Microstructural Control and Thermoelectric Properties of Misfit Layered

Mar 20, 2014 - ABSTRACT: We investigated the high-temperature thermoelectric properties of misfit layered n-type (LaS)1.20CrS2 and p-type (LaS)1.14NbS...
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Microstructural Control and Thermoelectric Properties of Misfit Layered Sulfides (LaS)1+mTS2 (T = Cr, Nb): The Natural Superlattice Systems Priyanka Jood,† Michihiro Ohta,*,† Hirotaka Nishiate,† Atsushi Yamamoto,† Oleg I. Lebedev,‡ David Berthebaud,‡ Koichiro Suekuni,§ and Masaru Kunii† †

Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan ‡ Laboratoire CRISMAT, UMR 6508 CNRS/ENSICAEN, 6 Boulevard du Maréchal Juin, F-14050 Caen Cedex 4, France § Department of Quantum Matter, ADSM, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8530, Japan S Supporting Information *

ABSTRACT: We investigated the high-temperature thermoelectric properties of misfit layered n-type (LaS)1.20CrS2 and p-type (LaS)1.14NbS2. The samples were prepared by CS2 sulfurization of 6 or 12 h duration and then consolidated using pressure-assisted sintering to produce randomly and highly oriented samples whose microstructures were tunable. Transmission electron microscopy analysis showed that perfectly layered structures containing some stacking faults had formed. The randomly and highly oriented natural superlattices provided ultralow lattice thermal conductivities (as low as ∼0.9 and ∼0.5 W K−1 m−1, respectively, at 950 K) perpendicular to the pressing axis. The improved electrical conductivities of the oriented CrS2 and NbS2 samples resulted in high power factors of 170 and 410 μW K−2 m−1, respectively. The highly oriented texture produced the highest thermoelectric figure of merit ZT of 0.14 at 950 K among the (LaS)1.20CrS2 system, whereas the weakly/randomly oriented texture produced the highest ZT of 0.15 at 950 K among the (LaS)1.14NbS2 system. These misfit layered sulfides exhibit phonon glass−electron crystal behavior and provide tremendous opportunities for further enhancing ZT by optimizing the thermoelectric properties.

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the carrier mobility in the electron crystal region (i.e., high S2/ ρ). There are three general methods of achieving PGEC behavior. The first is to reduce κlat by effective phonon scattering with rattling mode/low-energy vibrational mode in electron-crystal host structures such as clathrates,2,3 filled skutterudites,4,5 and tetrahedrites.6 The second involves inserting nanoinclusions (nanocomposites), which scatter heat-carrying phonons in bulk materials, with little effect on charge-carrier scattering, as for example, in PbTe/MTe7−10 (where M = alkaline earth elements) and ZnO/ZnAl2O4.11 The third is to use layered systems such as superlattice thin films12,13 and layered cobaltite oxides14−19 to separate the electron crystal from the phonon glass. Natural superlattice (layered) bulk oxides such as Na xCoO214−16 and Ca−Co−O17,18 have proven to be promising high-temperature thermoelectric materials. These superlattice systems are described through nanoblock integra-

INTRODUCTION A drastic increase in the demand for energy has generated extensive interest in sustainable energy generation technologies, essentially, thermoelectric power generation. Indeed, generating thermoelectric power from automobile and power plant wasted heat is a promising solution to the energy crisis and to global climate change. The potential of a thermoelectric material is determined by its dimensionless thermoelectric figure of merit, ZT = (S2/ρκtotal)T, where S, ρ, κtotal, and T are the Seebeck coefficient, electrical resistivity, total thermal conductivity, and absolute temperature, respectively. The total thermal conductivity arises from charge carriers transporting heat (i.e., electronic thermal conductivity, κel) and phonons traveling through the lattice (i.e., lattice thermal conductivity, κlat); therefore, κtotal = κel + κlat. κel can be estimated using the Wiedemann−Franz law: κel = L0T/ρ, where L0 is the Lorenz number. Efficient (i.e., high-ZT) thermoelectric materials should show low κtotal and high power factors (S2/ρ). The phonon glass−electron crystal (PGEC) concept is a strategy for developing high-ZT thermoelectric materials.1 In a PGEC system, the phonon glass region provides the disorder necessary to scatter phonons (i.e., low κtotal) without disturbing © 2014 American Chemical Society

Received: February 7, 2014 Revised: March 17, 2014 Published: March 20, 2014 2684

