Structural, electronic and thermal properties of TexCo4Sb11.75Te0.25

Structural, electronic and thermal properties of TexCo4Sb11.75Te0.25

Journal of Alloys and Compounds 809 (2019) 151477 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http:/...

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Journal of Alloys and Compounds 809 (2019) 151477

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Structural, electronic and thermal properties of TexCo4Sb11.75Te0.25 R. Chetty a, J. Tobola b, P. Klimczyk c, L. Jaworska c, K.T. Wojciechowski a, c, * a

Faculty of Materials Science and Ceramics, AGH University of Science and Technology, Al. Mickiewicza 30, 30-059, Krakow, Poland Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, Al. Mickiewicza 30, 30-059, Krakow, Poland c The Institute of Advanced Manufacturing Technology, Wroclawska 37a St., 30-011, Krakow, Poland b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 November 2016 Received in revised form 12 May 2019 Accepted 16 July 2019 Available online 27 July 2019

The aim of this work is the theoretical and experimental study of the influence of Te, introduced in two Wyckoff positions (2a and 24g) of the CoSb3 skutterudite structure, on the electronic and thermoelectric properties of the material. The samples with the nominal composition of TexCo4Sb11.75Te0.25, where x ¼ 0, 0.125, 0.25, 0.5 have been prepared using FAST and HPHT methods. Powder XRD analysis confirmed that Te can be introduced into both considered structural positions at high pressure of 7.8 GPa and can partially substitute Sb, as well as occupy the void at 2a site even if low pressure of 30 MPa is applied. Electrical and thermal conductivities vary with the increase of Te content up to x ¼ 0.25 and are almost invariant with further doping with Te. Changes in transport properties well correspond with alternations of the lattice parameter a. The electronic transport properties are coherently explained through electronic structure calculations results. Two opposing effects of Te doping were found, depending on the doping sites, since tellurium behaves either as a one-electron donor (24g site) or two-electron acceptor (2a site). The filling of structural voids with Te results in a decrease of the lattice thermal conductivity and the improvement of the power factor (a2s). The combined effects lead to enhancement of thermoelectric figure of merit to zT ¼ 1.02 at 673 K for Te0.25Co4Sb11.75Te0.25. © 2019 Elsevier B.V. All rights reserved.

Keywords: Thermoelectric materials Electronic properties Thermoelectric X-ray diffraction (XRD) High-pressure

1. Introduction Thermoelectric (TE) materials can be used for the construction of thermoelectric generators which convert directly heat into electrical energy and thermoelectric coolers which can be used for transformation of electricity into the flux of heat (heat pumps). The efficiency of these devices depends on the performance parameter of thermoelectric materials called the dimensionless thermoelectric figure of merit (zT ¼ a2sT/lT), where a- Seebeck coefficient, selectrical conductivity, T - absolute temperature, and lT - total thermal conductivity. One of the most promising groups of new thermoelectric materials are the compounds with the skutterudite structure. The general formula of skutterudites can be written as AxM4X12 (A can be a lanthanide, actinide or some s and p block element; M ¼ Co, Rh, Ir, Ni, Fe; X is a pnictogen atom such as P, As, Sb). The subgroup of compounds which do not contain the A element has a structural void at the 2a Wyckoff position and is called unfilled skutterudites. Consequently, compounds containing

* Corresponding author. Faculty of Materials Science and Ceramics, AGH University of Science and Technology, Al. Mickiewicza 30, 30-059, Krakow, Poland. E-mail address: [email protected] (K.T. Wojciechowski). https://doi.org/10.1016/j.jallcom.2019.07.189 0925-8388/© 2019 Elsevier B.V. All rights reserved.

the A atoms are called filled skutterudites (A atoms are named fillers). Among all, Sb-based skutterudites are most frequently studied because of their advantageous electrical properties such as relatively high Seebeck coefficient and high electrical conductivity. However, the lattice component of thermal conductivity in skutterudites is too high, which results in an unsatisfactory value of zT. In recent years, many research studies on skutterudites are undertaken to enhance their thermoelectric properties by using several approaches [1]. In the case of the “filling” approach, thermal conductivity can be significantly reduced as a result of the strong vibrations of atoms caged inside the voids, which scatter the heatcarrying phonons (so-called rattling effect) [2]. This concept was tested for single elements [3e14], two types of atoms (double-filled skutterudites) [15e18] and even multiple dopants [19e21] particularly for the reduction in thermal conductivity and as a result improvement of zT of materials. Substitutional doping can be a different method of improving both electronic properties, as well as reducing thermal conductivity due to point defect scattering. The Co and Sb atoms can be partially substituted by donors (elements with extra valence electron than host) and/or acceptors (elements with less valence electron) [22e33]. The combined use of both of these approaches is the subject of many papers [34e36].

