Thermoelectric Properties of Na2ZnSn5 Dimorphs with Na Atoms

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Thermoelectric Properties of NaZnSn Dimorphs with Na Atoms Disordered in Tunnels Masahiro Kanno, Takahiro Yamada, Takuji Ikeda, Hideaki Nagai, and Hisanori Yamane Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b04896 • Publication Date (Web): 28 Dec 2016 Downloaded from http://pubs.acs.org on December 29, 2016

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Chemistry of Materials

Thermoelectric Properties of Na2ZnSn5 Dimorphs with Na Atoms Disordered in Tunnels Masahiro Kanno,† Takahiro Yamada,*,†,‡ Takuji Ikeda,§ Hideaki Nagai‖, and Hisanori Yamane† †

Institute of Multidisciplinary Research for Advanced Material, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ‡

PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi 332-0012, Japan

§

Research Institute for Chemical Process Technology, National Institute of Advanced Industrial and Science and Technology, 4-2-1 Nigatake, Sendai 983-8551, Japan ‖

Advanced Coating Technology Research Center, National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba 305-8565, Japan Supporting Information ABSTRACT: Ingots of dimorphs (two polymorphs), hP-Na2ZnSn5 (metastable phase) and tI-Na2ZnSn5 (stable phase), were prepared from the melt of the constituent elements with the stoichiometric composition by furnace cooling from 773 K to 500 K within 1 hour (4.6 K min−1 in average) and slow cooling at a rate of 0.5 K min−1 from 703 K to room temperature, respectively. From the electrical conductivities, Seebeck coefficients, and thermal conductivities measured at 295 K for the ingots, the dimensionless figures of merit (ZT) of hP-Na2ZnSn5 and tI-Na2ZnSn5 were calculated to be 0.21 and 2.8×10−2, respectively. The lattice components of the thermal conductivities were estimated to be 1.10 W m−1 K−1 (hP-Na2ZnSn5) and 0.61 W m−1 K−1 (tI-Na2ZnSn5). Results of the crystal structure analysis by single-crystal X-ray diffraction of both phases demonstrated that the Na atoms in the tunnels of Zn/Sn frameworks had discrete (static) and large continuous (dynamic) positional disorder, which could play a role in reducing the lattice thermal conductivity due to phonon scattering. The disorder of 23Na nuclei was also evidenced by the solid-state nuclear magnetic resonance spectroscopy.

that large thermal vibration called “rattling” of the guest atoms scatters acoustic phonons which dominate heat transfer.7–9,20–22

■ INTRODUCTION Thermoelectric materials that can directly convert thermal energy into electrical energy have been attracting attention for potential utilization of waste heat. Recently, many new compounds have been actively investigated, and some new materials, exhibiting performances similar to those of Bi2Te3-based compounds at around room temperature, were reported.1–5 Since the performance of thermoelectric materials is evaluated with a nondimensional figure of merit ZT (= (σ×S2/κ) × T, σ: electrical conductivity, S: Seebeck coefficient, κ: thermal conductivity, T: absolute temperature), development of materials with high σ and S, and low κ is required.

Recently, ternary Zintl compounds, Na2+xTr2+xSn4−x (Tr = Ga, Al), were synthesized and their crystal structures and thermoelectric properties were characterized.23,24 In the crystal structures, (Ga or Al)/Sn atoms are tetrahedrally coordinated and form a 3D framework structure including helical tunnel spaces, and Na atoms are statistically located at a Na site in the tunnels with an occupancy of around 1/3. Sintered samples with relative densities of ca. 75% showed ZT values of 0.58–0.98 for Tr = Ga, x = 0.19, and of 0.15 for Tr = Al, x = −0.24. The thermal conductivities of the samples at room temperature were reported to be 0.52–0.68 W m−1 K−1 (Tr = Ga, x = 0.19) and 0.29 W m−1 K−1 (Tr = Al, x = −0.24). The possibility that large dynamic and static disorders of Na atoms in the tunnels of the compounds play a role in the reduction of the thermal conductivities has been pointed out.23,24

