Ultralow Lattice Thermal Conductivity in Ba23M10Ge10Sb25-Î´ (M

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Ultralow Lattice Thermal Conductivity in Ba23M10Ge10Sb25-# (M=Ga, In): Quarternary Compounds Containing Ba-centered Dodecahedra Ming-Yan Pan, Hongji Qi, Xiao-Cun Liu, Ming-Cheng Bai, and Sheng-Qing Xia Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01441 • Publication Date (Web): 22 Jun 2018 Downloaded from http://pubs.acs.org on July 7, 2018

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Chemistry of Materials

Ultralow Lattice Thermal Conductivity in Ba23M10Ge10Sb25-δ (M=Ga, In): Quarternary Compounds Containing Ba-centered Dodecahedra Ming-Yan Pan,†‡ Hongji Qi,*† Xiao-Cun Liu,§ Ming-Cheng Bai†‖ and Sheng-Qing Xia*‡ †Key Laboratory of Materials for High Power Laser, Chinese Academy of Science, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science, Shanghai, 201800, China ‡State Key Laboratory of Crystal Materials, Institute of Crystal Materials, Shandong University, Jinan, Shandong 250100, China §School of Materials Science and Engineering, Changzhou University, Changzhou, Jiangsu 213164, China. ‖School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China ABSTRACT: Two new quarternary compounds, Ba23Ga10Ge10Sb25 and Ba23In10Ge10Sb24.79, have been synthesized by using the high temperature solid-state reactions. These new phases exhibit huge unit cells (V = 17514.6 and 18455.8 Å3 for Ga- and In-containing compounds, respectively),which comprise 544 atoms and feature “onion-like” M20Ge20Sb30 clusters connected through the Sb-Sb bonds. Among these clusters, substantial isolated units such as Ba, Sb or Sb3 trimers are filled in order to maintain the charge-balanced system. With such a complex structure as well as the large unit cell and substantial heavy consitutent atoms, ultra-low lattice thermal conductivity (κl) was observed for Ba23Ga10Ge10Sb25, ranging from 0.2 to 0.38 W∙m-1∙K-1 from 323 to 823 K. As a result, a maximun zT of 0.27 was achieved at 823 K.

Introduction Since thermoelectric (TE) materials can achieve a direct energy conversion between heat and electricity, they have shown great potential on applications such as waste heat recovering and portable devices cooling. The efficiency of TE materials is evaluated by the figure of merit, ZT = S2σT/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the temperature, and κ is the thermal conductivity. The strategies of designing high ZT materials are various, typically compounds with “glass-like” thermal conductivity have innate advantages if the electrical properties are not destroyed (Slack 1995). For this reason, complex bulk materials composed of clathrates or Zintl phases, which naturally bear very low lattice thermal conductivity, have been especially focused recently.1,2 For clathrate compounds, the guest atoms can “rattle” around the centers in the oversized “cages”. This rattling motion is expected to enhance the anharmonicity and increase the low-energy acoustic phonon-phonon scattering probability, which may decrease the thermal conductivity to an extent typically as low as glass.3,4 For example, the thermal conductivity of the type-I clathrate Ba8Ga16Ge30, is 1.2 W∙m-1∙K-1 at room temperature.5 For antimony-based Zintl phases, low thermal conductivity and decent thermoelectric performance have been demonstrated as well. Take Yb14MnSb11 for example, its structure contains (MnSb4)9- tetrahedrons, (Sb3)7- polyanions, isolated Sb3anions and Yb2+cations, which thus causes significant non-harmonic effect and extremely low κl of 0.4 W∙m-1∙K-1