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Several reports describe attempts of doping of CoSb3 by Te using different synthesis procedures [23e25,37e39]. The substitution of Te on the Sb site provides one extra electron. It leads to an increase in both charge-carrier concentration and their effective mass. Additionally, the presence of Te in Sb site causes reduction of lattice thermal conductivity due to electron-phonon scattering [40]. Most of the research on skutterudites is focused on the exploration of the influence of electropositive elements as filler atoms. Only very few recent studies concern on electronegative atoms as fillers [41e45]. One of them describes the thermoelectric properties of skutterudites filled with chalcogenides: S and Se. The resulting materials showed very low thermal conductivities due to the significant influence of electronegative guest atoms on the lattice dynamics, in addition to the point defect scattering caused by substitutional impurities. Ortiz et al. [44] reported a p-type skutterudite material (Br-filled CoSb3), in which Br filler was responsible for a significant reduction of thermal conductivity without disturbing the intrinsic hole mobility. Most recently, several other electronegative filled CoSb3-based skutterudite compounds have been investigated and studied their influence of thermoelectric properties [46e50]. It seems that the concept of filling structural void with electronegative elements can be a new path for significant improvement of the thermoelectric figure of merit of skutterudite materials. This work is focused on the study of the influence of Te, acting both as an electronegative filler and donor impurity, on structural, electronic and thermoelectric properties of CoSb3. In our previous research [24,25], we have shown that introducing small amounts of Te into the CoSb3 structure leads to substitution of Sb by Te only and the composition of Co4Sb11.75Te0.25 corresponds to the solubility limit for this type of doping. Treating Co4Sb11.75Te0.25 material as a starting point, we have prepared the series of samples with an overabundance amount of Te, assuming that the excessive Te can be introduced into the void position (2a) under elevated pressure and temperature. This work demonstrates the dual role of Te atom in the skutterudite structure with the use of powder XRD, electronic structure calculations, and thermoelectric properties measurements.

of l ¼ 1.54056 Å, 2q ranged from 10 to 100 ). Microstructural analysis of samples was performed using scanning electron microscopy (NOVA NANO 200 SEM, FEI Europe Company). The electrical properties (Seebeck coefficient and electrical conductivity) of FAST sintered samples with a diameter of 10 mm and a height of 15 mm were investigated over a temperature range between 300 K and 673 K. Thermal conductivity was determined by the laser flash technique (NETZSCH LFA 457 MicroFlash®) in the same temperature range. For this measurement, the samples with a diameter of 10 mm and a height of 2 mm were used. 2.1. Electronic structure calculations The electronic band structure of the series TexCo4Sb11.75Te0.25 (x ¼ 0, 0.05, 0.1, 0.15 and 0.2) was computed by the charge selfconsistent Green function Korringa-Kohn-Rostoker method with the coherent potential approximation (KKR-CPA) [51,52] to incorporate explicitly the chemical disorder that accompanies the substitution of Te for Sb (TeSb) as well as the filling the structural void, also with Te (Tevoid). The exchange-correlation potential was treated by the Perdew-Wang [53] formula in the local density approximation (LDA) [54]. Within this formalism, the full charge self-consistent crystal potentials were constructed in the muffin-tin form. The position of the Fermi level was determined accurately by the generalized Lloyd formula. For all compositions, the experimental lattice parameters and atomic coordinates, determined by Rietveld refinements of the XRD patterns, were used. For converged crystal potentials (below 1 meV) and atomic charges (below 103e), total-, site- and orbital l-decomposed densities of states (DOS) were determined using a tetrahedron method for integration in the reciprocal k-space. It is worth noting that within the CPA approach, where electron scattering processes are described as average over all possible atomic configurations, the calculations become quite time-consuming when chemical disorder, treated as random, appears on crystallographic sites with high multiplicity, which is the case in the skutterudite type alloys, namely 24g (Sb sites) and 2a (the void site). 3. Results and discussion

