Synthesis, Crystal Structure, and Thermoelectric ... - ACS Publications

Article pubs.acs.org/cm

Synthesis, Crystal Structure, and Thermoelectric Properties of Na2+xAl2+xSn4−x (x = −0.38, −0.24) Masahiro Kanno,† Takahiro Yamada,*,† Hisanori Yamane,† and Hideaki Nagai‡ †

Institute of Multidisciplinary Research for Advanced Material, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba 305-8565, Japan

‡

S Supporting Information *

ABSTRACT: Single crystals of novel compounds, Na2+xAl2+xSn4−x with x = −0.38 and −0.24, were prepared by slow cooling from the melts of the constituent element metals from 1023 and 1123 K, respectively. Single-crystal X-ray diffraction analysis revealed that the compounds crystallize in hexagonal cells (space group P6122) (x = −0.38, a = 6.4050(6) and c = 6.1427(6) Å; x = −0.24, a = 6.3984(3) and c = 6.1529(3) Å). In the crystal structures, four-bonded Al and Sn atoms form frameworks in which helical tunnels are contained. Na atoms are statistically arranged at positions of a 6b site in the tunnels with occupancies of approximately 0.3. Polycrystalline bulk samples with a nominal composition X = −0.24 were prepared by heating compacts of a NaSn, Al, and Sn powder mixture at 623 K. The electrical conductivity and Seebeck coefficient of the polycrystalline samples changed from 3.09 × 103 to 1.03 × 104 S m−1 and from −222 to −185 μV K−1, respectively, with increasing temperature from 295 to 472 K. The thermal conductivity was 0.29−0.36 W m−1 K−1 at 295−371 K. The dimensionless figures of merit ZT were 0.15−0.21 at 295−371 K.

■. INTRODUCTION Thermoelectric materials, which can directly transform thermal energy into electrical energy, are attracting attention as effective devices for waste heat utilization. Waste heat is produced in a wide temperature range, and thermoelectric materials having high performance at various temperature regions have been investigated.1−4 The performance of a thermoelectric material is characterized by its dimensionless figure of merit, ZT, defined as (PF/κ)T (PF = S2σ; PF, power factor; S, Seebeck coefficient; σ, electrical conductivity; κ, thermal conductivity; T, absolute temperature). Performance of ZT ≥ 1 is generally required for practical application. Bi2Te3-based compounds, which can be practically used at ambient temperature, are materials with ZT ∼ 1 at temperatures lower than 500 K,5 whereas several materials such as PbTe-based,6 CoSb3-based,7 and SiGe-based compounds8 are known to be materials exhibiting high performance at temperatures higher than 500 K. To design high-performance thermoelectric materials by decreasing κ and increasing σ, a “phonon-glass electron-crystal (PGEC)” concept was proposed by Slack.9 For almost two decades, Zintl clathrates have been studied as candidates that satisfy the PGEC concept.10 In the crystal structures of the Zintl clathrates, cage-like three-dimensional (3D) framework structures are mainly constructed by the atoms of 13−15 group elements, and the atoms of alkali, alkaline-earth, or rare-earth metal elements are located inside of the cages. Recently, a ZT value of 1.45 at 520 K has been reported for Cu-doped Ba8Ga16Sn30.11 High ZT values of Zintl clathrates were considered to be realized by large local thermal vibration © XXXX American Chemical Society

(called “rattling” motion) of the atoms in the cages, which scatters long wavelength phonons, reducing κ.12 More recently, n-type polycrystalline sintered samples of Na2+xGa2+xSn4−x (x = 0.19) have been reported to have low thermal conductivities (0.52−0.68 W m−1 K−1 at 295 K) and high ZT values (0.58−0.98 at 295 K, 1.28 at 340 K).13 Helical tunnels, in which Na atoms are statistically distributed, are formed in a 3D framework of Ga and Sn atoms in the compound. Dynamic motion and static positional disorder of the Na atoms in the tunnels are regarded to exhibit a similar PGEC effect reported in Zintl clathrates. A compound isostructural with Na2+xGa2+xSn4−x (x = 0.19) would be expected in the Na−Al−Sn ternary system. However, to our knowledge, no ternary compound has been reported in that system. In the present paper, we studied the synthesis of novel ternary compounds, Na2+xAl2+xSn4−x (x = −0.38, −0.24), which are isotypic with Na2+xGa2+xSn4−x, and the crystal structures of which were analyzed by single-crystal X-ray diffraction (XRD). The thermoelectric properties were characterized for polycrystalline-sintered bulk samples of Na2+xAl2+xSn4−x (x = −0.24). Received: November 2, 2015 Revised: December 29, 2015

