Article pubs.acs.org/IC
Thermoelectric Properties of As-Based Zintl Compounds Ba1−xKxZn2As2 Kunihiro Kihou,* Hirotaka Nishiate, Atsushi Yamamoto, and Chul-Ho Lee* National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan S Supporting Information *
ABSTRACT: As-based Zintl compounds Ba1−xKxZn2As2 were prepared by solidstate reaction followed by hot pressing. Ba1−xKxZn2As2 (x ≤ 0.02) crystallizes in the α-BaCu2S2-type structure (space group Pnma) upon cooling from 900 °C, whereas it crystallizes in the ThCr2Si2-type structure (space group I4/mmm) for x ≥ 0.04. The lattice thermal conductivities are almost equivalent for both crystal structures with relatively low values of 0.8−1.1 W/mK at 773 K. The values are comparable to those of Sb-based Zintl compounds, though Ba1−xKxZn2As2 consists of As atoms, which are lighter than Sb atoms. The electrical resistivity and Seebeck coefficient decreases with increasing x, indicating successful hole doping by K substitution. The dimensionless figure-of-merit ZT is 0.67 at 900 K for x = 0.02, opening a new class of thermoelectric materials with the As-based 122 Zintl compounds.
1. INTRODUCTION
conductivity, causing low ZT values. Thus, very few studies on P-based compounds have been reported.13,14 Other crystal structures can be found in Zintl compounds with a 1:2:2 compositional ratio (termed 122 Zintl compounds). For example, there are the ThCr2Si2-type structure with the space group of I4/mmm (Figure 1a) and the α-
Thermoelectric power generation is expected to contribute to energy conservation through the generation of electricity from waste heat. However, to realize this technique, thermoelectric materials with improved efficiencies are required. Their performance is defined by the dimensionless figure-of-merit, ZT = S2T/ρκ, where S is the Seebeck coefficient, ρ is electrical resistivity, and κ is total thermal conductivity. Thus, to achieve high thermoelectric performance, the conflicting conditions of good electrical conductivity and poor thermal conductivity must be satisfied. A possible solution to this problem is the use of Zintl compounds.1 Zintl compounds consist of an anionic framework formed by covalent bonding and cations forming ionic bonds, which exhibits characteristics that are advantageous for thermoelectric materials. In general, they are semiconductors, sometimes with the narrow band gap required for a large Seebeck coefficient. Furthermore, their framework of covalent bonding results in good electric conductivity, and they often form complicated crystal structures, which contributes to a lowering of thermal conductivity. Zintl compounds of the CaAl2Si2-type crystal structure with the P3̅m1 space group have attracted attention as promising thermoelectric materials owing to their relatively high ZT values and their wide variety of compounds.2−10 For example, (Ca,Yb)Zn 2 Sb 2 , 2−4 SrZn 2 Sb 2 , 4 Yb(Zn,Cd) 2 Sb 2 , 3−5 Eu(Zn,Cd)2 Sb 2,3,4,6,7 Yb(Zn,Mn)2 Sb 2,3,4,8 Yb(Cd,Mn)2Sb 2,9 (Ca,Yb)Cd2Sb2,10 YbMg2Bi2,11 and Mg3(Sb,Bi)212 have been reported. In particular, Mg3(Sb,Bi)2 exhibits ZT = 1.51 at 716 K.12 The anion in most of the investigated materials is Sb. Compounds consisting of lighter atoms have attracted less attention because light atoms usually lead to high thermal © 2017 American Chemical Society
Figure 1. Schematic illustration of the crystal structures of BaZn2As2 in (a) β-phase (ThCr2Si2-type, I4/mmm) and (b) α-phase (α-BaCu2S2type, Pnma).
BaCu2S2-type structure with the space group of Pnma (Figure 1b).15,16 Although the thermoelectric properties of the CaAl2Si2-type Zintl compounds are good, those of ThCr2Si2type compounds are relatively poor.17 On the other hand, a few materials with the α-BaCu2S2-type crystal structure have been reported as promising thermoelectric materials.18,19 (Ba,Na)Cu2Se2 and BaZn2Sb2 crystallize in the α-BaCu2S2-type crystal structure and present ZT values of 1.0 at 773 K and 0.31 at 675 K, respectively.18,19 Received: January 30, 2017 Published: March 2, 2017 3709
DOI: 10.1021/acs.inorgchem.7b00232 Inorg. Chem. 2017, 56, 3709−3712
Article
Inorganic Chemistry
perpendicular to the applied pressure (see the Supporting Information).
