Nanostructural and Microstructural Ordering and Thermoelectric

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Nano- and Micro-structural ordering and thermoelectric property tuning in misfit layered sulfide [(LaS)] NbS x

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Priyanka Jood, Michihiro Ohta, Oleg I Lebedev, and David Berthebaud Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03365 • Publication Date (Web): 21 Oct 2015 Downloaded from http://pubs.acs.org on October 27, 2015

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Chemistry of Materials

Nano- and micro-structural ordering and thermoelectric property tuning in misfit layered sulfide [(LaS)x]1.14NbS2

Priyanka Jood,a Michihiro Ohta,*a Oleg I. Lebedev,b and David Berthebaudb

a

Research Institute for Energy Conservation, National Institute of Advanced Industrial Science and

Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan b

Laboratoire CRISMAT, UMR 6508 CNRS/ENSICAEN, 6 Boulevard du Maréchal Juin, F-14050 Caen

Cedex 4, France

Supporting Information available

ACS Paragon Plus Environment


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Abstract The potential to control the structural ordering at nanoscale in misfit layered systems can open new paths for achieving high thermoelectric performance in these systems. In the present work, we demonstrate that compositional changes can provide nanoscale tuning in misfit layered (LaxSx)1.14NbS2 (x = 0.90, 0.95, 1.00, 1.05, 1.10), leading to improved thermoelectric properties (investigated over the temperature range of 300 K to 950 K). The samples were prepared by CS2 sulfurization and consolidated using pressure-assisted sintering. It was revealed through transmission electron microscopy (TEM) analysis that non-stoichiometry promotes long range ordering of the layers and results in elongated lamella formations. The La deficient x = 0.95 sample was found to contain large strain induced stacking disorder, owing to which the long lamellas rolled up to form tubular structures, whereas, La rich x = 1.05 sample had a much ordered structure and only slight curling of lamella edges was observed. Improved structural ordering and textured grain growth in La rich sample resulted in a ~30% improved power factor (~460 µW K−2 m−1 at 950 K) and ZT (~0.2 at 950 K) in the in-plane direction, compared to the x = 1.00 sample. The origin of very low thermal conductivity (1.1 W K−1 m−1–2.5 W K−1 m−1 at 950 K) in our samples has been identified and discussed in detail.


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1. INTRODUCTION Thermoelectric power generation is considered as a promising technology for improving energy management and reducing the greenhouse gas emissions. In the past decade, the search for highperformance and environmentally friendly thermoelectric (TE) materials has accelerated, because electrical power generation from waste heat on an industrial scale requires less-hazardous materials with high figure of merit, ZT = S2T/ρκtotal (where, S is Seebeck coefficient, ρ is electrical resistivity, κtotal is thermal conductivity, and T is temperature) at temperatures above 600 K. A major challenge in achieving high ZT is the individual tuning of electron and phonon transports in the material. The phonon glass–electron crystal (PGEC)1 concept has proven to be a key strategy for developing high-ZT thermoelectric materials, where the phonon glass region provides the disorder necessary to scatter phonons (i.e., low κtotal) without disturbing the carrier mobility in the electron crystal region (i.e., high S2/ρ). Layered materials such as, homologous chalcogenides (for example, chalcogenide system Cs4[Bi2n+4Te3n+6]2, [(PbSe)5]m[(Bi2Se3)3]n3, Pb7Bi4Se134, [MTe]n[Bi2Te3]m (M = Ge, Sn, Pb)5-8) and misfit layered oxides (NaxCoO2, Ca-Co-O, and [Bi2A2O4][CoO2]b1/b2 (A = Ca, Sr, Ba))9-13, are an example of PGEC behavior and have shown great potential in terms of their thermoelectric performance. Misfit layered oxides, being less-hazardous and stable at 1000 K in air, were considered specially promising for high temperature applications, however, their ZT still remains limited by their high electrical resistivity. One expects lower electrical resistivity and higher power factor for sulfides owing to higher covalency, due to which the research focus has extended to layered sulfides in recent years3,9,14. Panascopic approach or all-scale hierarchical architecting has considerably enhanced the ZT ~2.2 of PbTe system15-17 and hence this approach can serve as a guideline for ZT enhancement in layered sulfides as well. Higher performance in layered sulfides is expected through proper optimizations of thermoelectric properties from atomic scale (for example, stacking faults tuning) to microscale (for example, microtexturing). Misfit layered sulfides, which are a part of the family (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)18, are slowly being considered as potential high temperature thermoelectric materials due to their use of less-hazardous and cost-effective constituent elements and their naturally modulated structure19. These compounds provide tremendous opportunities for ZT enhancement due to their PGEC behaviour, where the CdI2-type TS2 host layer forms a high mobility