A

DOI: 10.1021/acs.chemmater.5b04239 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials

■. EXPERIMENTAL SECTION Sn (shot, 99.99%, Rare Metallic Co., Ltd.), Al (piece, 99.9999%, Rare Metallic Co., Ltd.), and Na (lump, 99.95%, Nippon Soda Co., Ltd.) were used as starting materials for preparation of single crystals. Sn was weighed in air, and Al and Na were weighed in an argon gas-filled glovebox (MBraun, O2, H2O < 1 ppm) in molar ratios of Na:Al:Sn = 1:3:6 (sample A) and 1:6:3 (sample B). These metals were loaded in sintered boron nitride (BN) crucibles (inside diameter = 6 mm; depth = 18 mm in inner volume, Showa Denko K.K., 99.5%) and sealed in stainless-steel tubes (SUS316, inner diameter of 10.5 mm, length of 90 mm) with stainless-steel caps in the glovebox. Sample A was heated at 1023 K and cooled from 1023 to 973 K in 2 h, from 973 to 473 K in 100 h and to room temperature in the furnace. Sample B was heated at 1123 K for 10 h and cooled from 1123 to 373 K in 100 h and to room temperature in the furnace. The single crystals included in the samples were observed and analyzed with an electron-probe micro analyzer (EPMA, JEOL, JXA-8200 system). Because single crystals were unstable in air and reacted with moisture, they were taken from the samples and sealed in a glass capillary (an outer diameter of 0.5 mm) in the glovebox for XRD. Mo Kα radiation (λ = 0.71075 Å) with a graphite monochromator and an imaging plate X-ray camera (Rigaku, Model R-AXIS RAPID-II) was used. XRD data collection and unit-cell refinement were performed by the RAPID-AUTO program.14 Analytical absorption correction was applied with the NUMABS program.15 The crystal structure was refined by full-matrix least-squares on F2 using the SHELX2014 program.16 The VESTA program was used for illustration of the crystal structure.17 For the preparation of polycrystalline-sintered bulk samples, Sn (powder, 99.9%, Rare Metallic Co., Ltd.), Al (powder, 99.9%, Rare Metallic Co., Ltd.), and NaSn prepared by ourselves as described below, were used as starting materials. All the following sample manipulations and measurements were carried out in an Ar gas atmosphere. For the synthesis of NaSn, equivalent moles of the Na lump and Sn shots were loaded in a BN crucible, and sealed in a stainless-steel tube. The tube was heated to 898 K, cooled to 498 K at a rate of −8 K h−1, and then to room temperature by shutting off the power to the furnace. The obtained NaSn bulk was pulverized with a mortar and pestle. Sn, Al, and NaSn powders were weighed, mixed, and pressed into rectangular compacts (ca. 14 × 3 × 2 mm3) at a pressure of approximately 200 MPa. The compacts were loaded in BN crucibles, sealed in stainless steel tubes, and heated at 623 K for 10 h. Pulverization of the obtained samples, compaction, and heating at 623 K for 36 h were repeated two times to prepare polycrystalline bulk samples. The powder XRD patterns were measured at room temperature using a Bragg−Brentano type diffractometer (Bruker, D2 PHASER, Cu Kα) for the obtained polycrystalline samples pulverized and set in an Ar gas-filled sample holder with a kapton film window. The lattice parameters of the phases were refined by TOPAS software.18 For the measurement of thermoelectric properties, five polycrystalline bulk samples prepared with the same composition were pulverized and pressed into a pair of disc compacts (ca. ϕ10 × 3.5 mm3). The discs were individually put in BN crucibles, sealed in a stainless steel tube (SUS316, an inner diameter of 16.5 mm, a length of 100 mm), and heated at 623 K for 36 h. The σ and S were measured at 295−472 K by

the direct current four-probe method and by the thermoelectric-power temperature-difference method, respectively, using self-made cells. The κ was measured at 295−371 K by the hot-disk method19 using a commercial instrument (Hot Disk AB).

■. RESULTS AND DISCUSSION Synthesis and Crystal Structure. Black metallic-luster granular single crystals with a maximum size of 100 μm were formed in the matrix of the sample prepared by heating the starting materials of Na, Al, and Sn with molar ratio of Na:Al:Sn = 1:3:6 at 1023 K (sample A). Single crystals were also grown in the sample prepared from the starting materials of Na:Al:Sn = 1:6:3 at 1123 K (sample B). As shown in Figure 1, needle or elongated plate single crystals with a length of about 0.5−2 mm were included in the metal matrix of sample B. These crystals were unstable in air.