Based on the “Phonon-Glass Electron Crystal (PGEC)” concept proposed by Slack as a guide to discover the lowκ and high-σ materials, Zintl clathrates and skutterudite compounds have been studied, and high ZT values over 1 at 450–650 K for some compounds have been reported.6–19 Clathrate compounds have polyhedral-cage frameworks formed by “host atoms” mainly of group 13 and 14 elements, and “guest atoms” of alkali, alkaline-earth, and rare earth elements are encapsulated in the cages. Some clathrate compounds, such as α- and β-Ba8Ga16Sn30, show low thermal conductivities.14–18 It is typically considered

Dimorphs, hP-Na2ZnSn5 (metastable phase) and tINa2ZnSn5 (stable phase), have been synthesized and their crystal structures have been determined by Stegmaier et al.25 hP-Na2ZnSn5 transformed into tI-Na2ZnSn5 on heating at around 523 K, while no reverse phase transition was

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■ EXPERIMENTAL SECTION Sodium (lump, 99.95%, Nippon Soda Co. Ltd.), Zn (wire, 99.99%, Nilaco Co. Ltd.), and Sn (shot, 99.99%, Rare Metallic Co. Ltd.) were used as starting materials. Zn and Sn were weighed in air, and Na was weighed in an argon gasfilled glove box (MBraun, O2, H2O < 1 ppm) in a stoichiometric composition of Na : Zn : Sn = 2 : 1 : 5. These metals were placed in a BN crucible（99.5%,φ12 × 18 mm3, Syowa Denko K.K.） and sealed in a stainless-steel (SUS) container in the glove box. Polycrystalline ingots of hPNa2ZnSn5 were prepared by heating at 773 K for 10 h and by furnace cooling to room temperature, in which it took ca. 1 h from 773 K to 500 K (4.6 K min−1 in average). The cooling temperature profile is shown in supplemental information (Figure S1). Polycrystalline tI-Na2ZnSn5 ingots were obtained by heating at 703 K for 10 h and by cooling at a rate of 0.5 K min−1 to room temperature. The tINa2ZnSn5 ingots were annealed at 523 K for 48 h. The obtained ingots of both phases were polished into disc shapes (ca. φ12× 3.5 mm3) with lapping films. Since hP- and tI- Na2ZnSn5 are unstable in air, all manipulations were carried out in the glove box. The crystalline phases of the ingots were identified by X-ray diffraction measured using a Bragg-Brentano type powder diffractometer (Bruker AXS, D2 PHASER, CuKα). The cell parameters of the crystalline phases were refined with the TOPAS software.26 Single crystals were taken from the ingots and sealed in glass capillaries (an outer diameter of 0.5 mm) in the glove box for XRD analysis using Mo Kα radiation (λ = 0.71073 Å) with a multilayered confocal mirror and a CMOS detector on a single-crystal X-ray diffractometer (Bruker AXS, D8 QUEST). The measurement temperatures were set to 300, 250, 200, 145, and 90 K. The XRD data collection, unit-cell refinement, and absorption correction were performed with the Bruker Instrument Service V4.2.0 and APEX2 (Bruker AXS Inc., 2014).27 The crystal structure was refined using full-matrix least-squares on F2 with the SHELXL-2014 program.28 Visualization of the crystal structures was performed using the software package VESTA.29

Figure 1. Crystal structures of hP-Na2ZnSn5 (a) and tI-Na2ZnSn5 (b).

observed on cooling.25 hP-Na2ZnSn5 (hexagonal, P6122) is isostructural with Na2+xTr2+xSn4−x (Tr = Ga, Al) and the framework structures formed by a crystallographic Zn/Sn mixed site with a composition of Zn/Sn = 1/5 (Zn and Sn atom are disordered) as shown in Figure 1a. In the framework structure, spiral tunnels are formed along the c axis and a crystallographic Na site with an occupancy of 1/3 is located in the tunnels. tI-Na2ZnSn5 (tetragonal, 42) has a framework structure constructed from a Zn site and two Sn sites (Zn and Sn atoms are ordered) and spiral tunnels parallel to the a or b axis are alternately stacked in the c axis direction, as shown in Figure 1b. Na atoms reside in the tunnels with a site occupancy of 1/2. The thermoelectric properties of hP- and tI-Na2ZnSn5 have not been reported, but we expect high thermoelectric properties of both hP- and tI-Na2ZnSn5 to be similar to those of Na2+xTr2+xSn4−x (Tr = Ga, Al) because of the structural similarity.