at room temperature.6 Besides, the inclusion of “rattling” cages and heavy Sb atoms may lead to even lower κl, and the aliovalent atoms may break the conventional tetracoordinated framework rule of clathrates.7-11 Inspired by above progresses, two new quaternary compounds, Ba23Ga10Ge10Sb25 and Ba23In10Ge10Sb24.79(3), were discovered. With high-quality crystals obtained from the metal-flux reactions, which have approximate dimensions of 5 × 5 × 5 mm for Ba23Ga10Ge10Sb25 by using Pb as the flux (Figure S1), the thermoelectric properties of Ba23Ga10Ge10Sb25 were studied. On the basis of measured electrical transport properties, Ba23Ga10Ge10Sb25 is a p-type semiconductor and has a narrow band gap of about 0.18 eV. Owing to its unique crystal structure, this compound exhibits ultralow lattice thermal conductivity between 0.2 and 0.38 W∙m-1∙K-1 over the measured temperature range from 323 to 823 K. Without any optimizaiton, the compound itself already bears a maximum figure of merit value of 0.27 at 823 K. Experimental Procedures Synthesis. All manipulations were performed in an argon-filled glovebox. Starting materials were used as received: Ba (Alfa, 99%), Ga (Alfa, 99.999%), In (Alfa, 99.999%), Ge (Alfa, 99.999%), Pb (Alfa, 99.99%), Sb (Alfa, 99.999%). In order to obtain high-quality single crystals of the title compounds, materials Ba8M16Ge30 (M = Ga, In) were first synthesized by using stoichiometric elements with high-temperature solid-state reactions.12,13 Ba8M16Ge30 is not only a source of germanium, but also

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serves as necessary precursors for the preparation of Ba23M10Ge10Sb25-δ. Direct stoichiometric reactions through the elements only resulted in some simple phases such as Ba2Sb3 and Ge. The typical synthetic procedure can be described as following: Ba8M16Ge30 and other elements with a ratio of Ba8M16Ge30:Ba:M:Sb = 0.1:3:1:5 were first loaded in a graphite crucible, which was sealed in fused silica tubes under vacuum. The container was heated to 1000 °C and homogenized at this temperature for 20 hrs, which was then slowly cooled down to 400 °C at a rate of 5 °C/hr. At this temperature, the furnace was powered off and the products were checked in the glovebox, which proved the high yield of Ba23M10Ge10Sb25-δ crystals. The products are stable to air, but quickly decompose when exposed to water or moisture air. However, if kept in dry air even for a month, the crystal only shows slight oxidation on the surfaces, which is still brittle and shinning after polishing. In order to obtain larger single crystals, Pb-flux reactions were tried, which resulted in bulk crystals of Ba23Ga10Ge10Sb25 with sizes of about 5 × 5 × 5 mm. A loading ratio of Ba:Ga:Ge:Sb:Pb = 3:1:1:5:25 was applied for this Pb-flux reaction and the following procedure was used: the reactants were loaded in an alumina crucible, which was sealed in a fused silica tube under vacuum. The container was moved to a programmable furnace, and heated to 900 °C in a rate of 200 °C/h. After homogenizing at this temperature for 20 h, the reaction was slowly cooled down to 500 °C at a rate of 2 °C/h, and the excessive flux was quickly decanted by centrifuge. Interestingly, only one bulk crystal with regular shape was left in the crucible when the reaction was done. Single crystal structure determination. Single crystals of the title compounds were selected in the glovebox and cut in the Paratone-N oil to suitable sizes, which were then mounted on glass fibers for structure determination. Single crystal data collections were performed on a Bruker SMART APEX-II CCD area detector with graphitemonochromated Mo-Kα radiation (λ =0.71073 Å) at 173 K using ω scans. The frame width was 0.5° and the exposure time is 15 s per frame. Data reduction and integration, together with global unit cell refinements were done by the INTEGRATE program incorporated in APEX2 software.14 Semi-empirical absorption corrections were applied using the SCALE program for the area detector.14 The structure was solved by the direct methods and refined by the full matrix least-squares methods on F2 using SHELX.15 The structures were refined to converge with all atoms treated with anisotropic displacement parameters. During the structure refinements with anisotropic thermal parameters, the atomic displacement parameters (ADP) for Ba8 and Ba9 are slightly larger than other Ba atoms in both Ba23Ga10Ge10Sb25 and Ba23In10Ge10Sb24.79. Difference Fourier map were plotted (Figure S2) and the residual electron density around Ba8 and Ba9 atoms indicated small splits. We have tried the split-model for Ba(8) and Ba(9) with anisotropic refinements. For Ba(8), the split fashion did improved the Ueq (Å2) from 0.02525(17) to 0.0212(8), however, for Ba(9) the split is too small to