Figure 1. Optical micrograph of the sample prepared from starting materials with a composition of Na:Al:Sn = 1:6:3 (sample B). Single crystals of Na2+xAl2+xSn4−x (x = −0.24) were grown.

Elemental analyses were carried out by EPMA for the crystals taken from sample A and sample B. The molar ratios of Na:Al:Sn in the single crystals were determined to be 1.6(3):1.7 (2):4.4(2) for sample A and 1.79(8):1.75(2):4.25(8) for sample B, indicating that the crystals were novel ternary compounds in the Na−Al−Sn system. Large deviations of the molar ratio for single crystals from sample A were probably due to the surface roughness of the small crystal grains. The XRD reflections of the single crystals taken from samples A and B were indexed with hexagonal unit-cell parameters of a = 6.4050(6) and c = 6.1427(6) Å (crystal A) and a = 6.3984(3) Å and c = 6.1529(3) Å (crystal B), respectively. The observed conditions of systematic extinction were consistent with the conditions of space groups P61, P65, P6122, and P6522. The crystal structures were analyzed with the structure model of Na2+xGa2+xSn4−x (space group, P6122). Because the Frack parameters were refined to be 0.6(4) and 0.7(5) for crystal A and crystal B, respectively, a merohedral twin (1 0 0, 0 1 0, 0 0−1) was taken into account in the final refinements. The reliability factors R1 and the values of S for all data were 1.2% and 1.13 for crystal A, and 1.3% and 1.12 for crystal B, respectively. Details of the data collection and refinement, atomic coordinates, and displacement parameters are listed in Tables 1, 2, and 3, respectively. In the structural refinement, Al atoms at the Al/Sn site and Na atoms at the Na site were constrained with the same value, assuming a valence-precise compound represented by B


Article


Na atoms are statistically located with occupancies of 0.270(4) (x = −0.38) and 0.293(5) (x = −0.24) at the Na sites in the tunnels. The Al−Sn network is represented focusing on the hexagonal helical tubes in Figure 2b. The wall of the tubes is formed by five-membered Al/Sn rings connected with their edges. Na site resides at the center of the hexagonal crosssection of the tubes. The crystal structure of Na2+xAl2+xSn4−x, showing an open framework structure with four-bonded Al and Sn atoms, is related to the crystal structures of Sn-based clathrate and clathrate-like compounds. These clathrate(-like) compounds have closed-cage structures, which are mainly built of fourcoordinated Sn atoms. As the compounds having the Sn- and Al/Sn-polyhedral cages in which alkali atoms are included, K6Sn25, A8Sn44 (A = Rb, Cs), A8Al8Sn38 (A = K, Rb), A3Na10Sn23 (A = K, Rb, Cs), and Rb5Na3Sn25 have been reported.20−25 The cell parameters and compositions x of Na2+xAl2+xSn4−x (x = −0.38, −0.24) are plotted in Figure 3 together with the parameters of the single crystals and polycrystalline samples reported for Na2+xGa2+xSn4−x (0 ≤ x ≤ 0.25).13 The a- and caxis lengths and cell volume were changed by −0.10%, + 0.17%, and −0.04%, respectively, with increasing x from −0.38 to −0.24. Similar dependences of x on the cell parameters and volume were shown for those of Na2+xGa2+xSn4−x, although these values were not plotted on the same lines. The cell volumes of Na2+xAl2+xSn4−x (x = −0.38, −0.24) were larger than those of Na2+xGa2+xSn4−x (0 ≤ x ≤ 0.25). This may be related to the radii of the Sn atom (1.54 Å) and the Al atom (1.43 Å), which are larger than the radius of the Ga atom (1.41 Å).26 The distances between the adjacent Al/Sn−Al/Sn sites are 2.7855(5) Å ( × 2) and 2.8014(4) Å ( × 2) for x = −0.38, and 2.7863(5) Å ( × 2) and 2.7998(4) Å ( × 2) for x = −0.24. Those distances are close to Al/Sn−Al/Sn distances of the Al/ Sn cages in K8Al8Sn38 (2.7210−2.8337 Å) and Sn−Sn distances of Sn cages in K6Sn25 (2.7951−2.8602 Å). The Al/Sn−Al/Sn distances in the diagonal line of the helical tunnels are 6.0524(7) Å for x = −0.38 and 6.0509(7) Å for x = −0.24. The closest distances between the Na site and Al/Sn site are 3.0651(9) Å for x = −0.38 and 3.0641(9) Å for x= −0.24, which are consistent with the distance between Na and Sn atoms (3.07 Å) in Na9Sn4 and significantly shorter than closest distances between K atom and Sn atom of the cages in K8Al8Sn38 (3.7996 Å) and K6Sn25 (3.6247 Å).20,23,27 The distances between the nearest-neighbors, the secondneighbors, and the third-neighbors of the Na sites are 1.420(6) Å, 2.664(9) Å, and 3.647(8) Å for x = −0.38, and 1.432(6) Å, 2.684(10) Å, and 3.669(9)Å for x = −0.24, respectively. Recently, Yamanaka et al. have reported a novel Si clathrate compound, Na30.5Si136, prepared under high-pressure.28 Two