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La1.14NbO4. The as-obtained ternary oxides are not pure phases (Figure S1 in the Supporting Information). These ternary oxide powders were placed into quartz boats and set into a quartz reaction tube, which was evacuated and purged with Ar gas. The powders were heated to 573 K at 25 K min−1 under Ar gas flow. A mixture of CS2 and Ar gases was introduced into the quartz tube as soon as the powders reached 573 K. The CS2 gas was obtained by bubbling Ar carrier gas through CS2 liquid (99.9%, Kanto Chemical). The powders were heated at 10 K min−1 to 1073 K under CS2/Ar gas flow and then sulfurized at 1073 K for 6 h. They were then cooled at 10 K min−1 to 573 K under CS2/Ar gas flow and then gradually cooled to room temperature under Ar gas flow. The flow rate of Ar gas was fixed at 10 mL min−1. The obtained powders were ground well and sulfurized again under the same conditions at 1073 K for another 6 h to investigate the effects of sulfurization duration on the microstructure and thermoelectric properties. Two batches of sulfurized powder were obtained using this method: the first was only sulfurized for 6 h and the second was sulfurized for 12 h. The sulfurized powders were placed into quartz tubes. The tubes were evacuated to 7 × 10−3 Pa and then sealed. They were placed into a furnace and heated at 10 K min−1 to 1323 K, where it was further annealed for 24 h and subsequently cooled at 10 K min−1 to homogenize the powders. The sample nomenclature used in this paper is as follows: the 6 and 12 h sulfurized (LaS)1.20CrS2 samples are denoted as LCS-6S and LCS-12S, respectively, and the 6 and 12 h sulfurized (LaS)1.14NbS2 samples are denoted as LNS-6S and LNS12S, respectively. The (LaS)1.20CrS2 and (LaS)1.14NbS2 powders were placed into 10 or 15 mm diameter graphite dies that were then inserted into the sintering equipment (SPS-511S, Fuji Electronic Industrial) and heated at 10 K min−1. Sinterings were performed at 1223 K for 2 h under 30 MPa uniaxial pressure under vacuum (7 × 10−3 Pa) and subsequently cooled at 20 K min−1 to prepare high-density and oriented sintered compacts, which were cut into bars, disks, and plates for further measurements. The sintering curves for all the samples are shown in Figure S2 of the Supporting Information. The differential thermal analysis (DTA) on the sintered compacts was performed, showing the thermal stability of the materials with no significant peak (Figure S3 in the Supporting Information). Chemical Analysis. The sulfur, oxygen, and carbon contents of the sintered compacts were determined using chemical analysis at Toray Research Centre, Japan. Nondispersive infrared absorption method (EMGA-620W/C, HORIBA) was used to determine the O content, and infrared absorption method (EMIA-920V2, HORIBA) was used to determine the S and C contents. Powder X-ray Diffraction and Scanning Electron Microscopy. The crystal structure of the powders and the crystal structure and orientation of the sintered compacts were examined using X-ray diffractometry (XRD; Rint-Ultima, Rigaku) with Cu Kα radiation over the 2θ range 10−80°. The microstructures of the powders and sintered compacts were observed using scanning electron microscopy (SEM; JEOL JSM-6610LV, 20 kV). Transmission Electron Microscopy. Transmission electron microscopy (TEM), electron diffraction (ED), and high-resolution TEM (HRTEM) studies were performed using a 0.17 nm pointresolution FEI Tecnai G2 30 UT microscope operated at 300 kV. The TEM specimens were prepared by grinding the sintered compacts in an agate mortar, dissolving the ground samples in methanol, and spreading it onto a carbon holey grid. Electrical Transport Measurements. The Seebeck coefficient and electrical resistivity of the sintered compacts were simultaneously measured using temperature-differential and four-probe methods, respectively, (ZEM-3, ULVAC-RIKO) perpendicular (in-plane) and parallel (out-of-plane) to the pressing direction under a He atmosphere in the range 300−973 K. The bars used for the in-plane and out-of-plane measurements typically were ∼3 × 2 × 10 mm and ∼3 × 2 × 7 mm, respectively. The heating and cooling cycles provide reproducible Seebeck coefficient and electrical resistivity for all the sintered compacts. The sample preparations and electrical transport measurements were repeated. The experimental repeatability is