The electrical conductivity (σ) and Seebeck coefficient (S) were measured for the disk samples at 295−453 K by the direct current four-probe method and by the thermoelectric-power temperature-difference method, respectively, using self-made cells. The thermal conductivity (κ) was measured at 295−373 K by the hot disk method30 using a commercial instrument (Hot Disk AB, TPS-2500S). A hot disk sensor was sandwiched with two ingot disks prepared at the same condition.

In the present study, polycrystalline ingots of hPNa2ZnSn5 and tI-Na2ZnSn5 were prepared and their thermal properties were characterized by the measurement of electrical conductivity, Seebeck coefficient, and thermal conductivity under an Ar atmosphere. Lattice dynamical analyses for Zn and Sn atoms which form the frameworks and for Na atoms in the tunnels were performed with atomic displacement parameters obtained by low temperature single crystal X-ray diffraction (XRD). The local environment of 23Na nuclei was investigated by the solidstate nuclear magnetic resonance (NMR) spectroscopy.


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Table 1. Thermoelectric properties of hP-Na2ZnSn5 and tINa2ZnSn5 at 295±2 K. κcarrier κlattice Phase drelative σ S κ ZT (%) (S m−1) (µV K−1) (W m−1 K−1) hP 97.9 1.07×105 −111 1.87 0.77 1.10 0.21 tI 94.2 2.76×102 −455 0.61 2.0×10−3 0.61 2.8×10−2

ones reported for hP-Na2ZnSn5 (hexagonal, a = 6.451(1) Å and c = 6.237(1) Å).25 The XRD peaks of the tI-Na2ZnSn5 disk, except for the small peaks of NaSn2, were indexed with tetragonal cell parameters of a = 6.3375(1) and c = 22.3884(5) Å, which were similar to those reported for tINa2ZnSn5 (tetragonal, a = 6.336(1) Å and c = 22.382(1) Å).25 The densities of the disks of hP-Na2ZnSn5 and tI-Na2ZnSn5 were 5.10 g cm−3 and 4.91 g cm−3, respectively, corresponding to 98% and 94% of the theoretical densities of hPNa2ZnSn5 (5.207 g cm−3) and tI-Na2ZnSn5 (5.210 g cm−3) calculated from the crystal structures reported by Stegmaier et al.25 The irreversible transformation from hPNa2ZnSn5 to tI-Na2ZnSn5 by annealing a hP-Na2ZnSn5 ingot at the reported temperature (523 K for 48 h)25 was confirmed.

Figure 2. Photographic images of the disks of hP-Na2ZnSn5 (a) and tI-Na2ZnSn5 (b).

Solid state magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) measurements were carried out with an AVANCEIII 400WB spectrometer (Bruker Biospin K.K.) and using a 3.2 mm VT-MAS probe with zirconia rotors. 23Na triple-quantum (3Q) MAS NMR spectra with z-filter of hP- and tI-Na2ZnSn5 were recorded at a resonance frequency of 105.843 MHz with a rotor spinning rate of 20 kHz. A recycle delay time was set at 5 secs, and FID sizes were set at 2048 for F2 axis and 64 for F1 axis. Pure NaCl solid was used as a secondary reference material of chemical shift for 23Na nucleus.

The electrical conductivity (σ), Seebeck coefficient (S), and thermal conductivity (κ) of hP-Na2ZnSn5 and tINa2ZnSn5 at 295 K are summarized in Table 1. Temperature dependences of σ and S values from 295 K to 450 K, and κ values from 295 K and 375 K are shown in Figure 4. The σ and S values of hP-Na2ZnSn5 were 1.07×105 – 7.56×104 S m−1 and −111 – −142 µV K−1, respectively, while, those of tI-Na2ZnSn5 were 2. 59×102 – 3.93×102 S m−1 and −455 – −464 µV K−1. The negative Seebeck coefficients of hP-Na2ZnSn5 and tI-Na2ZnSn5 suggested that majority carriers of hP- and tI-Na2ZnSn5 were electrons.