Table 1. Crystal Data and Refinement Results for Na2+xAl2+xSn4−x, x = −0.38 and −0.24a chemical formula formula weight, Mr/g mol−1 radiation, λ/Å temperature, T/K crystal system space group, Z Unit Cell Dimensions a/Å c/Å unit cell volume, V/Å3 calculated density, Dcal/Mg m−3 absorption correction absorption coefficient, μ/mm−1 crystal form crystal size/mm3 Limiting Indices h k l F000 θ range for data collection reflections collected/ unique Rint data/restraints/parameters goodness-of-fit on F2, S R1, wR2 (I > 2σ (I)) R1, wR2 (all data) largest diffraction peak and hole, Δρ/e Å−3

Na1.62(3)Al1.62(3)Sn4.38(3)

Na1.76(3)Al1.76(3)Sn4.24(3)

600.81

591.19

0.71075 293(2) hexagonal P6122, 1 6.4050(6) 6.1427(6) 218.23(5) 4.572

6.3984(3) 6.1529(3) 218.15(3) 4.500

numerical 12.519

12.154

black prism 0.089 × 0.093 × 0.097

black prism 0.052 × 0.073 × 0.113

−8 ≤ h ≤ 8 −8 ≤ k ≤ 8 −7 ≤ l ≤ 6 258 3.67−27.34 2130/171

−8 ≤ h ≤ 8 −8 ≤ k ≤ 8 −6 ≤ l ≤ 7 254 3.31−27.47 2162/172

0.0438 171/1/14 1.128 0.0108, 0.0216 0.0120, 0.0219 0.299, −0.262

0.0583 172/1/14 1.119 0.0126, 0.0263 0.0134, 0.0266 0.302, −0.335

R1 = ∑||F0| − |Fc||/∑|F0|. wR2 = {[∑w[(F0)2 − (Fc)2]2]/ [∑w(F02)2]}1/2; w = [σ2(F0)2]−1. a

[Na+]2−x[(4b)Al−]2−x[(4b)Sn]4−x as similar as Na2+xGa2+xSn4−x. The occupancies in the Na site and the Al/ Sn site were refined to be 0.270(4) and 0.270(4)/0.730(4) for crystal A and 0.293(5) and 0.293(5)/0.707(5) for crystal B, respectively. The resulting chemical formulas of crystal A and crystal B were Na1.62(3)Al1.62(3)Sn4.38(3) and Na 1.76(3) Al 1.76(3) Sn 4.24(3) , respectively. These formulas (Na2+xAl2+xSn4−x, x = −0.38(3) (crystal A), −0.24(3) (crystal B)) are in good agreement with the compositions measured by EPMA within experimental error. The crystal structure of Na2+xAl2+xSn4−x (x = −0.24) is shown in Figure 2. Al/Sn atoms are tetrahedrally coordinated to neighboring Al/Sn atoms and construct a 3D framework structure with helical tunnels extending in the c-axis direction.

Table 2. Atomic Coordinates and Equivalent Isotropic Displacement Parameters, Ueq, for Na2+xAl2+xSn4−x for x = −0.38, −0.24 atom

site

occupancy

Al/Sn Na

6b 6b

0.270(4)/0.730(4) 0.270(4)

Al/Sn Na

6b 6b

0.293(5)/0.707(5) 0.293(5)

x x = −0.38 0.76495(3) 0.0887(7) x = −0.24 0.76492(4) 0.0902(8)

y

z

Ueq*/Å2

0.52990(6) 0.1773(15)

0.2500 0.2500

0.01910(13) 0.081(5)

0.52984(7) 0.1804(16)

0.2500 0.2500

0.01915(15) 0.080(5)