2. Experimental and theoretical calculations details 3.1. Structural analysis and characterization of microstructure The samples with the nominal composition of TexCo4Sb11.75Te0.25 (x ¼ 0, 0.125, 0.25, 0.5) were prepared by solid-state reaction synthesis from elements. High-purity substrates in the form of powders and granules: Co (99.99%), Sb (99.999%), and Te (99.999%) (Alfa Aesar) were used. All the samples were weighed and put into quartz ampoules, which were sealed under vacuum. The ampoules were placed in the furnace and slowly heated to the temperature of 800  C (just below the peritectic decomposition of CoSb3). Then the samples were annealed for one week in the furnace, and next cooled down to the room temperature. The resulted ingots were ground into fine powders using mortar and pestle. Next, the powders were sintered under vacuum in graphite dies with a bore diameter of 10 mm by Field Assisted Sintering Technique (FAST). FAST hot-pressing was carried out at 735  C for 15 min with a pressure of 30 MPa. The relative density of all the samples determined using the Archimedes’ method, was about 99%. The Te0.5Co4Sb11.75Te0.25 sample was pressed at room temperature into a pellet with a diameter of 15 mm and the height of 5 mm. Then the pellet was sintered in the high-pressure high-temperature (HPHT) press at p ¼ 7.8 GPa, and T ¼ 500  C for 10 min. The sample has broken into pieces after the HPHT treatment. The structural properties of the materials were characterized by X-ray diffraction (XRD) (Philips X'Pert X-ray diffractometer, Cu Ka1

Fig. 1 displays the XRD pattern for all the samples. It confirms that the main phase has a skutterudite structure. A trace of impurity

Fig. 1. Powder XRD pattern for TexCo4Sb11.75Te0.25 (x ¼ 0.0, 0.125, 0.25, 0.5).

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phase related to CoSbTe and Sb can be observed for the sample Te0.5Co4Sb11.75Te0.25 only. Rietveld refinement of diffraction patterns (FullProf program [55,56]) was carried out for the estimation of the lattice parameter a, and determination of the Te occupancy at the void site (2a position). The initial parameters were assumed from our previous results [24,25] of the Te-doped CoSb3 by fixing the Te on the Sb site as 0.25, and the occupancy factor of Te at the void site was refined. A good profile fitting of the diffraction patterns was achieved by simultanous refining the parameters such as zero shift, lattice parameters, atomic coordinates of Sb/Te and the occupancy of Te at the void position. Fig. 2 shows the demonstrative results of refinement for the Te0.125Co4Sb11.75Te0.25 sample. The analysis of Rietveld refinement results confirm the presence of Te at the void site (2a position); however, the determined occupancy values of Te at this position are lower than the nominal content. The occupancy of voids by Te rises with the increase of nominal Tevoid content (from the value of ~0.05 for the Te0.125Co4Sb11.75Te0.25 sample to ~0.13 for the Te0.25Co4Sb11.75Te0.25 sample). For the Te0.5Co4Sb11.75Te0.25 sample the presence of small amount of impurity phase is observed. It could be the result of exceeding the solubility limit of Te in CoSb3. To confirm the conclusion that Te can fill the void, the Te0.5Co4Sb11.75Te0.25 sample sintered under high pressure of 7.8 GPa has been prepared. Fig. 3 shows the comparison plot of low-angle XRD patterns for FAST and HPHT treated samples. One can observe the significant extinguishing of low-angle Bragg reflections such as (110), (210), (211) and (220), which is a signature for filling of the void at 2a site by the foreign atom [24,57,58]. This effect clearly indicates that under high pressure Te enters the void site. The assessed occupancy of the void by Te in HPHT sample is about 0.7. The lattice parameter a determined from the Rietveld refinement is plotted as a function of nominal Te content showed in Fig. 4a. One can observe that a value rises with the increase of Te content up to x ¼ 0.25 in TexCo4Sb11.75Te0.25, and further sustains its value for the sample x ¼ 0.5. The increase of a value may be the result of filling the structural void by Te. The interpolated lines for the linear rise and saturation regions of a value with Te content (shown in Fig. 4a) suggest that the solubility limit is reached for the Te0.25Co4Sb11.75Te0.25 nominal composition. Furthermore, lattice parameter a ¼ 9.116 Å for the HPHT treated sample (Fig. 4a-square symbol), is much higher than the CoSb3

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Fig. 3. Comparison of low-angle Bragg reflections for the FAST and HPHT sample.