BaZn2As2 crystallizes in the ThCr2Si2-type structure as a high temperature β-phase and in the α-BaCu2S2-type structure as a low temperature α-phase (Figure 1).20 The β-phase can be obtained by quenching from high temperature. Although many Zintl compounds with the CaAl2Si2-type structure, especially Sb-based compounds, have been reported as thermoelectric materials, there are relatively few 122 Zintl compounds with other crystal structures that show promise as thermoelectric materials. In particular, there are no reports on As-based 122 Zintl compounds as high-performance thermoelectric materials. Consequently, we explored the thermoelectric properties of Ba1−xKxZn2As2 in this study.
3. RESULTS AND DISCUSSION A single phase α-BaCu2S2-type structure (space group Pnma, αphase) was obtained for Ba1−xKxZn2As2 (x ≤ 0.02) (Figure 2).
2. EXPERIMENTAL SECTION Polycrystalline bulk samples of Ba1−xKxZn2As2 were synthesized by solid-state reactions. First, we synthesized BaAs and KAs as precursors. The starting materials Ba (3N, chunks) or K (3N, chunks), and As (6N, 1−5 mm chunks) were mixed at a stoichiometric ratio and placed in a silica tube (BaAs) or an alumina crucible (KAs). The silica tube was then sealed and heated at 750 °C for 24 h in a box furnace. The alumina crucible was encapsulated in a screw-top stainless-steel container and heated at 650 °C for 20 h. The thus obtained BaAs and KAs were mixed with Zn (3N, powder) and As (6N, 1−5 mm chunks) at a stoichiometric ratio to synthesize Ba1−xKxZn2As2. The mixtures were ground and pressed into pellets at room temperature. The pellets were placed in an alumina crucible and encapsulated in a screw-top stainless-steel container. Then, they were heated at 1000 °C for 20 h, cooled to 900 °C, and quenched. All mixing and grinding processes described above were carried out in a glovebox filled with dried Ar gas to prevent oxidation. The box furnace was installed in a drafter for precaution against leaking. To make the pellets denser, hot pressing was performed. The pellets were ground, wrapped in expanded graphite sheets, and loaded into graphite dies. The dies were heated at 900 °C for about 10 min at a uniaxial pressure of 70 MPa under Ar gas flow. Then, the pressure was released, maintaining a temperature of 900 °C for about 20 min, followed by furnace cooling. Powder X-ray diffraction was performed at room temperature using a Rint-2000 (RIGAKU) diffractometer. Cu Kα radiation was used at 40 kV and 30 mA. Scattering angle 2θ was moved with a scan rate of 0.1°/min. Hot-pressed samples were pulverized for the measurements. The densities of the hot-pressed samples were determined by the Archimedes method. Their relative densities were all in the range 94− 98% The resistivity and Seebeck coefficient were measured simultaneously using a ZEM3 (ADVANCED-RIKO) instrument from room temperature up to 970 K under 0.5 atm He gas. The resistivity was measured by the four-probe method under an equilibrium temperature state. The Seebeck coefficient was measured by applying a temperature gradient across a sample (the so-called temperature difference method) with Pt-PtRh thermocouples as probes. Typically, samples were 1.5 × 1.5 × 8 mm in size and rectangular parallelepiped in shape. Measurements were conducted using heating/cooling cycles to confirm the stability of the samples. The thermal conductivity was calculated using κ = DCpds, where D, Cp, and ds are the thermal diffusivity, the specific heat, and the sample density, respectively. The thermal diffusivity and specific heat were measured by a laser flash method using a LFA457 (Netzsch) instrument under Ar gas flow. The samples used for the measurements were disks 10 mm in diameter and 2 mm in thickness. Samples were coated with graphite. The specific heat of one of the samples was measured using a differential scanning calorimetry with a DSC404 F3 (Netzsch) instrument under Ar gas flow to confirm the consistency. The sample was a disk 5 mm in diameter and 1.25 mm in thickness. Systematic examinations of the resistivities, Seebeck coefficients, and the thermal diffusivities were conducted along the direction parallel to the applied pressure on a hot press. We confirmed that a similar ZT value was obtained by measurements along the direction
Figure 2. Powder X-ray diffraction patterns of Ba1−xKxZn2As2. Asterisks depict α-phase as an impurity in β-phase above x = 0.04.