■ RESULTS AND DISCUSSION Thermoelectric properties of hP- and tINa2ZnSn5. Micrographs of the disks of hP-Na2ZnSn5 and tI-Na2ZnSn5 are shown in Figure 2, and the XRD patterns measured for the powdered disks are shown in Figure 3. The XRD peaks from the hP-Na2ZnSn5 disk, except for the small peaks of Sn at 30.6˚ and 32.1˚, could be indexed with hexagonal cell parameters of a = 6.4534(2) Å and c = 6.2436(3) Å. The lattice parameters were identical to the

(a)

Intensity, I (a.u.)

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


Stegmaier et al. have calculated the electronic structures of hP- and tI-Na2ZnSn5 with ordered models based on density functional theory.25 A band gap of ca. 0.5 eV of tI-Na2ZnSn5 shown by the calculation was consistent with the low electrical conductivity of tI-Na2ZnSn5. They mentioned that the shortcomings for the calculated DOS of hP-Na2ZnSn5 with a pseudo-gap came from the ordered structural model.25 But if the result suggested a smaller band gap of hP-Na2ZnSn5, this might be related to the higher electrical conductivity of hP-Na2ZnSn5.

Sn

Simulation (hP-Na2ZnSn5)

(b)

NaSn2

Simulation (tI-Na 2ZnSn5)

10

20 30 40 50 Diffraction angle, 2θ (Cu-Kα) / degree

60

Figure 3. XRD patterns of the hP-Na2ZnSn5 ingot and the calculated pattern of hP-Na2ZnSn5 (a), and the tI-Na2ZnSn5 ingot and the calculated pattern of tI-Na2ZnSn5 (b).



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Figure 4. Electrical conductivities (a), Seebeck coefficients (b), thermal conductivities (c), and dimensionless figures of merit, ZT (d) of hP-Na2ZnSn5 (circles) and tI-Na2ZnSn5 (squares). The broken lines in (c) show extrapolated values. The alternate long and short dashed lines indicate the estimated carrier and lattice components of the thermal conductivities of hP-Na2ZnSn5. The thermal conductivities of tINa2ZnSn5 are regarded as being the lattice thermal conductivities because the carrier thermal conductivities of tI-Na2ZnSn5 are negligibly small (ca. 2×10−3 W m−1 K−1). Closed marks in (d) are obtained from the measured S, σ, and κ values and open marks are estimated from the measured S and σ values and the extrapolated κ values.

0.50 W m−1 K−1 at 300–600 K).14 The mechanism of slight increase in thermal conductivity has not been clarified.

The thermal conductivity (κ) of hP-Na2ZnSn5 was 1.87 W m−1 K−1 at 295 K and monotonically increased to a value of 2.12 W m−1 K−1 at 373 K. tI-Na2ZnSn5 showed a κ value of 0.61 W m−1 K−1 at 295 K and 0.69 W m−1 K−1 at 375 K (Figure 4c). Assuming that the carrier component of thermal conductivity, κcarrier, obeys the Wiedemann–Franz law (κcarrier = LσT) with the Lorenz number value, L, of 2.45×10−8 W Ω K−2, the values of κcarrier for hP- and tINa2ZnSn5 were estimated to be 0.77 W m−1 K−1 and 2.01×10−3 W m−1 K−1, respectively. Considering that the total thermal conductivity, κtotal, is the sum of the carrier component (κcarrier) and lattice component (κlattice), the κlattice (= κtotal−κcarrier) values of 1.10 W m−1 K−1 for hPNa2ZnSn5 and 0.61 W m−1 K−1 and for tI-Na2ZnSn5 were obtained (Table 1). These κlattice are significantly low; in particular, the κlattice of tI-Na2ZnSn5 is comparable to some of the lowest κlattice values reported for the tin basedclathrate compounds (ca. 0.35–0.50 W m−1 K−1 for αBa8Ga16Sn30 and β-Ba8Ga16Sn 30).14,15,31

Na2+xTr2+xSn4−x (Tr = Ga, Al) are isostructural with hPNa2ZnSn5 and the values of κtotal−κcarrier measured for the polycrystalline sintered samples with relative densities of ca. 75% have been reported to be 0.26 W m−1 K−1 (Tr = Ga) and 0.27 W m−1 K−1 (Tr = Al).23,24 In these cases, the κtotal−κcarrier values did not simply correspond to the κlattice values because of the contribution of other factors such as grain boundaries and pores in the samples. The thermal conductivities κtotal of the Na2ZnSn5 dimorphs measured for the ingot disks with a relative density of 94–98 % are close to the intrinsic values.