tion, implying that both the electrical and thermal transports can be individually controlled in a complex crystalline field, giving more opportunities to enhance ZT.17,19 However, the high electrical resistivity due to the high electronegativity of oxygen atoms still remains a limiting factor for these compounds. The covalency increases moving from oxides to sulfides; therefore, sulfides are expected to show lower electrical resistivity, leading to higher power factor.20 Layered sulfides are becoming potential candidates as bulk thermoelectric materials for high/medium-temperature applications. Han and Cook21 first discovered that layered sulfide TiS2 shows a high S2/ρ of ∼1495 μW K−2 m−1 at 300 K. Imai et al.22 also reported that TiS2 single crystal shows a high S2/ρ of ∼3710 μW K−2 m−1 along with a high κtotal ≈ 6.7 W K−1 m−1 at 300 K. The guest transition metals or rock-salt-type MS layers (where M = Pb, Bi, or Sn) are intercalated into the van der Waals gap of layered TiS2 to reduce κtotal.23−26 In the intercalated TiS2, the host TiS2 layer provides high S and the electron pathway (i.e., low ρ) and the MS layer reduces κlat because of increased lattice disorder. The intercalation enhances the ZT, resulting in a high n-type ZT ≈ 0.45 at 800 K for polycrystalline Cu0.02TiS226 and a promising n-type ZT ≈ 0.38 at 673 K for polycrystalline (SnS)1.2(TiS2)2.23 In other work, Miyazaki et al.27 reported a p-type ZT of 0.11 for polycrystalline NbS 2 -based misfit layered sulfide (Yb2S2)1+mNbS2 at 300 K. These natural superlattice sulfides are part of a large family of misfit layered chalcogenides (MX)1+m(TX2)n (M = Pb, Bi, Sn, Sb, rare-earth elements; T = Ti, V, Cr, Nb, Ta; X = S, Se; n = 1, 2, 3).28 The structural and physical properties of the misfit sulfides were investigated in the 1990s.28−30 However, few works to date have reported the thermoelectric properties of these compounds. The lack of three-dimensional lattice periodicity in the misfit layered compounds generates new features in properties, such as the effects of incommensurability on the lattice dynamic and electrical transport, making layered misfit sulfides an interesting platform not only for ZT enhancement but also from the fundamental perspective. We prepared polycrystalline samples of misfit layered sulfides (LaS)1.20CrS2 and (LaS)1.14NbS2 and investigated their hightemperature thermoelectric properties in the range 300−973 K. The structure of the misfit layered sulfides consists of alternating layers of rock-salt-type LaS and CdI2-type NbS2/ CrS2. The bulk-oriented compacts were obtained by sulfurizing the respective ternary oxides using CS2 gas, and subsequently pressure-assisted sintering them. CS2 gas is a powerful sulfurizing agent for rare-earth oxides and titanium oxide, allowing low-temperature formation of rare earth sulfides31,32 and titanium disulfide.33 These results prompted us to use CS2 sulfurization in order to prepare (LaS) 1.20 CrS 2 and (LaS)1.14NbS2. We report here the effects of the synthesis procedure on the thermoelectric properties and microstructures of the (LaS)1.20CrS2 and (LaS)1.14NbS2.

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EXPERIMENTAL SECTION

CS2 Sulfurization and Sintering. Commercial La2O3 (99.9% purity, Nippon Yttrium), Cr2 O3 (99.9%, Kojundo Chemical Laboratory), and Nb2O5 (99.99%, Kojundo Chemical Laboratory) were used as the starting materials for preparing ternary oxides. The binary oxide powders (∼5.0 g in total) were mixed in their stoichiometric amounts and were heated at 10 K min−1 to 1123 K, where they were allowed to react for 24 h in ambient atmosphere and were subsequently cooled at 10 K min−1 to obtain La1.20CrO3 and 2685

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confirmed to be sufficient (Figures S4 and S5 in the Supporting Information). The uncertainty of the Seebeck coefficient and electrical conductivity measurements is ∼5%. The Hall coefficient of the sintered compacts was measured using a homemade system under a magnetic field from 0 to +2.3 T at room temperature. The samples typically were ∼5 × 5 × 0.4 mm. In-rich In−Ga paste was used to attach Cu contact wires to the samples. The Hall coefficient was also measured using a physical property measurement system (PPMS, Quantum Design) under a magnetic field from −5 to +5 T. The samples typically were ∼4 × 3 × 0.4 mm. The results obtained using both methods are consistent (Table S1 in the Supporting Information). Thermal Transport Measurements. The total thermal conductivity (κtotal) was calculated using the density (d), the thermal diffusivity (D), the heat capacity (CP) of the sintered compacts, and the equation κtotal = DCPd. The thermal diffusivity was directly measured and the heat capacity was indirectly derived using a standard sample (Pyroceram 9606) and laser flash method (LFA 457 MicroFlash, Netzsch) over the range 300−973 K under Ar flowing at 100 mL min−1. The samples used for the out-of-plane measurements were typical ∼10 mm diameter, ∼2 mm thick coins, and those used for the in-plane measurements were 2 mm thick, ∼6 × 6 mm square plates. The heating and cooling cycles enable the repeatable thermal diffusivity for all the sintered compacts. The sample preparations and electrical transport measurements were repeated. The experimental repeatability is confirmed to be sufficient (Figures S4 and S5 in the Supporting Information). The thermal diffusivity data are provided in Figure S6 in the Supporting Information. The heat capacity was also measured using differential scanning calorimetry (DSC-60, Shimadzu) under Ar flowing at 50 mL min−1 over the range 323−623 K. The sintered compacts were crushed into fine powders of which ∼55 mg was used for the measurements. An error of ∼10% is found between the heat capacity measurements performed using both methods (Figure S7 in the Supporting Information). The DSC-based heat capacity measurements were used to determine the thermal conductivity. The densities of the sintered compacts were determined using Archimedes’ method. All the samples show a density >93% of the theoretical density (Table S2 in the Supporting Information). The uncertainty of the thermal conductivity is estimated to be within 8%, taking into account the uncertainties for D, CP, and d. The combined uncertainty for all measurements involved in the calculation of ZT is around 15%.