Fig. 4. The lattice parameter (a), electrical conductivity (s) and lattice thermal conductivity (lL) at 323 K as a function of a nominal Te content.

Fig. 2. Rietveld refinement results for the Te0.125Co4Sb11.75Te0.25.

lattice parameter (a ¼ 9.034 Å [24]) and also than that one of Te0.25Co4Sb11.75Te0.25 sample (a ¼ 9.05 Å). A significant rise of lattice parameter in addition to the extinguishing of lower angle Bragg reflections confirms the Te enters the void position. Backscattered electron micrographs from the SEM are shown in Fig. 5. All the samples exhibit uniform microstructure, and high density after the FAST treatment. The SEM pictures well agree with the obtained relative densities (~99%). There is a possibility that excessive tellurium could form secondary phases or exist at the interface between the grains as atomic segregations, like in the Cefilled CoSb3 [59]. However, our microstructural investigations did not reveal presence of secondary phases or segregation of Te at the interface between grains. The average grain size of the samples is

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Fig. 5. SEM images for TexCo4Sb11.75Te0.25 (a) x ¼ 0.0, (b) x ¼ 0.125, (c) x ¼ 0.25, (d) x ¼ 0.5.

between 5 and 40 mm and increases with the increase of Te content. The increase in grain growth may affect the physical properties of the material. Noteworthy, the electron mobility of the TE materials increases with larger grains due to the lowering of the grain boundary scattering of charge carriers, which leads to the increase of electrical conductivity. On the other hand, such microstructural effect may also result in a rise in lattice thermal conductivity due to reduced boundary scattering of heat-carrying phonons. 3.2. Electronic and thermal transport properties 3.2.1. Electronic structure The KKR-CPA method allowed calculating the electronic density of states DOS in TexCo4Sb11.75Te0.25 alloys, accounting for chemical disorder on two crystallographic sites. In order to investigate the influence of the void filling with different Te content on overall DOS shape and mostly on the Fermi level (EF) position, the concentration of Te on Sb (24g) sites was fixed, according to experimentally investigated samples, whereas the occupancy of the structural void (2a) by Te varied from x ¼ 0 to x ¼ 0.20.

Fig. 6. Total and partial densities of states of the TexCo4Sb11.75Te0.25 with x~0.