The lattice thermal conductivity of both compounds slightly increased with increasing temperature. Similar κlattice variation with temperature was observed in βBa8Ga16Sn30 (κlattice: ca. 0.35 W m−1 K−1 at 300 K and ca. 0.45 W m−1 K−1 at 500 K)15, 32, but not in α-Ba8Ga16Sn30 (κlattice: ca.


The dimensionless figure of merits (ZT) of hP-Na2ZnSn5 and tI-Na2ZnSn5 calculated from the measured σ, S, and κ values are shown in Figure 4d. The ZT value of hPNa2ZnSn5 was 0.21 at 295 K and increased to 0.28 at 371 K, whereas the ZT values of tI-Na2ZnSn5 were 2.8×10−2 at 297 K and 3.2×10−2 at 373 K. The ZT values estimated from the measured σ and S values and the extrapolated κ values, indicating break lines in Figure 4d, were 0.31 at 395 K for hP-Na2ZnSn5 and 4.4×10−2 for tI-Na2ZnSn5. Optimization of carrier concentration and mobility by doping are probably effective in enhancing the thermoelectric performance.

100

Low temperature single crystal XRD and lattice dynamical analysis. XRD data were analyzed

Na(hP)

2

80

60

Na(tI)

-3

Equivalent isotropic displacement parameters,

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


Ueq / 10 Å

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Na(hP, ref.25) Na(tI, ref.25)

40 Zn/Sn (hP, ref.25)

20

Zn/Sn(hP)

Zn, Sn1, Sn2 (tI, ref. 25)

Zn(tI)

using the structure models of hP-Na2ZnSn5 (space group P6122) and tI-Na2ZnSn5 (space group I42d) reported by Stegmaier et al.25 The reliability factors R1 and the values of GOF (goodness-of-fit) for all data of the refinements were 1.01–1.15 % and 1.322–1.414 for hP-Na2ZnSn5, and 1.33– 1.49 % and 1.175–1.229 for tI-Na2ZnSn5, respectively. Details of the data collections and refinements are listed in Tables S1 and S2. Atomic coordinates and equivalent displacement parameters are summarized in Tables S3 and S4, and anisotropic displacement parameters are shown in Tables S5 and S6.

equal to the ratios of Na2+xTr2+xSn4−x (Ueq(Na)/Ueq(Tr/Sn) = 4.1 (Tr = Ga) and 4.2 (Tr = Al)).23,24 The Ueq values of the Na site and Zn/Sn site of hP-Na2ZnSn5 are larger than those of tI-Na2ZnSn5 at 90–300 K.

Lattice parameters of hP- and tI-Na2ZnSn5 monotonically decreased with decreasing temperature, and no phase transitions were observed (Figure S1). Equivalent atomic displacement parameters (ADPs), Ueq, of hP- and tI-Na2ZnSn5 at measured temperatures are shown in Figure 5. The Ueq values of Na sites in the tunnels were larger than those of Zn/Sn or Zn and Sn sites which formed the framework structures of hP-Na2ZnSn5 and tI-Na2ZnSn5. The Ueq ratios of the Na sites and the Zn/Sn or Zn and Sn sites, Ueq(Na)/Ueq(Zn/Sn), were 3.8 for hP-Na2ZnSn5 and 4.0 for tI-Na2ZnSn5 at 300 K. These ratios were nearly

In the cage framework structures of clathrate and skutterudite-type compounds, guest atoms are encapsulated in their relatively large cages. Lattice dynamical analyses were performed based on the temperature dependence of the ADPs,31,33–39 assuming that the motion of the guest atom is independent of the collective motion of the host structure because of the weak bonding between the guest atoms and the host structure. The ADPs of the guest atom can be modeled as an Einstein oscillator using equation (1), and the host structure can be approximated by the Debye expression with equation (2), as shown

Sn1(tI)

0 0

50

100 150 200 Temperature, T / K

250

Na

K, Rb, Cs, Ba, Eu

hP-Na2ZnSn5

θE (K) d (Å) 118(3) 0.118(4)

tI-Na2ZnSn5

108(2)