Figure 1. Powder X-ray diffraction (XRD) patterns for the 6 and 12 h sulfurized (a) (LaS)1.20CrS2 and (b) (LaS)1.14NbS2 powders. Out-ofplane XRD patterns for the (c) (LaS)1.20CrS2 and (d) (LaS)1.14NbS2 sintered compacts.

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RESULTS AND DISCUSSION Morphology and Microstructure. Figure 1a,b shows the powder XRD patterns for the (LaS)1.20CrS2 and (LaS)1.14NbS2 sulfurized with CS2 at 1073 K for 6 and 12 h, respectively. No impurity peaks attributable to secondary phases are detected, indicating that 6 and 12 h CS2 sulfurizations are efficient methods of producing these single-phase sulfides. SEM images of the nonsulfurized La1.20CrO3 and La1.14NbO4 oxides and the (LaS)1.20CrS2 and (LaS)1.14NbS2 powders sulfurized with CS2 for 6 and 12 h are shown in Figure 2. The average particle diameter of the starting material oxides is ∼300 nm (Figure 2a,b) and those of the LCS-6S and LNS-6S are ∼1 and ∼6 μm (Figure 2c,d), respectively, indicating that 6 h sulfurizations have enhanced the growth of the grains in both samples. No significant differences are observed in the SEM images of the LCS-6S and LCS-12S powders (Figures 2c,e, respectively). Table 1 shows the sulfur, oxygen, and carbon contents in all four samples. Extending sulfurization results in higher C content in the LCS-12S sample owing to the decomposition of CS2 gas. The higher C content is believed to be responsible for impeding the grain growth, as Ohta et al.34 previously suggested. The SEM image also reveals that the LNS-6S powder is mainly comprised of platelet-like particles

Figure 2. Scanning electron micrographs of the nonsulfurized (a) La1.20CrO3 and (b) La1.14NbO4; 6 h sulfurized (c) (LaS)1.20CrS2 (LCS6S) and (d) (LaS) 1.14 NbS 2 (LNS-6S); 12 h sulfurized (e) (LaS)1.20CrS2 (LCS-12S) and (f) (LaS)1.14NbS2 (LNS-12S) powders.

and a few presence of spherical ones (Figure 2d). Extending the sulfurization to 12 h, as for LNS-12S (Figure 2f), decreases both the platelet proportion and size, producing mostly irregularly shaped particles whose average diameter is ∼2 μm. Although it is unclear what promotes the LNS-6S platelet morphology, higher O content and lower C content can be assumed to play a role (Table 1). The average O content is 0.8 wt % for all the samples except LNS-6S, whose O content is 3 2686

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pressure applied during sintering and giving the highly preferred (00l) orientation. The carbon in the LCS-12S and LNS-12S, on the other hand, inhibits grain growth even during sintering,34 producing weakly oriented compacts, whose grain sizes are 1−2 μm (Figure 3c,d, respectively). Only the d-spacing corresponding to the (00l) peaks shows any significant shift. The d-spacings corresponding to the (003) peaks for LCS-6S and LCS-12S are calculated as 3.640 and 3.676 Å, respectively; the latter is closer to the d-spacing reported for the single crystal (3.67 Å).35 The d-spacing of (LaS)1.14NbS2 also shifts. The d-spacings corresponding to the (006) peaks for LNS-6S and LNS-12S are calculated as 3.816 and 3.836 Å, respectively; the latter is again closer to the dspacing reported for the single crystal (3.84 Å).36 Longer sulfurization leads to d-spacing close to the value of single crystal, presumably due to better homogeneity and reduced oxygen impurity in the samples. Peak positions and d-spacing of all the peaks are presented in Tables S3 and S4 in the Supporting Information. The HRTEM image and corresponding ED pattern (Figure 4) refer to the perfect, relatively defect-free region of LCS-12S,

Table 1. Chemically Analyzed Sulfur, Oxygen, and Carbon Contents in the (LaS)1.20CrS2 Sintered Compacts Obtained from 6 h Sulfurization (LCS-6S) and 12 h Sulfurization (LCS-12S) and the (LaS)1.14NbS2 Sintered Compacts Obtained from 6 h Sulfurization (LNS-6S) and 12 h Sulfurization (LNS-12S) chemical composition (wt %) sample