*

Ueq = (∑i ∑j Uij ai*aj*aiaj)/3. C


Article

Chemistry of Materials Table 3. Anisotropic Displacement Parameters, Uij/Å2, for Na2+xAl2+xSn4−x for x = −0.38, −0.24 site

U11

U22

Al/Sn Na

6b 6b

0.01888(16) 0.080(7)

0.0217 (2) 0.030(5)

Al/Sn Na

6b 6b

0.0189 (2) 0.069(6)

0.0211(3) 0.036(6)

U33 x = −0.38 0.01764(19) 0.117(13) x = −0.24 0.0182(2) 0.124(13)

U23

U13

U12

0 0

−0.00094(14) −0.084(7)

0.01087(11) 0.015(2)

0 0

−0.00119(15) −0.078(7)

0.01056(13) 0.018(3)

Figure 2. Schematic drawings of the crystal structure of Na2+xAl2+xSn4−x (x = −0.24). (a) General view: Na atoms are drawn in orange, Al in green, and Sn in purple showing occupancies with the area of the atom spheres. (b) Helical chain of Na sites in a helical tunnel of Al/Sn atoms: displacement ellipsoids are drawn at the 75% probability level. Na site and Al/Sn site are drawn in orange and purple, respectively.

Na atoms, which reside on three Na sites with the occupancies of 0.169−0.32, are encapsulated in Si hexakaidecahedral cages (Na2@Si28). The nearest-neighbor and the second-neighbor distances of Na sites in the tunnels of Na2+xAl2+xSn4−x are similar to the distances between neighbors of Na sites (1.263− 1.551 Å) and between Na atoms located two sites away (2.310−2.678 Å) in the Si hexakaidecahedral cages, respectively. The third-neighbor distances of Na sites of Na2+xAl2+xSn4−x are close to the Na−Na distance (3.659 Å) in Na metal.29 As shown in Figure 2b, the atomic displacement ellipsoids of the Na atoms are largely distorted along the helical tunnels. On the other hand, the ellipsoids of the Al/Sn atoms are close to being isotropic and spherical. The values of the equivalent isotropic atomic displacement parameters Ueq of the Na site are 0.081(5) Å2 (x = −0.38) and 0.080(5) Å2 (x = −0.24) and are 4.2 times larger than those of the Al/Sn site (0.01910(13) Å2, x = −0.38 and 0.01915(15) Å2, x = −0.24) (Tables 2 and 3). These Ueq values and their ratios are close to those of Na2+xGa2+xSn4−x (x = 0.24) (Ueq = 0.079(3) Å2 (Na), 0.01911(18) Å2 (Ga/Sn), Ueq(Na)/Ueq(Ga/Sn) = 4.1).13 The low thermal conductivity of Na2+xGa2+xSn4−x (x = 0.19) reported in the previous study was regarded to be the consequence of the dynamic and static disorders of Na

Figure 3. a- and c-axis lengths and the cell volumes of the single crystals (filled symbols) and polycrystalline samples (open symbols) of Na2+xAl2+xSn4−x (red circles) and Na2+xGa2+xSn4−x (blue squares).13

atoms.13 Judging from these similarities in the crystal structures, a low thermal conductivity was expected for Na2+xAl2+xSn4−x (x = −0.38, −0.24). Thermoelectric Properties of Polycrystalline Bulk Samples. A sample prepared by heating Na, Al, and Sn with a molar ratio of Na:Al:Sn = 2:2:4 in a BN crucible at 1123 K, and by slow cooling, was heterogeneous and mainly consisted of NaSn2, Sn, NaSn, and NaSn5. Na2+xAl2+xSn4−x was contained as a minor phase. Polycrystalline bulk samples of Na2+XAl2+XSn4−X (X: nominal composition) for the characterization of thermoelectric properties were synthesized by heating compacts of Al, Sn, and NaSn powder mixtures at 623 K. The samples were prepared with compositions of the starting mixtures with X = −0.38, −0.24, 0.0, and 0.21 of the molar ratio, in which X = −0.38 and −0.24 were the same compositions of the single crystals analyzed in the previous section, and X = 0.21 was a composition close to that of the Na2+xGa2+xSn4−x (x = 0.19) sample which showed the highest D


Article

Chemistry of Materials ZT value (0.98 at 295 K). Polycrystalline samples were prepared by two repetitions of the procedure of pulverization, compaction, and heating at 623 K. Because the mass change of the samples after heating was less than 0.2 mass %, the prepared polycrystalline samples were regarded to maintain the original composition of the starting mixtures. The powder XRD patterns of bulk polycrystalline samples synthesized at 623 K are shown in Figure 4. All peaks, except

Figure 5. Electrical conductivity (σ) and Seebeck coefficient (S) of a polycrystalline sample of Na2+xAl2+xSn4−x (x = −0.24).