Cs8Na16Ge136* Rb8Na16Ge136* Ba8Ga16Ge30*

117 127

θE (K)

0.026(3)

Ba8Ga16Ge30* α-Eu8Ga16Ge30* β-Eu8Ga16Ge30* α-Ba8Ga16Sn30* β-Ba8Ga16Sn30* Ba16Ga32.1Sn103.9* Cs8Ba16Ga39.7Sn96.3* Rb9.9Ba13.3Ga36.4Sn99.6* K2.0Ba14.0Ga30.4Sn105.6*

300

Figure 5. Equivalent isotropic displacement parameters (Ueq) of Na, Zn, Sn and Zn/Sn sites of hP-Na2ZnSn5 (circles) and tINa2ZnSn5 (squares).

Table 2. Einstein (θE) and Debye (θD) temperatures and disorder parameters (d) of hP- and tI-Na2ZnSn5 and some reported clathrate compounds. Compound

Sn2(tI)

Zn, Ga , Sn θD (K) 183(2)

d (Å) 0.096(1)

Zn Sn1 Sn2 ave.

233(5) 200(5) 222(10) 209(12)

0.061(3) 0.062(2) 0.069(3) 0.063(2)

41.8 42.9 121 (Ba1) 72 (Ba2) 51 45 75 (Eu1) 45 (Eu2) 64 78 (Ba1) 60 (Ba2) 67 34 (Cs) 64 (Ba) 53 (Rb) 65 (Ba/Rb) 70 (Ba/K)

Ref.

θD (K) this study this study

230 220 300

[37] [37] [42]

275 214 245

[34] [42] [42]

195 194–203

[41] [17]

155 154

[43] [43]

156

[43]

162

[43]

*Disorder parameters, d, were not reported.


Ga, Ge

site Zn/Sn

Chemistry of Materials below:35,40

–5.5

Ueqhost (T) =

h 2

3h T 4π2mkBθD

coth E + d2, θ

8π2mkB θE

(1)

T

/

2 [ θ D

12.5

+ D ] + d2, θ

4T

(2)

where h and kB are the Planck constant and the Boltzmann constant, m is the atomic mass, θE and θD are the Einstein temperature and Debye temperature, and d describes the temperature independent disorder parameter.

B

23.4

The temperature dependences of Ueq of the Na and Zn/Sn sites of hP-Na2ZnSn5 and the Na, Zn, and Sn sites of tI-Na2ZnSn5 determined by single-crystal structure refinement were well fitted with the equations (Figure 5). The characteristic temperatures and disorder parameters of Na atoms were θE = 118(3) K and dNa = 0.188(4) Å for hPNa2ZnSn5, and θE = 108(2) K and dNa = 0.026(3) Å for tINa2ZnSn5, while those of Zn/Sn, Zn, and Sn atoms of the frameworks were θD = 183(2) K and dZn/Sn = 0.096(1) Å for hP-Na2ZnSn5, and average values of θD = 209(12) K and dZn, Sn = 0.063(2) Å for tI-Na2ZnSn5 (Table 2).

A

chemical shift, δ F2 / ppm

23

Figure 6. Na 3QMAS NMR spectra which superimposed hPNa2ZnSn5 (A) and tI-Na2ZnSn5 (B) obtained at R.T. CS and QIS lines indicate the chemical shift and the quadrupolar-induced shift, respectively. The contour line level of both spectra is standardized and the minimum threshold value is set to ca. 15% of the maximum of each peak. Left and upper sides show the 1D spectra of resonance peaks of hP-Na2ZnSn5 and tI-Na2ZnSn5 obtained by the projection to F1 and F2 axes.

The θE of Na atoms of hP-Na2ZnSn5 and tI-Na2ZnSn5 were comparable to the θE of Na atoms of some Ge clathrate compounds, Cs8Na16Ge136 (117 K) and Rb8Na16Ge136 (127 K),37 and significantly larger than the θE of heavier Ba and Eu atoms of α- and β-Ba8Ga16Sn30 (60–78 K)17,41 and α- and β-Eu8Ga16Ge30 (45–75 K).42 The θD of Zn and Sn atoms tetrahedrally connected each other in the framework structures of hP-Na2ZnSn5 and tI-Na2ZnSn5 were similar to or slightly larger than the θD of Ga/Sn atoms in a similar coordination environment of α- and β-Ba8Ga16Sn30 (194–203 K)7,41 and Ba16Ga32.1Sn103.9, Cs8Ba16Ga39.7Sn96.3, Rb9.9Ba13.3Ga36.4Sn99.6, and K2.0Ba14.0Ga30.4Sn105.6 (154–162 K),43 whereas the θD were 60–98% of the θD of the tetrahedrally coordinated Ge and Ga/Ge atoms of the Ge clathrate compounds of Cs8Na16Ge136, Rb8Na16Ge136, Ba8Ga16Ge30, and α- and β-Eu8Ga16Ge30 (214–300 K)37,42 (Table 2).