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carrier pathway and the intercalated NaCl-type MS layer creates disorder and is responsible for phonon scattering. This kind of incommensurate framework, allows individual tailoring of power factor (S2/ρ) and κtotal (κtotal = κel + κlat, the electronic and lattice contributions, respectively) to realize an efficient thermoelectric material. These natural superlattice sulfides attracted much attention after the discovery of large power factor, S2/ρ~3710 µW K−2 m−1 in TiS2 single crystal20, 21 followed by reports on its intercalated compounds22-24, such as n-type Cu0.02TiS225 and (SnS)1.2(TiS2)226,with ZT ~0.45 (at 800 K) and ~0.35 (at 670 K), respectively. These intercalated compounds provide very low κlat, for example ~0.9 W K−1 m−1 and ~0.4 W K−1 m−1 at 670 K in the in-plane and out-of-plane direction of (SnS)1.2(TiS2)2, respectively, with the out-ofplane value even lower than κmin. Furthermore, the κlat of these intercalated compounds is ~50% lower than their single layered counterpart, i.e. TiS2 system26-28, making them more favourable than the latter. In a separate report, Miyazaki et al.29, 30 reported the room-temperature properties of p-type (Ln2S2)pNbS2 (Ln = rare-earth elements) with NbS2 as the host layer. Among all the samples, Yb deficient (Yb2xS2x)pNbS2 had the highest ZT ~0.11 at 300 K. This motivated us to investigate the effects of non-stoichiometry in the LaS layer of (LaS)1.14NbS2 and to also facilitate nanoscale tuning of thermoelectric properties. This work is in continuance of our previous study on the effects of microscale tuning (microtexturing) on the high temperature thermoelectric properties of (LaS)1.14NbS2 and (LaS)1.2CrS231. The electronic structure and physical properties of (LaS)1.14NbS2 and related compounds have been studied in depth in the 1990’s18,32. Using the composite structure approach, it was determined that LaS and NbS2 substructures belong to Cm2a and Fm2m space group, respectively and mutually modulate each other incommensurately33. The complete structure including the modulations was then described in a (3+1)dimensional superspace group, which also revealed that the largest modulations are for the La atoms and the S atoms of NbS232, 34. This compound attains its stability through a large charge transfer from the LaS layer to the NbS2 layer and the conduction is by holes in the dz2 band of the NbS2 layer. Therefore, both the layers have their own role to play in the whole electronic arrangement, and it is critical to understand how the compositional changes in the layers affect the thermoelectric properties in these systems. This would also open the path towards identifying appropriate sites for doping/substitution to further enhance ZT. We prepared polycrystalline samples of misfit layered sulfides (LaxSx)1.14NbS2 (x = 0.90, 0.95, 1.00, 1.05, 1.10) and investigated their high-temperature thermoelectric properties in the range of 300 K to 950


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K. The bulk 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 and Ti based oxides, allowing low-temperature formation of rare earth and Ti sulfides27, 35, 36. In this report, an account of successful nanoscale tuning in misfit sulfides has been provided. We show that compositional variations improve the power factor in LaS-NbS2 system by realizing long range ordering of layers and reducing the nanoscale defects.

2. EXPERIMENTAL SECTION 2.1. Synthesis and sintering Commercial La2O3 (particle size: ~1 µm, 99.9% purity, Nippon Yttrium) and Nb2O5 (~1 µm, 99.99%, Kojundo Chemical Laboratory) were used as starting materials for the synthesis of ternary oxides with La:Nb = 1.14x:1.00 (x = 0.90, 0.95, 1.00, 1.05, 1.10). For example, 2.865 g of La2O3 and 2.162 g of Nb2O5 were used to prepare ~5.0g of x = 0.95 powder. The binary oxide powders were mixed in their stoichiometric amounts and 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 the ternary oxides with La:Nb = 1.14x:1.00 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 (LaxSx)1.14NbS2 powders were ground well and sulfurized again under the same conditions at 1073 K for another 6 h to improve the homogeneity. The sulfurized powders were then 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.



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The (LaxSx)1.14NbS2 powders were placed into 10-mm- or 15-mm-diameter graphite dies which were then inserted into the sintering equipment (SPS-515S, Fuji Electronic Industrial). Sintering were performed at 1223 K for 2 h under 30 MPa uniaxial pressure under vacuum (7 × 10−3 Pa) to prepare high-density compacts. The heating and cooling rates were 10 K min−1 and 20 K min−1, respectively. The sintered samples were cut into bars, coins, and plates for further measurement. For each material, we prepared three sintered samples with different sizes for electrical and thermal transport measurements in the direction perpendicular (in-plane) and parallel (out-of-plane) to the pressing direction. The cylindrical samples of 10 mm diameter × 10 mm length were cut into bars and plates for out-of-plane electrical transport measurements and in-plane thermal transport measurements, respectively. The disks of 15 mm diameter × 2 mm thickness were cut into bars for in-plane electrical transport measurements. The disks of 10 mm diameter × 2 mm thickness were used for out-of-plane thermal transport measurements. The densities of the sintered compacts were determined using Gas pycnometer method (AccuPyc II 1340, Micromeritics). All the samples show a density >94% of the theoretical density (Table S1 in the supporting information).

2.3. Powder X-ray diffraction, scanning electron microscopy, and transmission electron microscopy The crystal structure of the powders was examined using X-ray diffractometry (XRD; Miniflex, Rigaku) with Cu Kα radiation over the 2θ range 10–80°. The grain orientation in the sintered compacts was examined using XRD (Rint-Ultima, Rigaku). Lattice parameters of powder samples were refined using Integrated X-ray Powder Diffraction Software (Rigaku). The microstructures of the powders and sintered compacts were observed using scanning electron microscopy (SEM; JEOL JSM-6610LV, 20 kV). Transmission electron microscopy (TEM), including electron diffraction (ED), and high-resolution TEM (HRTEM) studies were performed using a FEI Tecnai G2 30 UT microscope operated at 300 kV, having 0.17-nm-point-resolution and equipped with EDAX EDX detector. The TEM specimens were prepared by grinding the sintered compacts in an agate mortar, dissolving the ground samples in butanol, and spreading it onto a Cu holey carbon grid.