Fig. 6 presents DOS of TexCo4Sb11.75Te0.25 with x~0, where Te placed in the structural void can be regarded as an acceptor impurity. We observe that the DOS shape of Co4Sb11.75Te0.25 reminds that one of binary CoSb3 semiconductor, presumably due to quite close site-decomposed DOS of Sb and Te (Fig. 6). The main difference between both cases comes from the position of the Fermi level, which is shifted from the gap (CoSb3) to the conduction states (Co4Sb11.75Te0.25) due to one electron more of Te with respect to substituted Sb. Hence, such a system is expected to behave as a strongly doped n-type semiconductor. However, Te impurity located in the void forms a huge p-DOS peak at the valence band edge due to its strongly repulsive crystal potential. Looking in more details, Te in the void behaves like a double electron acceptor. Bearing in mind this simple picture that one extra electron delivered with one Te atom replacing Sb can be compensated by two extra holes delivered by Te atom filling the void, one would then expect that the Fermi level will be located in the middle of the bandgap in TexCo4Sb11.75Te0.25 for x~0.125. But, as above-mentioned and looking at DOS features (Fig. 6), it appears that at finite Te contents, the situation is more complex. With increasing x, the Te-DOS peak tends to broaden, and its higher energy tail falls into the former energy gap. For x ¼ 0.05 and x ¼ 0.10, as shown in Fig. 7 (a) & (b), the energy gap narrows systematically, but the Fermi level is still located near the conduction band edge. For x ¼ 0.15 (Fig. 7 (c)), EF already crosses the former energy gap, since the concentration is higher than the ratio x ¼ 0.125 allowing to fully compensate electrons by holes. It is worth noting that even at x ¼ 0.15 and x ¼ 0.20 (Fig. 7d), Te in the void still constitutes high DOS peaks at the valence band edge, which presumably have non-bonding (or even anti-bonding) character, since Te-states are poorly hybridized with Co and Sb/Te states in this energy range. Such electronic structure features detected in TexCo4Sb11.75Te0.25 seems to be energetically unfavorable and remains in line with the solubility limit of observed experimentally. It is interesting to note that when the amount of tellurium at the void site Tevoid exceeds half of the amount of tellurium TeSb in the antimony position, the Fermi level will be located within the valence band (e.g. the DOS for the x ¼ 0.15 and 0.20 (Fig. 7c and d)). Such material should exhibit p-type behavior. However, the experimental results for the samples with the nominal content of x ¼ 0.25 and 0.50 still showed n-type conductivity. This observation would indicate that Te content at the void site is lower than the nominal composition. As it was discussed previously, the estimated Te content is about xexp ¼ 0.05e0.13 in for the samples with the nominal composition of x ¼ 0.125e0.25. These results suggest that synthesis of p-type skutterudites for ratio Tevoid/TeSb > 0.5 can be difficult likely due to unfavorable electronic structure features. 3.2.2. Electrical conductivity The temperature-dependent electrical conductivity for all the samples is shown in Fig. 8. The decrease in electrical conductivity with the rise of temperature indicates a strongly degenerated semiconductor (or metallic-like) behavior for all the samples. Electrical conductivity changes from the value of ~8  104 Sm1 at 323 K for the Co4Sb11.75Te0.25 sample to ~2  105 Sm1 at 323 K for the Te0.5Co4Sb11.75Te0.25 sample. Electrical conductivity at 323 K increases with an increase of Te amount to the nominal composition of Te0.25Co4Sb11.75Te0.25 and is almost invariant with a further rise of Te content (shown in Fig. 4b). This result is in good correlation with the analogous behavior of lattice parameter a (Fig. 4a). The KKR-CPA calculations well support the increase of electrical conductivity with increased occupancy of the void by Te. Fig. 7 shows that the bandgap is narrowing systematically with the

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Fig. 7. Total and partial densities of states of the TexCo4Sb11.75Te0.25 with x ¼ 0.05 (a), 0.10 (b), 0.15 (c) and 0.20 (d).

thermopower in the investigated temperature range, indicating that the majority carriers are electrons. Seebeck coefficient decreases with an increase of Tevoid content, which reflects the opposite behavior of electrical conductivity. It can be the direct effect of the increase in DOS near the Fermi level due to additional Tevoid states, and/or due to bipolar carriers generated from the new states which are responsible for narrowing the bandgap. The maximum absolute value of the Seebeck coefficient of 222 mVK1 at 673 K is measured for the Co4Sb11.75Te0.25 sample. The thermopower is systematically decreasing from the value of 160

Fig. 8. Temperature dependent electrical conductivity for the TexCo4Sb11.75Te0.25 (x ¼ 0.0, 0.125, 0.25, 0.5).

increase in the amount of Tevoid due to additional states created by this impurity. For the x ¼ 0.10 the band gap is closed. The Fermi level still lies within the range in which DOS of conduction bands and also on increasing DOS slope is high. This is consistent with the observed rise in electrical conductivity and n-type metallic-like behavior. 3.2.3. Seebeck coefficient The temperature-dependent Seebeck coefficient for all the samples is shown in Fig. 9. All the compositions show a negative

Fig. 9. Temperature dependent Seebeck coefficient for the TexCo4Sb11.75Te0.25 (x ¼ 0.0, 0.125, 0.25, 0.5).