The κ value was 0.29 W m−1 K−1 at 295 K and gradually increased to 0.36 W m−1 K−1 at 371 K as plotted in Figure 6. The ZT were 0.15 at 295 K and 0.21 at 371 K and would reach 0.37 at 472 K by using the extrapolated value of κ shown in Figure 6.

Figure 4. Powder XRD patterns of the polycrystalline samples prepared from the starting materials with nominal atom ratios of Na:Al:Sn = 2 + X:2 + X:4 − X (X = 0.21, 0, −0.24, and −0.38).

very small ones of Sn at 2θ = 30.6° and 44.8°, of the sample of X = −0.24 were indexed with hexagonal unit-cell parameters of a = 6.39522(14) and c = 6.15197(18) Å, which were consistent with the cell parameters measured for the single crystal of Na2+xAl2+xSn4−x (x = −0.24), (hexagonal, a = 6.3984(3), c = 6.1529(3) Å), as shown in Figure 3. The powder XRD intensities were also explained by those calculated with the crystal structure parameters determined by single crystal structure analysis. In the XRD patterns of the samples with X = −0.38, 0, and 0.21, diffraction peaks of other crystalline phases such as Sn and NaSn5 (X = −0.38) and NaSn, Sn, and Al (X = 0, 0.21) were observed besides the peaks of Na2+xAl2+xSn4−x. The refined lattice parameters and cell volumes of Na2+xAl2+xSn4−x measured from the powder XRD patterns of the samples slightly deviated from those measured for the sample of X = −0.24 (Supporting Information Figure S1). Thus, the actual composition, x values, of Na2+xAl2+xSn4−x contained in the samples prepared with the nominal compositions X = −0.38, 0, and 0.21 of the starting molar ratio were regarded to be around −0.24. The electrical conductivity (σ) and Seebeck coefficient (S) of the polycrystalline sintered sample of x = −0.24 are shown in Figure 5. The density of the sample used in the characterization was 3.38 Mg m−3, which corresponded to approximately 75% of the theoretical density of Na2+xAl2+xSn4−x (x = −0.24) determined by single-crystal XRD (4.50 Mg m−3). The σ was 3.09 × 103 S m−1 at 295 K, and monotonically increased to 1.03 × 104 S m−1 at 471 K, showing semiconducting behavior. The S value was −222 μV K−1 at 295 K and the absolute value gradually decreased to −185 μV K−1 at 472 K. The negatives values indicated that electrons were major carriers in Na2+xAl2+xSn4−x (x = −0.24). The PF was 1.5 × 10−4 W m−1 K−2 at 295 K and increased to 3.5 × 10−4 W m−1 K−2 at 472 K.

Figure 6. Thermal conductivity (κ) and dimension less figure of merit (ZT) of a polycrystalline sample of Na2+xAl2+xSn4−x (x = −0.24).

If the carrier thermal conductivity (κcarrier) is proportional to the electrical conductivity (σ) and absolute temperature (T), as represented by the Wiedemann−Franz law (κcarrier = LσT) with the Lorenz number value of L = 2.45 × 10−8 W Ω K−2 derived from the free-electron model, the κcarrier of the Na2+xAl2+xSn4−x (x = −0.24) sample at 295 K is calculated to be 0.02 W m−1 K−1 from the σ value (3.09 × 103 S m−1). Assuming that total thermal conductivity, κtotal, is the sum of the κcarrier and thermal conductivities contributed by other factors (phonons, grain boundaries, and pores), the κtotal−carrier is estimated to be 0.27 W m−1 K−1. On the other hand, the κcarrier and κtotal−carrier of Na2+xGa2+xSn4−x (x = 0.19) polycrystalline samples with the relative density of 73% have been reported to be 0.30 and 0.26 W m−1 K−1, respectively, from the σ (4.2 × 104 S m−1) and κ (0.56 W m−1 K−1) measured at 295 K.13 The κtotal−carrier of Na2+xAl2+xSn4−x (x = −0.24) is consistent with that of Na2+xGa2+xSn4−x (x = 0.19). These significantly low κtotal−carrier values are probably caused due to enhancement of phonon scattering by large disorder of Na atoms located in the tunnels, although the effects from grain boundaries and pores in the samples should also be considered. The ZT of the Na2+xAl2+xSn4−x (x = −0.24) polycrystalline sample was 0.15 at 295 K, which was about 1/6 smaller than that of the Na2+xGa2+xSn4−x (x = 0.19) polycrystalline sample (0.98 at 295 K).13 The difference between these values was attributed to the difference of the σ values. The increase of x E