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2.3. 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.

2.4. Electrical transport measurements The Seebeck coefficient and electrical resistivity of the compacts samples were simultaneously measured using temperature-differential and four-probe methods, respectively, (ZEM-3, Advance Riko) perpendicular (in-plane) and parallel (out-of-plane) to the pressing direction under a He atmosphere over the temperature range of 300 K to 950 K. The bars used for the in-plane and out-of-plane measurements typically were ~3 mm × ~2 mm × ~10 mm and ~3 mm × ~2 mm × ~7 mm, respectively. Seebeck coefficient and electrical resistivity were reproducible over heating and cooling cycles for both directions. The uncertainty of the Seebeck coefficient and electrical conductivity is estimated to be within ~5%. The Hall coefficient of the sintered compacts was measured using a homemade system under a magnetic field from 0 T to 2.3 T at room temperature (Table S2 in the supporting information). The samples typically were ~5 mm × ~5 mm × ~0.4 mm. In-rich In-Ga paste was used to attach Cu contact wires to the samples.

2.5. Thermal transport measurements The total thermal conductivity (κtotal) was calculated from the thermal diffusivity (D), and heat capacity (CP), density (d) of the sintered compacts, using equation κtotal = DCPd. The thermal diffusivity was directly measured using laser flash method (LFA 457 MicroFlash, Netzsch) over the temperature range of 300 K to 950 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 mm × ~6 mm square plates. The thermal diffusivity was reproduced over heating and cooling cycles for both directions. The thermal diffusivity data are provided in Figure S1 (a) in the supporting information.



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The heat capacity was measured using differential scanning calorimetry (DSC 404 F3 Pegasus, Netzsch) under Ar flowing at 50 mL min−1 over the temperature range of 330 K to 950 K (Figure S1 (b) in the supporting information). The sample dimensions used for the measurements were ~1-mm-thick, ~4 mm × 4 mm square plates. 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 12%. The sound velocities for longitudinal and transverse modes were measured at room temperature by the pulse-echo method with ultrasonic pulser/receivers (5077PR, Olympus), 5 MHz and 15 MHz longitudinal contact transducers (V110-RM and V113-RM, Olympus), 5 MHz transverse contact transducer (V156-RM, Olympus), and digitizing oscilloscope (WaveJet300A, Teledyne LeCroy). The typical dimensions of the square plates used for the in-plane measurements were ~6 mm × ~6 mm × ~2 mm. The typical dimensions of the coins used and for out-of-plane measurements were ~10 mm in diameter and ~2 mm thickness.

3. RESULTS AND DISCUSSION 3.1. Synthesis and sintering Figure 1 shows the X-ray diffraction (XRD) patterns of (LaxSx)1.14NbS2 (x = 0.90, 0.95, 1.00, 1.05, 1.10) powder samples where x = 1.00 represents the stoichiometric sample as per the earlier reports on its structural analysis18. No impurity peaks attributable to secondary phases are detected for x = 0.95, 1.00, 1.05 samples. However, for x > 1.05 and x < 0.95, impurities of La2S3 and NbS2 can be observed, respectively. From the XRD measurements, we confirm that single-phase samples can be obtained by adding or removing up to 5% of La in and from the NaCl-type LaS layer, similar to (Yb2xS2x)pNbS2 where up to 10% Yb off-stoichiometry is possible without impurities29. The d-spacing corresponding to the (00l) peaks and the lattice parameters as determined using the orthorhombic space group Ccca (68)37 are presented in Table 1. The d-spacing for x = 1.05 is calculated as 0.3845 nm, whereas x = 1.00 and x = 0.95 show a smaller value of 0.3836 nm. In addition, the lattice parameters of x = 0.95 (c = 2.290(2) nm) and x = 1.00 (c = 2.286(8) nm) are also slightly smaller than for x = 1.05 (c = 2.300(6) nm) sample, which could be due to strain induced defects in the former (also observed by TEM as shown in the later section). These


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lattice parameter values are only shown for comparison purposes, as for more accurate evaluation separate space groups for LaS and NbS2 subsystem should be considered during lattice refinement (Table S3 in the supporting information). As observed from the powder XRD patterns, the non-stoichiometric samples (x = 0.95 and 1.05) appear more crystalline with higher peak intensities compared to the stoichiometric x = 1.00. We also analyzed the extent of grain orientation using peak-intensity ratios for the most prominent peak, i.e. ΣI(006)/ΣI(hkl) through the XRD data (Figure S2 in the supporting information) of sintered compacts. The (00l) peakintensity ratios for x = 1.05 sample was the highest with 55%, followed by x = 1.00 with 41% and x = 0.95 with 38%. This shows that x = 1.05 has the strongest grain orientation perpendicular to pressing direction among all samples, which is also confirmed by SEM (discussed in the later section). Chemical analysis was performed on the crushed sintered samples and the results are presented in Table 2. The results reveal that unlike the non-stoichiometric samples, the sulfurization reaction in x = 1.00 is completed long before, resulting in the presence of excess C (0.13 wt. %) and S (31.1 wt. %) in the sample. Chemical analysis results show that, x = 1.05 sample has the least C (0 .04 wt. %) and O (0.99 wt. %) impurities and near-stoichiometric S (28.4 wt. %). TEM observations were performed on the crushed powders from the sintered compacts. HRTEM image and corresponding ED pattern of the x = 1.00 (LaS)1.14NbS2 sample are shown in Figure 2. Some distortions, imaged as the dark contrast planes (marked with white arrows) can be observed in the area marked as A in Figure 2, whereas the area B signifies a defect-free region with perfect atomic and layers arrangement. An overlaid structural model on the enlarged HRTEM image of the distorted area A depicts the nature of the defects which appear as strain induced compression of the LaS layers. In (LaS)1.14NbS2 system, La atoms are reported to have the largest modulation amplitudes which mainly refers to the displacements in the plane of the layers34. This implies that the LaS layers (or NaCl type MX layers, in general) are the most prone to any disorder in the lattice. For instance, in our previous work we showed stacking faults related to the absence of LaS layers in the (LaS)1.2CrS2 lattice31. In another work, it was suggested that most of the stacking disorder occurred in the BiS layer of misfit (BiS)1+δ(NbS2)n compound38. TEM observations for (LaxSx)1.14NbS2 (x = 0.95 and 1.05) samples are shown in Figures 3 and 4, respectively. Figures 3 (a) and (b) shows low magnification TEM image of x = 0.95 sample viewed along