S

O

C

LCS-6S LCS-12S LNS-6S LNS-12S

34.1 33.8 27.4 31.1

0.73 0.88 2.49 0.75

0.02 0.14 0.05 0.13

times higher. The S content is near-stoichiometric in both LCS samples (∼3.2) and in the LNS-12S sample (∼3.1) but is slightly lower in the LNS-6S sample (∼3.0). Figure 1c,d shows the XRD patterns for the surfaces perpendicular (out-of-plane) to the pressing directions of the (LaS)1.20CrS2 and (LaS)1.14NbS2 sintered compacts. All the samples except LCS-12S show a strong normalized orientation index of the (00l) plane, indicating that the crystalline c-axis is preferably oriented along the out-of-plane direction. The extent of grain orientation was analyzed using the peak-intensity ratios for the most prominent peaks, i.e., ΣI(003)/ΣI(hkl) for the LCS and ΣI(006)/ΣI(hkl) for the LNS systems. The 6 h sulfurizations produce preferably orientated textures; LNS-6S and LCS-6S both show (00l) peak-intensity ratios of 60%, whereas the 12 h sulfurizations produce less-oriented LNS-12S and randomly oriented LCS-12S showing peak-intensity ratios of 41 and 25%, respectively. The calculated (simulated) (00l) peak-intensity ratios for a perfectly randomly oriented sample are ∼23% for (LaS)1.14NbS2 and ∼17% for (LaS)1.20CrS2. The SEM images of the sintered compacts presented in Figure 3 strongly support the XRD patterns. The grains in the LCS-6S and LNS-6S grew to >20 μm and self-arranged into a layered structure (Figures 3a,b, respectively). This natural layering of the grains is due to the anisotropic nature of the atomic bonds in these materials.28 The intralayer covalent bonding is much stronger than the interlayer bonding, causing the grain growth to proceed in direction perpendicular to the

Figure 4. High-resolution transmission electron microscopy (HRTEM) image of LCS-12 taken along [110] zone axis of the (LaS)1.20CrS2 structure and corresponding electron diffraction pattern. Structural model superimposed on enlarged HRTEM image is given as inset in left bottom corner. La, Cr, and S are represented as blue, red, and yellow, respectively. White arrows indicate CrS2 intergrowth layers.

as viewed along the most informative [110] direction. The dark dots in the simulated image correspond to the atom columns (the darker and the lighter, smaller dots represent La and Cr columns, respectively, whereas the bright ones represent the channel). The structural model overlaid on the enlarged HRTEM image of a very thin area is given as the inset and closely corresponds with the proposed (LaS)1.20CrS2 structure. Despite the overall perfect crystal structure, a number of defects

Figure 3. Scanning electron micrographs of the fractured areas of the 6 h sulfurized (a) (LaS)1.20CrS2 and (b) (LaS)1.14NbS2 and the 12 h sulfurized (c) (LaS)1.20CrS2 and (d) (LaS)1.14NbS2 sintered compacts. 2687

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6S morphology, which shows the highest in-plane μ of 1.6 cm2 V−1 s−1 (Table 2). These results suggest that the electron transport properties in these incommensurate misfit compounds can be controlled by properly optimizing the number of layer stacking faults and by tuning the microstructure/ morphology. For the LCS-6S, ρ is highly anisotropic between the two directions over the entire temperature range measured, as shown in Figure 5a. The in-plane ρ is lower than the out-ofplane ρ; for example, the in-plane and out-of-plane ρ are ∼170 and 280 μΩ m at 950 K, respectively. As shown in Table 2, the highly oriented LCS-6S texture promotes higher μ (1.6 cm2 V−1 s−1) in the in-plane direction, while increased electron scattering at the interfaces between the layers results in lower μ (0.67 cm2 V−1 s−1) and higher ρ in the out-of-plane direction. The signs of the Seebeck coefficient (S) (Figure 5b) and Hall coefficient (Table S1 in the Supporting Information) are negative for all the sintered (LaS)1.20CrS2 compacts, indicating n-type carrier transport. No significant difference in S is observed in either direction for the oriented LCS-6S and nonoriented LCS-12S samples at 950 K. S is in the range ∼−150 to ∼−170 μV K−1 in both directions for all the samples at 950 K. All the samples show an anomalous S around 600 K, which might be related to the semiconductor-metal transition (Figure S8 in the Supporting Information). The power factors, S2/ρ, for the LCS-6S and LCS-12S sintered compacts are shown in Figure 5c. LCS-6S shows the highest S2/ρ of ∼170 μW K−2 m−1 in the in-plane direction, which is >50% higher than its out-of-plane counterpart (∼80 μW K−2 m−1) and ∼22% higher than the S2/ρ for LCS-12S (∼140 μW K−2 m−1 for both directions) at 950 K. The LCS-6S shows an in-plane S2/ρ higher than that for LCS-12S because of the slightly higher S due to the low n combined with the lowest ρ due to the high μ in the in-plane direction. The S2/ρ can be further enhanced by optimizing n by either tuning of the stacking faults and/or appropriately doping. Figure 5d shows the temperature dependences of the total thermal conductivity (κtotal) and lattice thermal conductivity (κlat) of the LCS-6S and LCS-12S samples measured in both directions. κlat is obtained by subtracting the electronic thermal conductivity (κel) from κtotal, and κel is estimated using the Wiedemann−Franz law with L0 = 2.44 × 10−8 W Ω K−2. The large grain size and oriented texture of LCS-6S reduce the number of phonons scattered at grain boundaries in the inplane direction; therefore, the LCS-6S shows the highest κlat ≈ 1.1 W K−1 m−1 and κtotal ≈ 1.2 W K−1 m−1 at 950 K. This inplane κtotal is ∼8% higher than that of the randomly oriented counterpart, LCS-12S. Both samples show very low κtotal in the out-of-plane direction, the lowest being for the LCS-6S (0.92 W K−1 m−1) at 950 K, which is mainly from the ultralow κlat (0.84 W K−1 m−1) and is attributed to the fact that the interfaces between the intercalation LaS and host CrS2 layers effectively scatter the heat-carrying phonons. The modulated-structure-induced phonon scattering and softening of the transverse velocity also play a major role in low κlat.24 The low κtotal for this misfit layered sulfide are comparable to those of the best-known conventional thermoelectric materials such as PbTe.7−10 These results show that misfit layered compounds are more favorable than the single-layered compounds such as TiS222,33 for obtaining low κlat. The ZT for LCS-6S and LCS-12S calculated from their respective S, ρ, and κtotal in both directions are shown in Figure