result in a reasonable model and after refinements they converged to the same postion again. Since such an improvement is not very significant, these two atoms are still treated with the ordered fashion for the sake of concise structure description. For Ba23Ga10Ge10Sb25, there are three residual peaks left, ca. 15 e/Å3, and all less than 1.1 Å away from atoms Sb7, Sb8, and Sb11, respectively. Thus, for the next refinements cycles, Sb8 and Sb11 were refined with freed occupation factors, which resulted in marginal improvement on their displacement parameters and occupancies around 70% and 50% for Sb8 and Sb11, respectively. Then partially occupied Sb were considered in refining the “Q” atoms, and the occupancies of Q2 plus Sb8 almost equals 100% after considering the site symmetry, so does Q3 and Sb11. Thus, the occupancies of Q2 and Sb8, Q3 and Sb11 are constrained to sum to one, which are supported by corresponding difference Fourier maps. For Sb7 atom, disorder can also be observed, which splits into two sites with the restrains of equal displacement parameters. With this consideration, Q1 was refined as Sb7A. The split of Sb11 in Ba23In10Ge10Sb24.79(3) is a little different. There are two residual peaks around Sb11, one is 13.53 e/Å3 and about 1.118 Å away from Sb11, and the other is 13.23 e/Å3 and 1.234 Å away. Refinements on these two Q atoms plus Sb11 only sum up to about 72% occupancy at this site. With such a model, the structures were finally refined to converge. In the last refinement cycles, the atomic positions for the two compounds were standardized using the program Structure TIDY.16,17 Atomic positions and anisotropic displacement parameters are provided in Table S1. Selected bond lengths are given in Table S2. Further information in the form of CIF have been deposited with Fachinformationszentrum Karlsruhe, 76344 EggensteinLeopoldshafen, Germany, (fax: (49) 7247−808−666; email: [email protected])−depository CSD-number 433666 and 434101 for Ba23Ga10Ge10Sb25 and Ba23In10Ge10Sb24.79, respectively. X-ray diffraction. Powder X-ray diffraction patterns were taken at room temperature by a Bruker AXS X-ray powder diffractometer using Cu-Kα radiation. The data were recorded in a 2θ mode with a step size of 0.02° and the counting time of 10 seconds. In Figure S3, the comparison between experimental and simulated data indicates the purity of the samples. In addition, the result of X-ray diffraction measurement on Ba23Ga10Ge10Sb25 bulk crystal is shown in Figure S4, and the indices of crystal face is marked. From the XRD pattern, it can be speculated that the bulk crystal may not be strictly single, yet it is still dense enough for the property measurements. Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC). The thermal stability was tested on a polycrystalline sample of Ba23Ga10Ge10Sb25 (mass: 75.688 mg) with the MettlerToledo TGA/DSC/1600HT instrument under the protection of high-purity argon gas. Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) experiments were performed as well and the