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c-axis direction depicting a tubular formation. ED patterns referring to areas of the curled region of the plane section of the sample viewed along the most informative [001] and [110] direction are shown in Figure 3 (b). ED patterns in this case are superposition of differently oriented crystal parts and exhibit complexity with features typical for curled structures. A number of defects similar to stoichiometric sample, such as the insertion of darker contrast plane (marked by white arrows) were observed in HRTEM image of the curled region (area A) (Figure 3 (c)). These defects are not observed in the edge (area B) of the tubes, which exhibits rather well-ordered layers (Figure 3 (d)). Figure 4 (a) shows the low magnification bright field TEM image of x = 1.05 sample along c-axis. A spread of dark contrast stripes across the whole region due to misorientation among the ab plane can be seen. Darker areas correspond to the curled edges of plane sections as schematized on the inset drawing. Due to this disorder, it is very difficult to obtain information about the stacking direction because of complex contrast image projected on the plane. HRTEM images in Figure 4 (b) show a magnified area of the image in Figure 4 (a), where lamellas are observed in-plane close to [001] zone axis (area A) and crossplane along the c-axis (area B) with the corresponding Fourier transform (FT) in inset. The HRTEM image in Figure 4 (c) and corresponding ED pattern refers to areas of the cross section of x = 1.05 sample viewed along the most informative [110] direction and shows an overall perfect crystal structure as no particular defect are observed. Fourier patterns taken from different areas (A and B) of the same crystal evidence different orientation of part A and B which are out of plane and close to [110] zone axis correspondingly. As it can be seen in Figure 4 (b) and (c), ED patterns along two different directions can be taken from the same crystal without practically any tilting of the crystal and they demonstrate well-ordered structure free of any extended defects or stacking faults. The tubular formation has been reported previously by a few authors in the NbS2 misfit system and it is evidently a result of strong inter-layer stacking disorder39, 40 which is mostly related to the MX layer38. In our case, the rolling up of lamellas in the x = 0.95 and 1.05 samples is due to two reasons: 1) the elongated lamella formation originating from the long range ordering of the layers as a compensation of the nonstoichiometry, and 2) strain induced stacking disorder. La deficient x = 0.95 sample, contains large stacking disorder (seen as dark contrasts planes) which explains the complete tubular formations. On the contrary, La rich x = 1.05 sample appears to have a much ordered structure compared to the former and hence only a slight curling up of the lamella edges is observed for this sample. A relatively small tension is


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required to curve the layers in the misfit system due to their large flexibility39. The reason why extra La atoms would provide more order to the layers is hard to depict, however, a similar behaviour was reported previously where Cr-doping yielded a more ordered structure in (BiS)1.2(TiS2)241. Stoichiometric x = 0 sample does not exhibit the tubular formation or curled lamella edges due to the absence of elongated lamellas, even though it has stacking disorders, as shown in the TEM images. SEM images of sulfurized (LaxSx)1.14NbS2 (x = 0.95, 1.00, 1.05) powders are shown in Figures 5 (a), (b), and (c). The microstructure and morphology correlates well with the nanoscale features observed by TEM. The La deficient x = 0.95 sample comprises of inhomogeneous morphology consisting of large platelet like grains (average diameter ~5 µm) and small irregular shaped grains. Whereas, the La rich x = 1.05 sample consists of a homogenous lamellar morphology with average grain size of ~6 µm. The large grain growth in both the x = 0.95 and 1.05 samples stems from the long range order of the layers. The x = 1.00 sample has a particle size much smaller than the x = 0.95 and 1.05samples which is due to two reasons: 1). the absence of the long range ordering and 2). the presence of relatively larger C content (0.13 wt%) impeding the grain growth42. The SEM images of the fractured area of the sintered compacts relate quite well to the SEM images of powder sample. The x = 0.95 sample shows large grain size (>20 µm) with the least oriented texture (Figure. 5 (d)). Stoichiometric x = 1.00 sample shows a small grain size and slight degree of grain orientation perpendicular to the pressing direction (Figure. 5 (e)). The x = 1.05 sample exhibits a much uniform grain distribution where the grains grew to ~10 µm and self-arranged into a layered structure (Figure. 5 (f)). These results comply with the XRD analysis of grain orientation discussed above. The natural layering of the grains under pressure is due to the anisotropic nature of the atomic bonds in these materials18. The covalent bonding within the layers (intralayer) is much stronger than the bonding between the layers (interlayer), owing to which the crystals deflect until they align along the layers under pressure. Furthermore, non-stoichiometry induces long lamellas in-plane which helps to texture the grain growth during sintering. The larger carbon content (0.13 wt%) in the x = 1.00 sample (Table 2), on the other hand, inhibits grain growth even during sintering42.