such as the insertion of an extra bright contrast plane (marked by the white arrows in Figure 4) are observed in the HRTEM image and ED pattern (i.e., the streaks along the c-axis). The bright contrast planes exhibit coherent stacking-fault-induced intergrowth with the (LaS)1.20CrS2 structure. The HRTEM image contrast analysis based on the image simulation results and the analysis of ED pattern generated along the [001] zone axis (not shown here) suggest that these layers are CrS2 (monoclinic C2/m (12), a = 0.346 nm, b = 0.579 nm, c = 1.0699 nm, α = 95.54°, ICSD 75420). The CrS2-layer lamellas vary from two layers to several nanometers wide, spreading throughout the material. The additional CrS2 layers can be interpreted as a deficiency of LaS layers; such crystal defects could be responsible for the unique physical properties, particularly thermoelectric properties. Thermoelectric Properties of (LaS)1.20CrS2. It is quite evident from previous reports that vacancies exist at La sites such that the exact charge balance between La3+, Cr3+, and S2− is maintained; hence, the chemical composition of this phase becomes (La0.94 □0.06S)1.20CrS2 (□ represents a metal vacancy).30,37 Furthermore, this compound exhibits its semiconductor behavior through charge transferred from the LaS layer (acting as an electron reservoir) to the CrS2 one such that no excess electrons (which would otherwise cause metallic conductivity) remain in the LaS layer.38 From the room-temperature Hall effect measurements and assuming parabolic bands and single-band conduction, we estimated n from the formula: n = 1/eRH, where e represents the electronic charge and RH is the Hall coefficient. Table 2 Table 2. Room-Temperature Electron Mobility (μ) and Carrier Concentration (n) Measured in the In-Plane and Out-of-Plane Directions for the (LaS)1.20CrS2 Sintered Compacts Obtained from 6 h Sulfurization (LCS-6S) and 12 h Sulfurization (LCS-12S) sample LCS-6S LCS-12S

in-plane out-of-plane in-plane out-of-plane

μ (cm2 V−1 s−1)

n (1020 cm−3)

1.6 0.67 0.57 0.56

2.8 3.6

shows the n estimated for all the samples and Table S1 in Supporting Information shows the measured RH. The n for LCS-6S (2.8 × 1020 cm−3) is less than that for LCS-12S (3.6 × 1020 cm−3), presumably because the LCS-6S has more stacking faults (i.e., fewer LaS layers) than the LCS-12S. A detailed TEM study is necessary and is underway to validate this hypothesis. The temperature dependence of the electrical resistivity (ρ) of the CrS2 system is shown in Figure 5a. The LCS-6S and LCS-12S both exhibit semiconductor behavior from room temperature to ∼600 K; the ρ decreases as the temperature increases. For temperatures above 600 K, they become metallic; the ρ increases with temperature. An optical bandgap of ∼1.2 eV and an activation energy of ∼0.56 eV have previously been suggested for this system.28 The room-temperature electron mobility, defined as μ = 1/ neρ, is shown in Table 2. Despite the highly oriented LCS-6S showing the lower n (Table 2), it shows an in-plane ρ of ∼170 μΩ m at 950 K, which is about ∼20% lower than that for the randomly oriented LCS-12S (∼210 μΩ m) (Figure 5a) because of the reduced carrier scattering from the highly textured LCS2688

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Figure 5. Temperature dependences of the (a) electrical resistivity (ρ), (b) Seebeck coefficient (S), (c) power factor (S2/ρ), and (d) total thermal conductivity (κtotal) and lattice thermal conductivity (κlat) measured perpendicular (in-plane) and parallel (out-of-plane) to the pressing direction for the LCS-6S and LCS-12S sintered compacts.