This demonstrates the successful transition from the high temperature β-phase to the low temperature α-phase by furnace cooling. Upon doping, the ThCr2Si2-type structure (space group I4/mmm, β-phase) was stabilized for x ≥ 0.04 with αphase as an impurity of volume fraction less than 3.5%. A mixture of both α- and β-phases was obtained for x = 0.03, indicating that this is the critical composition. The crystal structure changes from α- to β-phase could be due to weakening of covalent bonding between Zn and As atoms by hole doping and ionic radius of K+ slightly larger than that of Ba2+, which can destabilize ZnAs cages surrounding Ba atoms in α-phase. The temperature dependences of resistivity and Seebeck coefficient are typical of heavily doped degenerated semiconductors (Figure 3). Resistivity increases with increasing temperature and decreases rapidly with doping from 145 (x = 0.00) to 6.8 μΩm (x = 0.02) at room temperature. A discontinuous increase is observed at x = 0.04 owing to the
Figure 3. Temperature dependences of (a) electrical resistivity and (b) Seebeck coefficient for Ba1−xKxZn2As2. 3710
DOI: 10.1021/acs.inorgchem.7b00232 Inorg. Chem. 2017, 56, 3709−3712
Article
Inorganic Chemistry
using κe = LT/ρ, where L is the Lorenz number (2.44 × 10−8 W/ΩK2). The obtained κL values range from 1.9 to 2.4 W/mK at room temperature. κL decreases with heating due to the increase of Umklapp phonon scattering and finally reaches 0.8− 1.1 W/mK around 773 K. Remarkably, κL is almost independent of K content, although the crystal structure is quite different for the α- and β-phases. The present κL is also comparable with other Sb-based Zintl compounds crystallized in the CaAl2Si2-type crystal structure different with the present α- and β-phases,2−10 despite that Ba1−xKxZn2As2 consists of As, which is lighter than the Sb atom. These suggest that covalent bonds between Zn and As atoms induce strong anharmonic interatomic potentials, which are responsible for the low κL. The obtained ZT values are shown in Figure 6. The ZT at x = 0.00 is low due to high resistivity. Those at x = 0.01 and 0.02
crystal structure transition from α- to β-phases, and then the resistivity decreases again down to 4.4 μΩm (x = 0.10). A positive value of the Seebeck coefficient indicates a p-type semiconductor. Decrease with doping is also observed for the Seebeck coefficient from 200 (x = 0.00) to 73 μV/K (x = 0.02) and finally to 26 μV/K (x = 0.10) at room temperature, indicating the increase in hole carrier density with K doping in both the α- and β-phases. The degenerated semiconducting feature in α-phase is also demonstrated by band structure calculations which exhibit a hole pocket around the Γ point upon hole doping.21 Figure 4 shows the temperature dependence of the power factor S2/ρ. The power factor for x = 0.00 is relatively low due
Figure 4. Temperature dependence of the power factor for Ba1−xKxZn2As2.
Figure 6. Temperature dependence of ZT for Ba1−xKxZn2As2.
to high resistivity. The highest value is obtained for x = 0.02, where it increases with heating, reaches the maximum value of 1.1 mW/mK2 at 660 K, then remains almost constant in the high temperature region. The high-power factor can be originated from the band degeneration of Zn and As p orbitals at valence band maximum.21 The power factor above x = 0.04, on the other hand, is relatively low. This can be due to the excess hole doping and nondegenerative band structure at valence band maximum in β-phase.20 The thermal conductivity is shown in Figure 5. κ increases with K doping from 2.0 (x = 0.00) to 3.0 W/mK (x = 0.02) at room temperature due to the increases in electronic thermal conductivity (κe). An increase is also observed in the β-phase from 3.1 (x = 0.04) to 4.0 W/mK (x = 0.10). The lattice thermal conductivity (κL) was estimated by subtracting κe from κ. The Wiedemann−Franz law was applied for calculating κe
are almost the same, reaching a maximum value of 0.67 at 900 K for x = 0.02. All other compositions exhibit similar ZT values ranging from 0.19 to 0.27 around 755 K.
4. CONCLUSION We studied the thermoelectric properties of Ba1−xKxZn2As2 above room temperature. We prepared α- and β-phase samples by solid-state reactions followed by hot pressing and furnace cooling. The highest value of ZT obtained was 0.67 at 900 K for x = 0.02 (α-phase), opening a new class of thermoelectric materials with the α-BaCu2S2-type As-based 122 Zintl compounds.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00232. Thermoelectric properties along the directions parallel and perpendicular to the pressure applied in the hotpress process and the lattice parameters of Ba1−xKxZn2As2 (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Kunihiro Kihou: 0000-0003-3065-4147 Notes
Figure 5. Temperature dependences of (a) total thermal conductivity and (b) lattice thermal conductivity for Ba1−xKxZn2As2.
The authors declare no competing financial interest. 3711
DOI: 10.1021/acs.inorgchem.7b00232 Inorg. Chem. 2017, 56, 3709−3712
Article
Inorganic Chemistry
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ACKNOWLEDGMENTS We would like to acknowledge discussions with H. Usui and K. Kuroki. This work was supported by CREST, Japan Science and Technology Agency (JST) and by the New Energy and Industrial Technology Development Organization (NEDO) through the Thermal Management Materials and Technology Research Association (TherMAT).
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REFERENCES
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DOI: 10.1021/acs.inorgchem.7b00232 Inorg. Chem. 2017, 56, 3709−3712