3.2. Thermoelectric properties



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The hole mobility (µ) and carrier concentration (n) measured at 300 K for (LaxSx)1.14NbS2 (x = 0.95, 1.00, 1.05) are presented in Table 3. The Seebeck coefficient (S) (Figure. 6b) and the Hall coefficient (RH) (Table S2 in the supporting information) are both positive, indicating p-type carrier transport. The n of both, x = 0.95 and x = 1.05 samples (~1.3×1021 cm−3) is higher compared to x = 1.00 sample (~0.98×1021 cm−3). The higher n in x = 0.95 sample might be originating from the La vacancies. However, the higher n in x = 1.05 sample is slightly hard to interpret. It is justified to calculate the number of holes per unit volume from RH = (ne)−1, where e is the elementary electric charge. For x = 1.00, one calculates 0.1 h/Nb atom which means a donation of 0.90 e/Nb from the LaS part. For x = 1.05, the number of holes per Nb is 0.15, which means a slightly smaller charge transfer of 0.85 e/Nb from the LaS layer. It was suggested by Suzuki et al.43 that the excess electrons in the LaS layer can become localized due to the incommensurate lattice potentials generated by the incommensurate lattice modulations, which would imply smaller charge transfer. Another possible reason for slightly smaller n of x = 1.00 sample could be the small grain size and large grain boundary density which could act as carrier trapping sites. The room-temperature µ for all the samples shows anisotropic behaviors with highest anisotropy observed for the x = 1.05 sample. For example, the in-plane µ (~7.9 cm2 V−1 s−1) of the stoichiometric x = 1.00 is twice larger than its out of plane µ (~4.7 cm2 V−1 s−1), whereas, for x = 1.05 the difference is more than three times higher with in-plane and out-of-plane µ being ~9.3 and ~2.6 cm2 V−1 s−1, respectively. The highest µ measured among all the samples was for the in-plane direction of x = 1.05 (9.3 cm2 V−1 s−1), which is the closest from the single crystal value (~17 cm2 V−1 s−1)18. This is due to its comparatively better crystallinity (as observed through XRD) and it also indicates that the (00l) planes of the La rich x = 1.05 sample are preferentially oriented so that the electron transport in the in-plane direction is close to that in the single crystal. The µ decreases owing to increased carrier scattering in the out-of-plane direction. Although, x = 0.95 sample has a long range ordering of the layers in the in-plane direction, its µ is slightly lower compared to the stoichiometric x = 1.00 sample. This is due to its inhomogeneous morphology resulting in a mixed orientation of grains (Figure 5 (d)) and also due to the higher defect density (Figure 3 (c)) as observed in TEM images. As shown in Figure. 6 (a), the electrical resistivity (ρ) trends follow the carrier mobility ones. Owing to the large grain size, oriented grains, and higher n, the x = 1.05 sample shows the lowest in-plane ρ of ~16 µΩ m, followed by the x = 0.95 and x = 1.00 samples with ρ of ~19 µΩ m and ~22 µΩ m, respectively at


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950 K. The out-of-plane ρ is ~38% higher than the in-plane ρ for both, x = 0.95 and x = 1.00, whereas for x = 1.05 the out-of-plane ρ (~50 µΩ m) is more than three times higher than its in-plane ρ, at 950 K. The difference in anisotropy among the three samples mainly comes from the much enhanced carrier scattering at the interfaces of the well aligned grains in x = 1.05 sample. All the samples show anisotropic S in the whole temperature range measured; the in-plane S are higher than the out-of-plane ones (Figure. 6 (b)). The x = 1.00 and x = 1.05 samples show the highest S (~85 µV K−1), followed by x = 0.95 which shows S ~71 µV K−1, in the in-plane direction at 950 K. From the measured n and S, the carrier effective mass m* can be estimated using the following relation. =

/

∗

,(1)

where kb is the boltzman’s constant. The m* values are listed in Table 3 and lie between 0.7–2.2 m0 (where m0 is the free electron mass) These values are much smaller than those reported for (MS)1+m(TiS2)n (M = Pb, Bi, Sn)36 intercalated compounds. Band structure calculations of (LaS)1.14NbS2 reveal that strong intralayer energy dispersions of ~1.0 eV (caused by strong intralayer interactions) occur near the Fermi level, while smaller dispersions of 0.01 eV–0.05 eV occur in the interlayer direction32. This anisotropic band structure might be responsible for the anisotropy in S and m* in these compounds, the largest being for x = 1.05 due to the stronger alignment of grains and long range ordering of layers in-plane. Extra La atoms in x = 1.05 sample could also potentially increase the hole m* owing to the anharmonicities in the crystal lattice which is discussed later in the manuscript. La rich x = 1.05 sample not only has a higher S through m* enhancement, but also possess a lower ρ owing to better structural ordering. As shown in Figure. 6c, x = 1.05 shows the highest in-plane power factor, S2/ρ, of 460 µW K−2 m−1 at 950 K, which is about ~30 % higher than the x = 1.00 sample (320 µW K−2 m−1). This shows that Larichness in MS layer is quite effective in improving the power factor in misfit sulfides. However, the S2/ρ of NbS2 misfit system remains two times lower than the TiS2 misfit systems26, and can be enhanced by appropriate doping preferably on the Nb site of (LaxSx)1.14NbS2 with x = 1.05. To examine the variation in lattice thermal conductivity (κlat) among the three samples, we performed ultrasonic pulse echo measurements and evaluated the longitudinal (vl) and transverse (vt) sounds velocities. The average sound velocity (va) was then calculated as follows:

=

+

/

, (2)



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Table 4 summarizes the room-temperature vl, vt, and calculated va. The κlat is related to the volumetric heat capacity at constant pressure (Cv), average sound velocity (va), mean free path of phonon (lp) and is expressed as:

! = "# $% , (3) Here, we approximated Cv as equal to Cp44, 45. The estimated lp of (LaxSx)1.14NbS2 as determined from (3) is presented in Figure 7 (a). Figure 7 (b) and (c) shows the temperature dependences of the total thermal conductivity (κtotal) and κlat (determined by subtracting electronic thermal conductivity (κel) from κtotal) of (LaxSx)1.14NbS2 (x = 0.95, 1.00, 1.05) in both the directions, respectively. The κel was estimated using the Wiedemann–Franz law given as κel = LT/ρ, where L is the Lorenz number (2.44×10−8 WΩK−2). For all the samples in both directions, the κtotal and κlat is quite low and lie in the range from 1.1 W K−1 m−1–2.5 W K−1 m−1 and 0.8 W K−1 m−1–1.5 W K−1 m−1 at 950 K, respectively, which is of good advantage as thermoelectric materials. Moreover, the out-of-plane lp for all samples is ~70% smaller than the length of the c-axis (2.3 nm) and reach the interlayer distance (< 1 nm) between LaS and NbS2 layers, which shows that the interfaces between the layers effectively scatter the longer wavelength heat-carrying phonons (Figure 7 (a)). The micrograins in x = 0.95 and x = 1.00 samples are not completely oriented, so the thermal transport behavior in the out-of-plane direction is intermixed with that of the in-plane one, consequently resulting in a similar lp for both the directions. For instance, for x = 0.95 the in-plane and out-of-plane lp are ~1.4 nm and for x = 1.00 the in-plane and out-of-plane lp are ~1.0 nm. Similar lp in both directions results in similar κlat, i.e ~2.2 W K−1 m−1 and ~1.5 W K−1 m−1 for x = 0.95 and x = 1.00 at 300 K, respectively. The x = 0.95 sample has the largest lp owing to the long range ordering and its large grain size and shows the largest κlat and κtotal (~2.2 W K−1 m−1 and ~3.4 W K−1 m−1, respectively at 300 K) in the in-plane direction. Among all the samples, only x = 1.05 shows a big difference between its in-plane (~1.1 nm) and out-of-plane (~0.8 nm) lp. The increased interlayer and boundary scatterings due to highly oriented grains in x = 1.05 results in the lowest out-of-plane lp, and consequently, lowest κlat and κtotal (~1.0 W K−1 m−1 and ~1.4 W K−1 m−1 at 300 K and ~0.88 W K−1 m−1 and ~1.2 W K−1 m−1 at 950 K, respectively). One would expect x = 1.05 to show the largest in-plane lp among all samples due to the strong grain orientation, long range ordering, and


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smallest interlayer defect density. However, it’s in-plane lp being smaller than that of x = 0.95 suggests the possibility of larger in-plane phonon scattering due to other factors (studied below). To further evaluate the low κtotal of these compounds, we determined the Young’s modulus (E), which is related to the strength of the material’s atomic bonds, and Gruneisen parameter (γ), which determines the anharmonicity of the bonding arrangement. The E and γ (Table 4) can be calculated from sound velocity as follows: &=

'( ) * +

, (4)

./

, = - 0 , (5) /

where d is density of the material and vp is the poison’s ratio expressed as: % =

)( /1 + )( /1 +

, (6)

In general, the sound velocities of LaS(NbS2) system (vl ~3950 m s−1–4890 m s−1; vt ~1770 m s−1–2420 m s−1) (Table 4) are slightly higher than that reported for the Bi,Pb,Sn–S (TiS2) system (vl ~3660 m s−1–4110 m s−1; vt ~1120 m s−1–2350 m s−1)23, due to the stronger interatomic bonding which also makes them more mechanically strong than the latter. The stoichiometric x = 1.00 sample exhibits the lowest vl (3940 m s−1), vt (2060 m s−1), and E (54 GPa) among all the three samples in the in-plane direction. Both nonstoichiometric samples have ~30 % larger in-plane E compared to the stoichiometric one, meaning stronger interatomic bonding which is related to their long range structural ordering. The x = 1.00 samples has the smallest Gruneisen parameter γ (~1.9 for in-plane and ~2.2 for out-of-plane) among all, which shows least anharmonicity of the chemical bonds. The γ is the highest for La rich x = 1.05 sample (~2.1 for in-plane and ~2.7 for out-of-plane) followed by La deficient x = 0.95 sample (~2.0 for in-plane and ~2.2 for out-of-plane) and this could explain the lower lp of the former. We speculate that the excess localized electrons43 in La 5d orbitals of the La rich x = 1.05 sample create electron clouds near the La atoms resulting in higher anharmonicity46 of the bonds in the LaS layers. Furthermore, the changes in La content appear to promote the deviation of phonon vibrations from harmonic oscillations in the crystal lattice, which is expected due to the tubular and curled lamella formations. A large Gruneisen parameter (γ) leads to lower κlat (by increasing phonon-phonon Umklapp scattering), along with a larger S (due to increased m*) in x = 1.05 sample. The γ of our misfit system in general (~1.8–2.8) is much higher than that of conventional PbTe (~1.45)47 or other emerging thermoelectric materials,48,