6. The LCS-6S shows the highest ZT of 0.14 at 950 K in the inplane direction, which is 40% higher than its out-of-plane ZT,

Table 3. Room-Temperature Electron Mobility (μ) and Carrier Concentration (n) Measured in the In-Plane and Out-of-Plane Directions for the (LaS)1.14NbS2 Sintered Compacts Obtained from 6 h Sulfurization (LNS-6S) and 12 h Sulfurization (LNS-12S) μ (cm2 V−1 s−1)

sample LNS-6S LNS-12S

in-plane out-of-plane in-plane out-of-plane

11 6.1 7.9 4.7

n (1020 cm−3) 11 9.8

higher than that for LNS-12S because the LNS-6S contains larger grains. As shown in Figure 7a, the ρ trends are in line with the mobility ones. Similar to (LaS)1.20CrS2, the highly oriented LNS-6S shows the lowest ρ of 17 μΩ m, followed by the weakly oriented LNS-12S with ρ of 22 μΩ m at 950 K in the in-plane direction. The out-of-plane ρ is ∼40% higher than the in-plane ρ for both samples. The difference is mainly from the preferred alignment of the grains in the LNS-6S, resulting in higher μ. Similar to the electronic structure of (LaS)1.20CrS2, that of (LaS)1.14NbS2 has previously been described as a superposition of charge-transfer-stabilized LaS and NbS2 band structures.29 S (Figure 7b) and the Hall coefficient (Table S1 in the Supporting Information) are both positive, indicating p-type carrier transport. The LNS-6S shows n ≈ 11 × 1020 cm−3, which is nearly the same as that for LNS-12S, n ≈ 9.8 × 1020 cm−3. The LNS samples both show anisotropic S; the in-plane S are higher than the out-of-plane ones (Figure 7b). The LNS-6S and LNS-12S show the highest S (∼80 μV K−1) in the in-plane direction at 950 K. The LNS-12S and LNS-6S show S ≈ 70 and 60 μV K−1, respectively, in the out-of-plane direction. This anisotropic S has previously been reported for the misfit (SnS)1.20(TiS2)2 system and is said to arise from the anisotropic band structure.24 The Fermi level lies in the middle of the Nb 4dz2 bands in the NbS2 sandwich; hence, the Nb 4dz2 states

Figure 6. Temperature dependence of the thermoelectric figure of merit (ZT) measured perpendicular (in-plane) and parallel (out-ofplane) to the pressing direction for the LCS-6S and LCS-12S sintered compacts.

because it shows a high S2/ρ arising from its highly oriented texture in the in-plane direction. The out-of-plane ZT for LCS12S (0.13) is slightly higher than its in-plane ZT (0.11), which is mainly due to the lower out-of-plane κtotal because the S2/ρ is the same in both directions. Thermoelectric Properties of (LaS)1.14NbS2. The μ and n measured at 300 K for (LaS)1.14NbS2 are presented in Table 3. The μ for both samples shows anisotropic behaviors; the highest μ measured in the in-plane direction of LCS-6S is 11 cm2 V−1 s−1, which is not far from the μ of 17 cm2 V−1 s−1 for the single crystal,28 indicating that the (00l) planes of the polycrystalline samples are preferentially oriented so that the electron transport in the in-plane direction is similar to that in the single crystal. The μ decreases owing to increased carrier scattering in the out-of-plane direction. The μ for LNS-6S is 2689

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Figure 7. Temperature dependences of the (a) electrical resistivity (ρ), (b) Seebeck coefficient (S), (c) power factor (S2/ρ), and (d) total thermal conductivity (κtotal) and lattice thermal conductivity (κlat) measured perpendicular (in-plane) and parallel (out-of-plane) to the pressing direction for the LNS-6S and LNS-12S sintered compacts.