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Chemistry of Materials measured temperature range was from 300 to 1173 K with a heating rate of 10 K/min applied. The cooling rate down to 300 K was 10 K/min. Elemental Analysis. Energy Dispersive X-ray Spectroscopy was taken on the slected single crystals of the title compounds with the Hitachi FESEM-4800 field emission microscopy equipped with a Horiba EX-450 EDS (Figure S5 and Table S3). The energy dispersive spectra (EDS) were taken on the visibly clean surfaces of the title compounds and the measured compositions were averaged, i.e., Ba23Ga10.46Ge9.815Sb27 and Ba24.9In9.68Ge10Sb24.31. The OPTIMA 7000 Inductively Coupled Plasma- Atomic Emission Spectrometer (ICP-AES) was also used to quantitatively determine the composition of Ba23Ga10Ge10Sb25. Polycrystalline samples were dissolved in dilute nitric acid and hydrochloric acid at 323 K. The analytical spectral lines of Ba, Ga, Ge, Sb are 455.403, 294.364, 265.118 and 206.833 nm, respectively. The results show a mole ratio of Ba:Ga:Ge:Sb=23:11.22:9.45:27.1, comparable to those data from the EDS or single crystallographic measurements (Table S4). Seebeck Coefficient and Electrical Resistivity. A sample for Seebeck coefficient and electrical conductivity measures was cut from the bulk crystal of Ba23Ga10Ge10Sb25, which was polished into a rectangular bar with sizes of 4.40 × 1.84 × 1.96 mm inside the glovebox. The geometry density of the sample was about 93% of the theoretical value. The test was performed over a temperature range from 323 to 823 K by a Linseis LSR-3/1100 instrument under the protection of helium atmosphere. The measurements were carried out by using a modified dynamic method, for which a series of voltages ΔU due to different temperature gradients ΔT were measured. By linearly fitting the curve of ΔU-ΔT, the Seebeck coefficient was obtained by the slope. On the basis of the activation model, lnρ a can be linearly fitted to the inverse temperature with the equation lnρ = Eg/2kBT + f in the hightemperature region. As shown in Figure S6, the linearity of lnρ versus 1/T tends to be good from 323 to 823 K, featuring a band gap of about 0.18 eV. Thermal Conductivity. Thermal conductivity was measured by NETZSCH LFA457 instrument on a square plate sample with sizes of 3.84 × 3.68 × 0.94 mm, which was cut from the same crystal for the Seebeck coefficient measurements. Besides, the surface of the sample was protected by spraying coating graphite, which was then placed in a SiC sink inside the glovebox. The measurements were performed under an argon atmosphere over a temperature range from room temperature to 973K. A pyroceram 9606 (4 × 4 × 1 mm) standard sample was used as the reference for measuring the heat capacity. The thermal conductivity was calculated from κ=D*ρ*Cp (D: thermal diffusivity; ρ: density; Cp: heat capacity). Assuming acoustic phonon scattering as the main scattering mechanism, the Lorenz factor was approximated based on Eq. (1) - (3) using a single parabolic band (SPB) model (Figure S8).

=

−

(1)

=

(2)

%

= &

(3)

!"[]

Where η is chemical potential and λ=0. Carrier Concentration and Mobility. Measurements on the carrier concentration and mobility were performed by using an MMR K2500 Hall effect test system. The samples used for the thermal conductivity measurements were first polished into a thin plate with sizes of 3.84 × 3.68 × 0.4 mm, and then gold electrodes were glued at the four corners by using silver epoxy. All operations were done in glovebox. The results were shown in Table S5. Results and Discussion Structure Description. Ba23Ga10Ge10Sb25 and Ba23In10Ge10Sb24.79(3) are isotypic and both crystallize in the cubic space group Pn-3m (No. 224) with a huge unit cell containing 544 atoms. The crystallographic data and important information on the structure refinements were summarized in Table 1. Table 1. Selected crystal data and structure refinement parameters for Ba23M10Ge10Sb25-δ. Formula

[a]

Ba23Ga10Ge10Sb25

Ba23In10Ge10Sb24.79(3)

Fw / g∙mol

7625.67

8050.80

T/K

173(2)

Radiation, wavelength

Mo-Kα, λ=0.71073 Å

Crystal system

Cubic

Space group

Pn-3m(No. 224)

Unit cell parameters

a = 25.9697 (10) Å

–1

V/Ǻ

3

ρcalc / g∙cm

17514.6 (12), Z=8

18455.8 (5), Z=8

5.784

5.795

1.126

1.119

R

R1=0.0223 wR2=0.0449

R1=0.0253 wR2=0.0453

R [all

R1=0.0294 wR2=0.0469

R1=0.0319 wR2=0.0515

–3

Goof Final a indices [I>2σ(I)] Final a indices data]

a = 26.4268 (4) Å

[a] R1 = ∑||Fo| 2 2 2 2 1/2 Fc ) ]/∑[w(Fo ) ]] , 2 2 (Fo + 2Fc )/3; A and

2

– |Fc||/∑|Fo|; wR2 = [∑[w(Fo – 2 2 2 and w = 1/[σ Fo + (A∙P) + B∙P], P = B are weight coefficients.