■

might change the carrier densities and improve the conductivity of Na2+xAl2+xSn4−x; however, the polycrystalline sample of almost single phase Na2+xAl2+xSn4−x was obtained only at x = −0.24 in the present study. Besides the investigation of conditions to synthesize Na2+xAl2+xSn4−x at larger x values, optimization of carrier density by doping with other elements would realize high ZT. Preparations and characterizations of sintered samples with high density or large single crystals would provide intrinsic values of thermoelectric properties of Na2+xAl2+xSn4−x.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b04239.

■

a- and c-axis lengths and cell volumes of the polycrystalline samples. (PDF) X-ray crystallographic files. (CIF) X-ray crystallographic files. (CIF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.Y.). Author Contributions

All authors have given approval to the final version of the manuscript. All authors contributed extensively to the work presented in this paper. Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Snyder, G. J.; Toberer, E. S. Complex thermoelectric materials. Nat. Mater. 2008, 7, 105−114. (2) Tritt, T. M.; Subramanian, M. A. Thermoelectric materials, phenomena, and applications: a bird’s eye view. MRS Bull. 2006, 31, 188−198. (3) Sootsman, J. R.; Chung, D. Y.; Kanatzidis, M. G. New and old concepts in thermoelectric materials. Angew. Chem., Int. Ed. 2009, 48, 8616−8639. (4) Liu, W.; Jie, Q.; Kim, H. S.; Ren, Z. Current progress and future challenges in thermoelectric power generation: From materials to devices. Acta Mater. 2015, 87, 357−376. (5) Yamashita, O.; Tomiyoshi, S.; Makita, K. Bismuth telluride compounds with high thermoelectric figures of merit. J. Appl. Phys. 2003, 93, 368−374. (6) Heremans, J. P.; Jovovic, V.; Toberer, E. S.; Saramat, A.; Kurosaki, K.; Charoenphakdee, A.; Yamanaka, S.; Snyder, G. J. Enhancement of thermoelectric efficiency in PbTe by distortion of the electronic density of states. Science 2008, 321, 554−557. (7) Tang, X.; Zhang, Q.; Chen, L.; Goto, T.; Hirai, T. Synthesis and thermoelectric properties of p -type- and n -type-filled skutterudite RyMxCo4−xSb12 (R: Ce, Ba, Y ; M: Fe, Ni). J. Appl. Phys. 2005, 97, 093712. (8) Wang, X. W.; Lee, H.; Lan, Y. C.; Zhu, G. H.; Joshi, G.; Wang, D. Z.; Yang, J.; Muto, A. J.; Tang, M. Y.; Klatsky, J.; Song, S.; Dresselhaus, M. S.; Chen, G.; Ren, Z. F. Enhanced thermoelectric figure of merit in nanostructured n-type silicon germanium bulk alloy. Appl. Phys. Lett. 2008, 93, 193121. (9) Slack, G. A. In CRC Handbook of Thermoelectrics; Rowe, D. M., Ed.; CRC Press: Boca Raton, FL, 1995; p 407. (10) Takabatake, T.; Suekuni, K.; Nakayama, T.; Kaneshita, E. Phonon-glass electron-crystal thermoelectric clathrate: Experiments and theory. Rev. Mod. Phys. 2014, 86, 669−716. (11) Saiga, Y.; Du, B.; Deng, S. K.; Kajisa, K.; Takabatake, T. Thermoelectric properties of type-VIII clathrate Ba8Ga16Sn30 doped with Cu. J. Alloys Compd. 2012, 537, 303−307. (12) Toberer, E. S.; Zevalkink, A.; Snyder, G. J. Phonon engineering through crystal chemistry. J. Mater. Chem. 2011, 21, 15843. (13) Yamada, T.; Yamane, H.; Nagai, H. A thermoelectric Zintl phase, Na2+xGa2+xSn4−x with disordered Na atoms in helical tunnels. Adv. Mater. 2015, 27, 4708−4713. (14) RAPID-AUTO; Rigaku/MSC & Rigaku Corporation: The Woodlands, TX, USA and Akishima, Tokyo, Japan, 2005. (15) Higashi, T. NUMABS-Numerical Absorption Correction; Rigaku Corporation: Tokyo, 1999. (16) Gruene, T.; Hahn, H. W.; Luebben, A. V.; Meilleur, F.; Sheldrick, G. M. Refinement of macromolecular structures against neutron data with SHELXL2013. J. Appl. Crystallogr. 2014, 47, 462− 466. (17) Momma, K.; Izumi, F. VESTA: A three-dimensional visualization system for electronic and structural analysis. J. Appl. Crystallogr. 2008, 41, 653−658. (18) TOPAS; Bruker AXS GmbH: Karlsruhe, Germany, 2009. (19) Gustafsson, S. E. Transient plane source techniques for thermal conductivity and thermal diffusivity measurements of solid materials. Rev. Sci. Instrum. 1991, 62, 797−804. (20) Fässler, T. F.; Kronseder, C. K6Sn23Bi2 and K6Sn25 − two phases with chiral clathrate structure and their reactivity towards ethylenediamine. Z. Anorg. Allg. Chem. 1998, 624, 561−568. (21) Zhao, J.-T.; Corbett, J. D. Zintl phases in alkali-metal-tin systems: K8Sn25 with condensed pentagonal dodecahedra of tin. Two A8Sn44 phases with a defect clathrate structure. Inorg. Chem. 1994, 33, 5721−5726. (22) Nolas, G. S.; Chakoumakos, B. C.; Mahieu, B.; Long, G. J.; Weakley, T. J. R. Structural characterization and thermal conductivity of type-I tin clathrates. Chem. Mater. 2000, 12, 1947−1953. (23) Kröner, R.; Peters, K.; von Schnering, H. G.; Nesper, R. Crystal structure of the clathrates K8Al8Ge38 and K8Al8Sn38. Z. Kristallogr. New Cryst. Struct. 1998, 213, 675−676.