49

mainly due to its highly


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modulated structure lacking three-dimensional lattice periodicity. Large anharmonicity, along with other factors such as weak chemical bonds, and scattering at layer interfaces are responsible for their low thermal conductivity. The theoretical minimum limit of lattice thermal conductivity (κmin)50 is described as:

23 = 4

5

67 89

5 ∑3 3

;

A < =

(7)

where, 3 , D3 , and 89 are sound velocity and Debye temperature associated to the longitudinal and transverse modes, and the number density of atoms, respectively. D3 is determined as follows:

5 ,

D3 = 3 )6F 89 +

(8)

The κmin of stoichiometric (LaS)1.14NbS2 was determined to be ~0.4 W K−1 m−1 which is about four times lower than its in-plane and out-of-plane κlat. This suggests that there is a large room for reduction in κlat through increasing complexity of crystal structure (doping heavier elements) and by optimizing the structural ordering. The thermoelectric figure of merit (ZT) for the misfit layered sulfide, (LaxSx)1.14NbS2 (x = 0.95, 1.00, 1.05) in the in-plane and out-of-plane directions calculated from the measured ρ, S, and κtotal are shown in Figure. 8. The highest ZT of ~0.2 at 950 K was observed for the in-plane direction of La rich x = 1.05, because it possesses the highest power factor. The stoichiometric x = 1.00 has an in-plane ZT of ~0.15 followed by La deficient x = 0.95 with ZT ~0.1 at 950 K. The ZT of the LaS rich sample can be further boosted by following strategies; the tuning of n through doping for optimizing the power factor, microstructure tuning and grain size reduction to reduce κlat .

4. CONCLUSION Here, we successfully improved the thermoelectric performance of misfit layered sulfide (LaxSx)1.14NbS2 through varying the La content in the system. Chemical composition tuning provided elongated lamella formations due to long range ordering of layers. Tubular formation was observed for La deficient x = 0.95 sample as a result of large strain induced stacking disorder in this sample. On the other hand, La rich x = 1.05 sample had the best structural and long range ordering which provided a highly


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textured grain growth in-plane. This resulted in a 30% higher in-plane power factor and a 25% increase in ZT for La rich sample compared to the x = 1.00 one. In-depth phonon transport study revealed that the very low lattice thermal conductivity (κlat ~0.8 W K−1 m−1–1.5 W K−1 m−1 at 950 K) in these systems is a result of an effective phonon scattering from layer interfaces and large anharmonicity of atomic bonds, which can be tuned to decrease the κlat further. This study gives new insights into nanoscale disorder tuning of misfit layered sulfides and paves the path for further ZT enhancement in these systems.

ASSOCIATED CONTENT Supporting information The Supporting Information is available free of charge on the ACS Publications website at DOI: . Thermal diffusivity, heat capacity, X-ray diffraction pattern (XRD), sintered density, hall coefficient, and XRD peak indices and positions of (LaxSx)1.14NbS2 (x = 0.95, 1.00, 1.05) sintered compacts.

AUTHOR INFORMATION Corresponding Author *M. Ohta: E-mail: [email protected]

Notes The authors declare no competing financial interest.

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 M.O. performed the XRD experiments and measured the electrical and thermal transport properties. O.L. and D.B. performed the XRD and TEM experiments. All authors discussed the results and contributed to writing of the manuscript.

ACKNOWLEDGEMENTS The authors express our thanks to Mr. Hirotaka Nishiate of AIST for measuring the electrical and thermal transport properties. The work was supported by Bilateral Joint Research Projects between the



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Japan Society for the Promotion of Science and French Ministry of Foreign Affairs and International Development (JSPS-MAEDI SAKURA Program). At AIST, the work was also supported by the JSPS KAKENHI Grant Number 25420699. P.J. as an International Research Fellow of the Japan Society for the Promotion of Science acknowledges financial support from JSPS Grant Number 15F15068.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure captions

Figure 1 Powder X-ray diffraction patterns for (LaxSx)1.14NbS2 (x = 0.90, 0.95, 1.00, 1.05, 1.1) along with the reference pattern (ICDD #01-088-1585, Ref. 36)

Figure 2 Bright field HRTEM image of the stoichiometric (LaS)1.14NbS2 structure. Structural model superimposed on enlarged HRTEM image of area A (with defects) and area B (defect free) are shown in bottom left and right images, respectively. La, Nb, and S are represented as blue, yellow, and dark yellow, respectively. White arrows indicate defect area. Scale bar is 2 nm.