As mentioned in the experimental section, Ba8M16Ge30 serve as the important precursors for the preparation of the Ba23M10Ge10Sb25-δ phases. By a careful comparison on these two types of structures, the reasons are probably self-explanatory. As indicated in Figure 1, the anionic structure of Ba13Ga20Ge20Sb30 can be readily derived from Ba8Ga16Ge30.18 By hypothetically scissoring the basic (Ga,Ge)64 units from Figure 1a and connecting these clusters with some extra Sb atoms, the three-dimensional framework of Ba23Ga10Ge10Sb25 can be easily constructed, as indicated in Figure 1d. This first discovered



Ga20Ge20Sb30 cluster (Figure 1c), actually has the same topological structure as (Ga,Ge)60 if ignoring the six extra Sb5 atoms of the outer shell (The connections related to these Sb atoms are emphasized by purple sticks). The inner shell of the “onion-like” Ga20Ge20Sb30 cluster features the pentagonal dodecahedra Ge20 “core”, which are surrounded by 20 Ga atoms as the second “shell”. The outer “shell” is not closed and consists of 30 Sb atoms, which lead to 12 open cages filled by the Ba cations. Note that the difficulty of X-ray diffraction in differentiating Ga and Ge atoms is obvious, thus the corresponding Inanalogue was also synthesized, which confirmed the reasonability of such an interesting structural model.

In Ba23Ga10Ge10Sb25, the bonding lengths of the Ga-Ge and Ge-Ge pairs range from 2.4672(12) Å to 2.565(2) Å, which are comparable to those of Ba8Ga16Ge30. The Ga-Sb contacts vary from 2.6750(8) to 2.7461(7) Å, similar to those typical bonding distances as in BaGa2Sb2 (from 2.664 to 2.817 Å)19 or Ba3GaSb3 (from 2.697 to 2.759 Å).20 For the In-analogue Ba23In10Ge10Sb24.79(3), the In-Ge bond lengthes vary from 2.6798(8) to 2.756(2), a little longer than Ga-Ge bonds. The In-Sb distances range from 2.7992(7) to 2.8648(7), comparable to those reported ternary indium−antimony-based analogues.21,22

Figure 1. Side-by-side structural comparison between Ba8Ga16Ge30 and Ba23Ga10Ge10Sb25, showing the close relationship of these novel phases. Ba, Ga, Ge, and Sb atoms are represented by purple, green, blue and red spheres, respectively. a) Balland-stick plot of the crystal structure of Ba8Ga16Ge30, viewed down the a-axis. b) Novel (Ga,Ge)64 cluster scissored from the structure of Ba8Ge16Ge30 along the indicated direction. c) Close-up view of Ga20Ge20Sb30 cluster, which can be readily derived from the (Ga,Ge)64 cluster by repalcing part of the outer-shell Ga/Ge atoms with Sb atoms. The extra bonds created by six Sb5 atoms are indicated by purple colour. d) Ball-and-stick structure view of Ba23Ga10Ge10Sb25 along the [101] direction. Ba cations filling into the cages of Ga20Ge20Sb30 clusters are also shown for better illustration.