The ZT for the misfit layered sulfide, (LaS)1.14NbS2, are shown in Figure 8. The less-oriented LNS-12S shows the

mainly determine the transport properties. Strong interlayer energy dispersions of about 1.0 eV occur near the Fermi level, whereas smaller dispersions of 0.01−0.05 eV occur in the interlayer direction.29 As shown in Figure 7c, the LNS-6S shows the highest power factors, S2/ρ, of 410 and 230 μW K−2 m−1 at 950 and 300 K, respectively, which is about 5 times higher than that previously reported for (LaS)1.14NbS2 at room temperature.27 No reference on the high-temperature thermoelectric properties of (LaS)1.14NbS2 is available for comparison. Similar to CrS2 system, the textured LNS-6S shows an S2/ρ 22% higher than its weakly textured counterpart, indicating that oriented grain texture is quite favorable for producing misfit compounds showing higher S2/ρ. Figure 7d shows the temperature dependences of the total thermal conductivity (κtotal) and lattice thermal conductivity (κlat) of the LNS samples. The LNS-6S shows the largest κlat of ∼1.9 W K−1 m−1, owing to its large grains oriented in the inplane direction, which is twice as large as that of the LNS-12S (κlat ≈ 0.93 W K−1 m−1) at 950 K. The resultant κtotal for the LNS-6S and LNS-12S are 3.3 and 2.0 W K−1 m−1, respectively, at 950 K in the in-plane direction. The increased interlayer and boundary scatterings in LNS-6S result in the lowest κlat and κtotal (∼0.52 and ∼1.3 W K−1 m−1, respectively) at 950 K in the out-of-plane direction. The micrograins in the bulk LNS-12S sample are not completely oriented, so the thermal transport behavior in the out-of-plane direction is intermixed with that in the in-plane one, and κlat ≈ 0.93 W K−1 m−1 is similar in both directions at 950 K. This implies that in a weakly oriented sample the difference between the κtotal for the two directions is mainly governed by κel, unlike the highly oriented sample. The in-plane value is 20% higher than the out-of-plane one. The κel and κtotal for the LNS-12S sample can be further reduced by tuning the electron transport, which will simultaneously help improve S2/ρ. The κlat for the oriented LNS-6S can also be reduced by further tuning the microstructure.

Figure 8. Temperature dependence of the thermoelectric figure of merit (ZT) measured perpendicular (in-plane) and parallel (out-ofplane) to the pressing direction for the LNS-6S and LNS-12S sintered compacts.

highest ZT of ∼0.15 at 950 K in the in-plane direction. Although the LNS-6S shows an S2/ρ 22% higher than that of the LNS-12S, it also shows a 40% higher κtotal, which easily overcomes its high S2/ρ and ultimately produces a low ZT. This result shows that the microstructures of oriented/textured systems must be further tuned to reduce the κtotal and enhance the ZT. In weakly oriented LNS-12S, a further increase in ZT can be achieved by the reduction in n, which will not only reduce κel but will also enhance S2/ρ.

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CONCLUSIONS We have successfully synthesized misfit layered sulfides, (LaS)1.20CrS2 (n-type) and (LaS)1.14NbS2 (p-type), through CS2 sulfurization and pressure-assisted sintering. We produced a preferably oriented texture along the pressing direction and a 2690

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randomly (or weakly) oriented one for both materials. The naturally intercalated sulfides provide ultralow lattice thermal conductivities, as low as ∼0.5 W K−1 m−1 at 950 K, which are ∼50% lower than those for single-layered sulfides such as TiS2.33 The highly oriented texture produces the highest ZT of 0.14 at 950 K among the (LaS)1.20CrS2 system, while the weakly/randomly oriented texture produces the highest ZT of 0.15 at 950 K among the (LaS)1.14NbS2 system. Still, higher ZT is expected as further work is required in order to optimize the carrier concentrations by tuning the stacking faults of the layers and/or introducing appropriate dopants. In addition, this work shows that the thermoelectric properties of these layered sulfides are highly sensitive to microstructural tuning; therefore, further microstructural tuning is also expected to increase the ZT. These misfit layered sulfides are promising materials for high-temperature thermoelectric applications because they are environmentally benign, nontoxic, and stable at high temperatures. Furthermore, both n- and p-type materials can be realized from the same family of misfit layered sulfides, which adds to their advantages.

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ASSOCIATED CONTENT

S Supporting Information *

XRD patterns of starting oxide precursors, sintering densification curves, DTA analysis, experimental repeatability, Hall coefficient, thermal diffusivity, heat capacity, sintered density, XRD peak positions and d-spacings, clear view of the slight anomaly in the S. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*M. Ohta. E-mail: [email protected]. Author Contributions

The authors are arranged in order of their contributions. P.J. and M.O. designed the experiments and prepared the powders and sintered samples. P.J. and H.N. measured the Seebeck coefficients, electrical resistivities, and total thermal conductivities. H.N and K.S. performed the Hall coefficient measurements. M.K. performed differential thermal analysis. O.I.L. and D.B. performed the TEM experiments. P.J., M.O., and A.Y. analyzed the results. All authors discussed the results and contributed to writing of the paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Ms. Yasuko Takashima of AIST for operating the differential scanning calorimeter. This study was partially supported as part of the Japan-US Cooperation on Clean Energy Technologies funded by the Ministry of Economy, Trade and Industry (METI). The study was also supported by JSPS KAKENHI Grant Number 25420699 and the Thermal and Electric Energy Technology Foundation.

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REFERENCES

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