were collected for both hP-Na2ZnSn5 and tI-Na2ZnSn5 at room temperature (Figure 6). The 3QMAS NMR technique can decompose overlapped signal due to the large second-order quadrupolar interaction of a half-integer quadrupolar nuclei. The spectra widths (FWHM) along F2 axis were ca. 13 ppm for hP-Na2ZnSn5 and 6.5 ppm for tINa2ZnSn5, which are considerably broader than that of NaCl solid (ca. 0.8 ppm). Additionally, the spectra widths along F1 axis were ca. 14.3 ppm for hP-Na2ZnSn5 and 1.35 ppm for tI-Na2ZnSn5. In both peaks of hP- and tINa2ZnSn5, the differences of the chemical shift values between F1 and F2 dimensions were ca. 11–13 ppm, which was due to quadrupolar interaction.

The disorder parameters of Na and Zn/Sn atoms of hPNa2ZnSn5 were ca. 4.5 times (dNa) and 1.5 times (dZn/Sn) larger than those of tI-Na2ZnSn5. This can probably be attributed to a difference between the occupancies of the Na sites in hP-Na2ZnSn5 (Occ: 1/3) and tI-Na2ZnSn5 (Occ: 1/2), and to the difference between the Zn and Sn atom arrangements in hP-Na2ZnSn5 (a mixed Zn/Sn site) and tINa2ZnSn5 (Zn and Sn sites). The results of the lattice dynamic analysis demonstrate that the Na atoms included in the tunnel frameworks of hP-Na2ZnSn5 and tI-Na2ZnSn oscillate with large amplitudes as in the case of the guest Na atoms of the clathrate compounds. Since tI-Na2ZnSn5 having the smaller static disorder parameters (dNa = 0.026(3) Å and dZn, Sn = 0.063(2) Å) showed lower lattice thermal conductivity (0.61 W m−1 K−1) than that (1.10 W m−1 K−1) of the hP-Na2ZnSn5 with a disordered framework structure (dNa = 0.118(4) Å and dZn/Sn = 0.096(1) Å), the static disorder parameters of the Na atoms may not be directly related to the reduction of the thermal conductivity. 23

isotropic dimension, δ F1 / ppm

Ueqguest (T) =

2

7.5

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The resonance peak of tI-Na2ZnSn5 elongated along F2 axis with two shoulder peaks, indicating the presence of a typical quadrupolar interaction. However, a sharp resonance peak was observed in isotopic dimension (F1 axis), that is, there is a single Na site with high regularity in tINa2ZnSn5. On the other hand, the resonance peak of hPNa2ZnSn5 elongated along not only F2 axis but also F1 axis. This finding suggests that the Na distribution in hPNa2ZnSn5 consists of plural Na positions with different local environments. Namely, the Na distribution is highly inhomogeneous compared to that of tI-Na2ZnSn5. The degree of the inhomogeneity of Na distribution estimated by the 3QMAS NMR experiment corresponds to the magnitude relation of d values calculated from equation (1) qualitatively. In the present study, low lattice thermal conductivity of tI-Na2ZnSn5 (0.61 W m−1 K−1) which was approximately a half of that of hP-Na2ZnSn5 (1.10 W m−1 K−1) was demonstrated. tI-Na2ZnSn5 had smaller static disorder than hP-

In order to investigate the local structure around the Na nuclei with I = 3/2 spin, 23Na 3QMAS NMR spectra