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Chemistry of Materials such disorders also coincide well with the abnormal temperature parameters of Ba cations closely bonded, which can be clearly verified through the difference Fourier maps (Figure S2). There are totally 9 unique Ba sites in the title compounds, which show various coordination geometries as plotted in Figure 3. The Ba1 atom resides in the center of a Ge20 dodecahedron with Ba-Ge bonds between 3.447 and 3.579 Å. For Ba2 and Ba4, both cations reside in the distorted octahedrons formed by six Sb atoms. However, when it turns to Ba3 and Ba5, the coordination geometry become a capped semicage consisting of five Ga/In, five Ge and six Sb atoms. Ba6 resides in a snub disphenoid formed by eight Sb atoms. Ba7 and Ba9 possess similar coordination environment, which features pentagonal bipyramid formed by seven Sb atoms. The coordination number of Ba8 is also seven, but the geometry is different from Ba7 or Ba9, which can be described as a capped octahedron. More careful comparation reveals that both Ba8 and Ba9 sit off the equatorial plane formed by the Sb atoms. In Ba23Ga10Ge10Sb25, Ba8 is 0.35 Å above the rectangle plane composed of two Sb2 and two Sb5 atoms, and Ba9 sits 0.43 Å away from the pentagon plane constructed by one Sb7, two Sb5 and two Sb8 atoms. The coordination details of Ba9 are shown in the supporting information (Figure S8).

Figure 2. Disordered models on atoms Sb7, Sb8 and Sb11, which are closely related to the high-symmetry sites occupied by these atoms.

A careful analysis on the anionic structure also reveals there exists symmetry-related disorders on some Sb atoms, i.e., Sb7, Sb8 and Sb11. These atoms occupy the high-symmetry sites in a cubic system and thus the splitting around these sites will result in very complicated structural models. As indicated in Figure 2, the deviation of Sb7 from the mirror position may cause a two-fold splitting and the actual bonding distance between Sb10 and Sb7 is 3.07 Å, very reasonable for the Sb-Sb interactions observed in similar analogues.23,24 Similar case occurs on Sb8, which apparently acts as a bridge in connecting the Ga20Ge20Sb30 clusters to form the 3D structure. For Sb11, since it locates at the high-symmetry 6d site, the deviation from this position will lead to a four-fold splitting, which generates a tetrahedral disordered model (Figure 2c). It is worth of noting that

Figure 3. Coordination geometry plot for various Ba cations in Ba23Ga10Ge10Sb25 and related atoms are labeled in graph.



Thermal stability. The results of differential thermal analysis (DTA) and thermogravimetry (TG) measurements are presented in Figure S9. For Ba23Ga10Ge10Sb25, there is no significant mass loss observed over the whole measured temperature range, and an endothermic peak appearing at around 1050 K in the DSC curves suggests the decomposition of the compound. Subseque,nt powder XRD characterization after the cooling process indicates that the decomposing products were GeSb,25 BaSb226 and some other uncertain phases (Figure S10), which showed that the decomposition was not reversible.

excitation of carriers. An optimization of the carrier concentration by modulating the Ga/Ge ratio or by doping is possibly necessary in order to further improve the thermoelctric perforamce at high temperature region. In addition, this work can shed new lights on a novel type of clathrate-like compounds that feature very complex structures containing cage-like clusters, huge unit cells, large number of atoms, and ultralow lattice thermal conductivities.