Figure. 3 (a) Low magnification bright field TEM image of (La0.95S0.95)1.14NbS2 viewed along close to caxis. (b) Low magnification bright field TEM image of a single lamella showing remarkable changes of contrast within a crystallite. Inset of (b) shows ED patterns along main zone axis (close to ab plane and along c axis). High resolution TEM images of (c) curled region (area A) and (d) edge (area B) of the crystal marked by rectangles in (b) are also shown. White arrows in (c) depict dark contrast bending defect planes resulting from strains.

Figure. 4 (a) Low magnification bright field transmission electron microscopy (TEM) image of (La1.05S1.05)1.14NbS2 crystallite viewed along close to ab plane and corresponding electron diffraction (ED) pattern. Schematic of curled lamellas is given as inset, (b) High resolution TEM image of curled part of the crystal viewed along c-axis and corresponding FT patterns taken from different areas of the image (A- out of orientation and B - close to [110] zone axis), (c) bright field [110] High resolution TEM image and corresponding ED pattern showing a very well ordered structure.


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Figure. 5 SEM images of the sulfurized powder of (LaxSx)1.14NbS2 with (a) x = 0.95 (b) x = 1.00 and (c) x = 1.05, and fractured areas of (LaxSx)1.14NbS2 sintered compacts with (d) x = 0.95 (e) x = 1.00 and (f) x = 1.05, in the direction perpendicular to the pressing direction.

Figure. 6 Temperature dependences of the (a) electrical resistivity (ρ), (b) Seebeck coefficient (S), and (c) power factor (S2/ρ) measured perpendicular (in-plane) and parallel (out-of-plane) to the pressing direction for the (LaxSx)1.14NbS2 (x = 0.95, 1.00, 1.05) sintered compacts.

Figure. 7 (a) Phonon mean free path (lp) for (LaxSx)1.14NbS2 (x = 0.95, 1.00, 1.05) sintered compacts. Temperature dependences of (b) total thermal conductivity (κtotal), and (c) lattice thermal conductivity (κlat) measured perpendicular (in-plane) and parallel (out-of-plane) to the pressing direction.

Figure. 8 Temperature dependence of the thermoelectric figure of merit (ZT) measured perpendicular (inplane) and parallel (out-of-plane) to the pressing direction for the (LaxSx)1.14NbS2 (x = 0.95, 1.00, 1.05) sintered compacts.



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Table 1 Lattice parameters of (LaxSx)1.14NbS2 (x = 0.95, 1.00, 1.05) determined using orthorhombic space group Ccca (68). These lattice parameter values are rough estimates and only shown for comparison purposes. For more accurate evaluation separate space groups for LaS and NbS2 subsystem should be considered during lattice refinement. x in (LaxSx)1.14NbS2

Lattice parameters (nm)

d-spacing (nm)

a

b

c

0.95

2.311(6)

0.576(2)

2.290(2)

0.3836

1.00

2.311(4)

0.576(3)

2.286(8)

0.3836

1.05

2.320(5)

0.580(4)

2.300(6)

0.3845


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Table 2 Chemically analyzed sulfur, oxygen, and carbon contents in the (LaxSx)1.14NbS2 (x = 0.95, 1.00, 1.05) sintered compacts. x in

Chemical composition (wt%)

(LaxSx)1.14NbS2

S

O

C

0.95

29.5

1.29

0.04

1.00

31.1

0.75

0.13

1.05

28.4

0.99

0.04



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Table 3 Room-temperature carrier concentration (n), hole mobility (µ), and effective mass (m*) measured perpendicular (in-plane) and parallel (out-of-plane) for the (LaxSx)1.14NbS2 (x = 0.95, 1.00, 1.05) sintered compacts.

x in (LaxSx)1.14NbS2

Direction

n (1021 cm−3)

in-plane 0.95

µ (cm2 V−1 s−1)

m*/m0

7.3

1.9

3.8

0.70

7.9

1.8

4.7

1.2

9.3

2.2

2.6

0.98

1.3 out-of-plane in-plane

1.00

0.98 out-of-plane in-plane

1.05

1.3 out-of-plane


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Table 4 Room-temperature longitudinal sound velocity (vl), transverse sound velocity (vt), average sound velocity (va), Gruneisen parameter (γ), and Young’s modulus (E) for the in-plane and out-of-plane directions of (LaxSx)1.14NbS2 (x = 0.95, 1.00, 1.05) sintered compacts. x in (LaxSx)1.14NbS2

Direction

vl (m s−1)

vt (m s−1)

va (m s−1)

γ

E (GPa)

in-plane

4830

2420

2720

2.0

79

out-of-plane

4890

2240

2530

2.2

71

in-plane

3950

2060

2300

1.9

54

out-of-plane

4550

1920

2170

2.2

52

in-plane

4780

2360

2660

2.1

75

out-of-plane

4670

1770

2000

2.7

45

0.95

1.00

1.05



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Graphical abstract 213x184mm (144 x 144 DPI)


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Figure 1 889x742mm (150 x 150 DPI)



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Figure 2 148x261mm (150 x 150 DPI)


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Figure 3 189x269mm (150 x 150 DPI)



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Figure 4 161x370mm (150 x 150 DPI)


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Figure 5 104x53mm (300 x 300 DPI)



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Figure 6 2001x1612mm (72 x 72 DPI)


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Figure 7 1834x1517mm (72 x 72 DPI)



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Figure 8 813x843mm (150 x 150 DPI)


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Nanostructural and Microstructural Ordering and Thermoelectric

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