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mVK1 at 323 K for the Co4Sb11.75Te0.25 to the 122 mVK1 at 323 K for Te0.5Co4Sb11.75Te0.25. To check the influence of tellurium at the void position on thermoelectric properties, we have prepared under high pressure of 7.8 GPa an individual sample with the nominal composition of Te0.5Co4Sb11.75Te0.25. Fig. 10 displays the temperature dependencies of the Seebeck coefficient for HPHT treated Te0.5Co4Sb11.75Te0.25 sample for two heating and cooling cycles. During the first heating cycle, as the temperature rises, the Seebeck coefficient decreases probably due to the thermal excitation of carriers (growth of bipolar conduction) up to the temperature of 500 K. The absolute Seebeck coefficient further rapidly increases above the 500 K. This effect could be a result of the partial disinsertion of the Te at the void site. Next, the thermopower slowly decreases with in the first cooling cycle. During the second heating cycle of the measurement, the Seebeck coefficient followed a similar path of the first cooling cycle, but a sudden increase of Seebeck coefficient at 473 K is again observed. During the second cooling cycle, the Seebeck coefficient decreases with the decrease of temperature, but the absolute values are higher than during the first cooling cycle. We have repeated the XRD analysis of the samples just after thermoelectric measurements. The XRD pattern clearly shows the rise of the lower angle Bragg-reflections, which again confirms partial disinsertion of Te during the measurements. As above mentioned from the simple analysis of the KKR-CPA electronic structure calculations of the sample with Tevoid exceeding half of TeSb content one can expect that this material should rather behave as p-type conductor because the Fermi level is located in the valence bands. On the other hand, the former energy gap tends to be closed with increased Tevoid, making the system metallic like. Indeed, results of theoretical calculations of Bertini et al. [60], based on semi-classical Boltzmann transport theory predicted that, for the fully Te-filled skutterudtite (TeCo4Sb12) Seebeck coefficient values are positive for low T < 300 K (about few mV) only and slightly negative between 350 and 850 K. For higher temperatures, thermopower is close to 0. The experimental Seebeck coefficient (Fig. 10) results for the first heating cycle reflects a similar trend with the previously reported results [60], but the thermopower measured is higher. This effect can be due to differences in the chemical composition of the samples studied in the present work (Te0.5Co4Sb11.75Te0.25) and reported (TeCo4Sb12).

Fig. 10. Temperature dependent Te0.5Co4Sb11.25Te0.25 sample.

Seebeck

coefficient

for

the

HPHT

treated

The power factor a2s for all the samples is calculated from the temperature-dependent a and s data. Power factor is increasing with the rise of both Tevoid content and temperature. The maximum value of power factor a2s ¼ 5.5 mWm1K2 at 673 K is obtained for the Te0.25Co4Sb11.75Te0.25 composition. Further increase of Te content leads to the decrease of power factor due to the significant decrease of Seebeck coefficient. 3.2.4. Thermal conductivity The temperature-dependent total thermal conductivity (lT) is shown in Fig. 11. Total thermal conductivity for all the samples decreases with the rise of temperature. The lT values are obtained below 6 Wm1K1 in the entire temperature for all the samples. One can observe that the thermal conductivities for different samples are almost the same in the range of the measurement error. It is expected that the total thermal conductivity should increase because of the increase of charge carrier contribution in heat transport. The total thermal conductivity is a combination of both electronic and lattice thermal conductivities:

lT ¼ lel þ lL

(1)

The electronic part can be calculated using the WeidemannFranz relation (lel ¼ LsT), where L is the Lorenz number (2.44  108 WUK2). The lattice component of thermal conductivity can be determined from Eq (1) and is shown in Fig. 12. We have found that the lattice thermal conductivity lL is dominant over the electronic thermal conductivity lel. It also systematically decreases with the amount of Tevoid. We consider two possible explanations of this behavior: the influence of changes in the microstructure and filling the void at 2a position. However, it was observed that the grain size increases with the increase of nominal Tevoid content (see Fig. 5). Such an effect should lead rather to the enhancement of heat transport due to the lowering of boundary scattering of phonons. Therefore, we conclude that the decrease of lL is presumably caused by filling the structural void by Te impurity. One can observe that the lL decreases with the rise of Tevoid content up to x ¼ 0.25 and almost unchanged for x ¼ 0.5 (as shown in Fig. 4c). This result is in good correlation with changes in the lattice parameter and the electrical conductivity as a function of nominal Tevoid content. All these results together suggest that the

Fig. 11. The temperature dependent total thermal conductivity for the TexCo4Sb11.75Te0.25 (x ¼ 0.0, 0.125, 0.25, 0.5).