Thermoelectric properties. The temperature dependent electrical conductivity σ and the Seebeck coefficients S of Ba23Ga10Ge10Sb25 are shown in Figure 4(a). The electrical conductivity increases from 68 S∙cm-1 to 463 S∙cm-1 with the increasing temperature, which indicates semiconducting property. The band gap, estimated by the electrical resistivity with the Arrhenius equation, is very narrow, approximately 0.18 eV.27 The Hall effect measurements indicate that the majority carriers are holes with a carrier concentration of about 6.66×1018 cm-3 at room temperature, which is in agreement with the measured positive Seebeck coefficient. As shown in the figure, the Seebeck coefficient of Ba23Ga10Ge10Sb25 decreases from 117.7 to 96.8 μV∙K-1 when the temperature ascends from 323 to 823 K. This behavior can be understood by inclusion of the thermal excitation of charge carriers, which has been commonly seen in the narrow band-gap semiconductors.28 Figure 4(b) shows the total (κtot) and lattice (κlatt) thermal conductivity of Ba23Ga10Ge10Sb25, as a function of the temperature between 323 and 823 K. The κtot values range from 0.23 to 0.98 W∙m-1∙K-1, which is comparable to those clathrates or Zintl phases based thermoelectrics such as type-I Ba8Ga16Sn3029 and Yb14MnSb11.6 By assuming a single parabolic band model, the Lorenz factor (L) was estimated from the Seebeck coefficient, and the electronic component of the thermal conductivity can be calculated by using the Wiedemann-Franz law κe = LσT.30 Thus, the lattice thermal conductivity κl was obtained by directly subtracting κe from κtot if the bipolar effects were ignored. The lattice thermal conductivity at room temperature is only 0.2 W∙m-1∙K-1, even less than 1/4 of that of p-type Ba8Ga16Ge30 (0.9 W∙m-1∙K-1). The calculated κl ranges from 0.2 to 0.38 W∙m-1∙K-1 over the measured whole temperature range, significantly smaller than that of Ba8Ga16Ge30 (0.7 to 0.9 W∙m-1∙K-1).12 It has been reported that the oncenter rattles in type-I clathrates can cause a large reduction of κl by reducing the relaxation time of phonons at high temperatures.3 Together with the huge unit cell volume as well as the heavy constituent atoms, which may lead to anharmonicity in the phonon transport and have negative correlations with κl,31-33 a glass-like κl in Ba23Ga10Ge10Sb25 is able to realize. As shown in Figure 4(c), the calculated figure of merit for Ba23Ga10Ge10Sb25 approaches 0.27 at 823 K. The zT value of Ba23Ga10Ge10Sb25 is considerably higher than that of Ba8Ga16Ge30 at room temperature, however, it seems suppressed at high temperature owing to the thermal

Figure 4. Figure Caption Thermoelectric properties of the Ba23Ga10Ge10Sb25 sample.

Conclusions In conclusion, two novel Sb-based compounds, Ba23Ga10Ge10Sb25 and Ba23In10Ge10Sb24.79(3), were discovered and corresponding thermoelectric properties of Ba23M10Ge10Sb25 were reported. These compounds feature interesting M20Ge20Sb30 clusters that can be closely related


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Chemistry of Materials to the type-I clathrate Ba8Ga16Ge30, with which decent thermoelectric performance has been demonstrated. More importantly, with a very complex crystal structure containing such cage-like clusters, an extremely low lattice thermal conductivity can be realized. Owing to the thermal excitation of carriers, the resultant figure of merit seems suppressed at high temperature. However, with the carrier concentration optimized by modulating the M/Ge ratio or doping, these materials can be very promising for thermoelectric condidates in consideration of their very low thermal conductivity and excellent electrical conductivity.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] *Email: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT We would like to acknowledge the financial support recieved from the National Natural Science Foundationof China (NSFC) (Grants Nos. 11535010, 51771105, 51271098 and 21601021) and CAS Interdisciplinary Innovation Team.

Supporting Information Picture of Ba23Ga10Ge10Sb25 bulk crystal; Difference Fourier map of Ba8 and Ba9 in Ba23Ga10Ge10Sb25; Powder XRD patterns of the title compounds, Ba23Ga10Ge10Sb25 bulk crystal, and Ba23Ga10Ge10Sb25 polycrystalline sample after DSC measurement; EDX analysis on the composition of single crystals; Temperature-dependent electrical resistivity of Ba23Ga10Ge10Sb25; Temperature-dependent Lorenz number calculated from Seebeck coefficient; Coordination geometry of Ba atoms in Ba23Ga10Ge10Sb25; TG-DSC measurements on a polycrystalline sample of Ba23Ga10Ge10Sb25; Refined atomic coordinates, isotropic displacement parameters, and selected bond lengths of the title compounds; ICP-AES analysis on dissolved polycrystals of Ba23Ga10Ge10Sb25; Hall effect measurements of Ba23Ga10Ge10Sb25.

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Ultralow Lattice Thermal Conductivity in Ba23M10Ge10Sb25-Î´ (M

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