■. CONCLUSIONS Single crystals of novel intermetallic compounds, Na2+xAl2+xSn4−x (x = −0.38 and −0.24), were prepared by slow cooling from the melts of the constituent element metals at 1023 and 1123 K, respectively. Single-crystal X-ray diffraction analysis revealed that Na2+xAl2+xSn4−x (x = −0.38 and −0.24) crystallize in hexagonal cells (space group P6122). In the structure, four-connected Al/Sn atoms construct a 3D framework with helical tunnels in the c direction. About 3/10 positions helicoidally arranged in the helical tunnels are occupied by Na atoms. Dynamic and static disorders of the Na atoms similar to these reported for the isotypic compound Na2+xGa2+xSn4−x (x = 0.19) were expected to reduce its thermal conductivity. The electrical properties were characterized for the polycrystalline bulk samples of nominal composition X = −0.24 prepared by heating the compacts of Al, Sn, and NaSn powder mixtures at 623 K. The electrical conductivities were 3.09 × 103 S m−1 at 295 K and 1.03 × 104 S m−1 at 471 K. The Seebeck coefficient and thermal conductivity increased to −185 μV K−1 at 472 K and 0.36 W m−1 K−1 at 371 K, respectively. The ZT value was 0.21 at 371 K.

■

Article

ACKNOWLEDGMENTS

This work was supported by JSPS KAKENHI (Grant-in-Aid for Scientific Research (B)) under Grant No. 26288105 and was performed under the Cooperative Research Program of “Network Joint Research Center for Materials and Devices.” F


Article

Chemistry of Materials (24) Kröner, R.; Peters, K.; von Schnering, H. G.; Nesper, R. Crystal structure of the clathrates Rb8Al8Ge38 and Rb8Al8Sn38. Z. Kristallogr. New Cryst. Struct. 1998, 213, 669−670. (25) Bobev, S.; Sevov, S. C. Synthesis and characterization of A3Na10Sn23 (A = Cs, Rb, K) with a new clathrate-like structure and of the chiral clathrate Rb5Na3Sn25. Inorg. Chem. 2000, 39, 5930−5937. (26) Pauling, L. Atomic radii and interatomic distances in metals. J. Am. Chem. Soc. 1947, 69, 542−553. (27) Müller, W.; Volk, K. The structures of the phases Na9Sn4 and Na15Sn4. Z. Naturforsch. 1978, 33b, 275−278. (28) Yamanaka, S.; Komatsu, M.; Tanaka, M.; Sawa, H.; Inumaru, K. High-pressure synthesis and structural characterization of the type II clathrate compound Na30.5Si136 encapsulating two sodium atoms in the same silicon polyhedral cages. J. Am. Chem. Soc. 2014, 136, 7717− 7725. (29) Kittel, C. Introduction to Solid State Physics, 6th ed.; Wiley: New York 1986.

G


Synthesis, Crystal Structure, and Thermoelectric ... - ACS Publications

Recommend Documents