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figure of merit is shown in Fig. 13. The zT is increasing with the rise of temperature for all the samples. The zT value increases with the amount of Tevoid content up to x ¼ 0.25 and further decreases for the sample x ¼ 0.5. The maximum zT ¼ 1.02 is achieved at 673 K for the Te0.25Co4Sb11.75Te0.25. The improvement in thermoelectric properties is a consequence of increased electrical conductivity and maintaining relatively high Seebeck coefficient, as well as, low thermal conductivity caused by the partial filling the void by Te. 4. Conclusions

Fig. 12. The temperature-dependent lattice thermal conductivity for the TexCo4Sb11.75Te0.25 (x ¼ 0.0, 0.125, 0.25, 0.5).

maximum solubility of Te is reached for the Te0.25Co4Sb11.75Te0.25 sample. The behavior of lattice thermal conductivity by Te as the filler atom can be compared with the results obtained for Se and S filled CoSb3 samples [45,46]. At room temperature the lL value of ~3.8 W/ m-K for the sample Te0.25Co4Sb11.75Te0.25 is comparable to the value of ~3.1 W/m-K for the compound Se0.25Co4Sb11.75Se0.25 [46]. It can be noticed that the lL values of the Te filled samples are slightly higher than the Se and S filled samples, which may be due to the variation in the chemical composition and/or the differences in the chemical interactions between TeeSb, SeeSb, and SeSb atoms. This can be the possible reason for the systematic decrease of lL (T) for Te, Se, and S filled skutterudite samples respectively. Recently, it was reported that a significant reduction of lL for the S and Se filled samples, which is due to the cluster vibration effect in addition to the point defect scattering, electron-phonon scattering [45]. The same justification may also apply to the the present study. The temperature-dependent dimensionless thermoelectric

Structural property study of TexCo4Sb11.75Te0.25 (x ¼ 0.0, 0.125, 0.25, 0.5) samples confirmed that Te can partially enter the void position (2a site). Further, it is supported by the XRD diffraction pattern of HPHT treated Te0.5Co4Sb11.75Te0.25 sample. The electrical conductivity of TexCo4Sb11.75Te0.25 is increased with the rise of Tevoid content and showed a metallic-like behavior independently of x amount. Experimental findings are consistent with the results of KKR-CPA electronic structure calculations, which show that the Fermi level is located within the conduction band for the TexCo4Sb11.75Te0.25 (x ¼ 0.0, 0.05, 0.10) but with the tendency to close the energy gap with x increasing. From both the experimental and electronic structure calculations, we conclude that the amount of tellurium at the void position is lower than the nominal one, which is partially evidenced by the Rietveld refinement results. The absolute values of the Seebeck coefficient decrease with the Tevoid content and showed a negative sign, which indicates that the majority charge carriers are electrons. Further, a sample with nominal composition Te0.5Co4Sb11.75Te0.25 was prepared by HPHT and the influence of tellurium at the void position on thermopower was investigated. Temperature-dependent behavior of Seebeck coefficient shows that the position of Te at the void appears to be thermally unstable. Lattice thermal conductivity of TexCo4Sb11.75Te0.25 samples showed a systematic decrease with an increase of Tevoid content. Interestingly some correlation among lattice parameter, electrical conductivity, and lattice thermal conductivity dependencies with Tevoid content would suggest that occupancy of Te in the void site is limited to about 0.13. A substantial increase of power factor and low thermal conductivity lead to an increase of zT ¼ 1.02 at 673 K for the Te0.25Co4Sb11.75Te0.25 sample. On the whole, our experimental and theoretical results indicate that the achieving of p-type skutterudites for ratio Tevoid/TeSb > 0.5 can be difficult. Therefore, the present study of introducing tellurium in two Wyckoff positions (2a and 24g) of the CoSb3 skutterudite structure convinced us about a possible new path for the investigation and optimization of skutterudite TE materials. Acknowledgments Authors thank the Foundation for Polish Science (TEAM-TECH/ 2016-2/14 Grant, “New approach for the development of efficient materials for direct conversion of heat into electricity”), cofinanced by the European Union under the European Regional Development Fund for financial support to The Lukasiewicz Research Network e The Institute of Advanced Manufacturing Technology. References

Fig. 13. Temperature dependent thermoelectric figure of merit for the TexCo4Sb11.75Te0.25 (x ¼ 0.0, 0.125, 0.25, 0.5).

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