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Na2ZnSn5, while the dynamic disorder (Einstein and Debye temperatures) was similar to that of hP-Na2ZnSn5. These results imply that the low lattice thermal conductivity could be achieved from multiple factors, not only the static and dynamic disorders of the guest and host atoms, but also e.g. lattice anharmonicity and structural complexity, etc. From a crystallographic view point, the Na site (16e site) with a half occupancy in tI-Na2ZnSn5 can be regard as twofold splitting sites from 8d site in the spiral tunnel framework as shown in Fig. S3 in supporting information. The distance between the 16e and 8d sites is 0.435(9) Å. These crystallographic features are similar to those of “off-centered” type-I clathrates (space group, 3) 7,8,34,42,44, in which the splitting sites of guest atoms (24k or 24j sites) with a distance ca. 0.3–0.45 Å from the center (6d site) of cages are considered as an origin of the glasslike thermal properties.7,8 The off-symmetric position of the Na splitting site in tI-Na2ZnSn5 may also play a role in the reduction of thermal conductivity. In contrast, Na and Zn/Sn atoms in hP-Na2ZnSn5 are at the same symmetry 6b sites. Further studies on the related factors to the phonon transport are necessary.

framework structure. Meanwhile, a single Na site of tINa2ZnSn5 with relatively high regularity was confirmed.

■ ASSOCIATED CONTENT Supporting Information. This supporting Information is available free of charge via the Internet at http://pubs.acs.org. Detailed results of single crystal X-ray refinements of hPNa2ZnSn5 and tI-Na2ZnSn5 (crystal data, atomic coordinates, and atomic displacement parameters); Cooling temperature profile for the preparation of the hP-Na2ZnSn5 ingots; lattice parameters of hP-Na2ZnSn5 and tI-Na2ZnSn5(PDF), and four X-ray crystallographic files (CIF).

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (T. Yamada).

Author Contributions All authors contributed extensively to the work presented in this paper.

■ ACKNOWLEDGMENT This work was supported by JST PRESTO and JSPS KAKENHI (26288105) and was performed under the Cooperative Research Program of “Network Joint Research Center for Materials and Devices.” A part of this work was financially supported by the AIST–Tohoku University matching fund.

■ CONCLUSIONS Thermoelectric properties of hP-Na2ZnSn5 and tINa2ZnSn5 were investigated with ingots prepared from the melts of the constituent elements with a stoichiometric composition by cooling with different cooling rates. The electrical conductivity (σ), Seebeck coefficient (S), and thermal conductivity (κ) of hP-Na2ZnSn5 measured at 295 K were 1.07×105 Sm-1, −111 µV K−1, and 1.87 W m−1 K−1, respectively, and those of tI-Na2ZnSn5 were 2.76×102 Sm-1, −455 μVK-1, and 0.61 Wm−1K−1 at 295 K. The lattice thermal conductivity of the hP-Na2ZnSn5 and tI-Na2ZnSn5 (κlattice) were estimated to be 1.10 and 0.61 W m−1 K−1, respectively. The dimensionless figures of merit (ZT) at 295–373 K were 0.21–0.28 for hP-Na2ZnSn5 and 2.8×10−2–3.2×10−2 for tINa2ZnSn5.

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The atomic displacement parameters of the Zn/Sn, Zn, and Sn sites of the frameworks and the Na sites in the tunnels of the frameworks of hP-Na2ZnSn5 and tINa2ZnSn5 were refined based on the X-ray diffraction of the single crystals at 90–300 K, and the characteristic parameters and disorder parameters of the atoms were evaluated. The Einstein temperatures, θE, of the Na sites and the Debye temperatures, θD, of the Zn and Sn atoms were 118(3) K and 183(2) K for hP-Na2ZnSn5 and 108(2) K and 209(12) K for tI-Na2ZnSn5, respectively. tI-Na2ZnSn5, having an ordered Zn/Sn framework structure, showed disorder parameters of Na, Zn, and Sn atoms smaller than those of hP-Na2ZnSn5 with the disordered Zn/Sn framework structure. In the 23Na solid-state 3QMAS NMR experiments, considerable broadening of resonance peak widths along both chemical shift axis and isotropic dimension axis were observed for hP-Na2ZnSn5, indicating large disorder (temperature independence) of Na nuclei located on the Na site(s) in the tunnel space of the Zn/Sn



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Thermoelectric Properties of Na2ZnSn5 Dimorphs with Na Atoms

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