0
1.93 1.93
8π 2kB2
D
DOI: 10.1021/acs.inorgchem.8b00301 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 5. Thermal conductivity, lattice thermal conductivity, and thermoelectric figure of merit for Cu4Mn2+xTe4 (top) and Cu4−yInyMn2Te4 (bottom)
investigated here have rather low κ values
