Thermoelectric Properties of Cu2SnSe4 with Intrinsic Vacancy

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Thermoelectric properties of Cu2SnSe4 with intrinsic vacancy Wen Li, Siqi Lin, Xinyue Zhang, Zhiwei Chen, Xiangfan Xu, and Yanzhong Pei Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02416 • Publication Date (Web): 22 Aug 2016 Downloaded from http://pubs.acs.org on August 23, 2016

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Chemistry of Materials

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Wen Li1,#, Siqi Lin1,#, Xinyue Zhang1, Zhiwei Chen1, Xiangfan Xu2, Yanzhong Pei1, * Key Laboratory of Advanced Civil Engineering Materials of Ministry of Education, School of Materials Science and Engineering, Tongji Univ., 4800 Caoan Rd., Shanghai 201804, China. 2 School of Physics and Engineering, Tongji University, 1239 Siping Rd., Shanghai 200092, China. # These authors contribute equally to this work. *Email: [email protected]

Abstract Phonon scattering by point defects has been proven as an effective strategy for thermoelectric performance enhancements through reducing lattice thermal conductivity. This type of scattering largely relies on the mass and strain fluctuations between host and guest atoms, both of which can be maximized by vacancies as demonstrated in a few thermoelectric solid solutions showing a significant reduction in the lattice thermal conductivity. Here we show Cu2SnSe4, a new compound with intrinsic vacancies on the cation site, as a promising thermoelectric material due to the low lattice thermal conductivity of 0.6 Wm-1K-1 intrinsically resulting from the vacancies. A peak thermoelectric figure of merit (zT) of ~0.6 is achievable in this compound without relying on additional approaches such as nanostructuring or band engineering. This result demonstrates the existence of intrinsic vacancies as an important guidance for exploring new thermoelectric materials. 1. Introduction Without hazardous emissions and moving parts, thermoelectric has long been considered as a promising and environmental friendly technology for solving the crisis of the energy and environment. For a large-scale application, high performance thermoelectric materials are needed to ensure a high heat-to-electricity conversion efficiency. The performance of a thermoelectric material is determined by the figure of merit zT, zT=σS2T/(κE+κL), where S, σ, κE, κL and T are the Seebeck coefficient, the electrical conductivity, the electronic thermal conductivity, the lattice thermal conductivity and the absolute temperature, respectively. However, the coupling effects among S, σ and кE in a given material lead to the difficulty on enhancing zT1. Recently, increase of electronic performance (power factor, S2σ) for zT enhancements, through band convergence2-10 and the resonant doping11 has been demonstrated in various materials, such as PbTe2, 3, 8, 12, SnTe13, Mg2Si10, 14 and half-Heusler15 alloys. Besides this, reducing lattice thermal conductivity κL, the only one independent material property, has also been widely used to enhance zT of thermoelectric materials. This can be achieved by various approaches including alloying16, 17, nanostructuring18-21, and lattice anharmonicity22, 23, as demonstrated in various materials. Except the above mentioned strategies for enhancing the zT of existing thermoelectric materials, the exploration of new thermoelectric materials, especially for which are composed of the abundant and nontoxic elements has also attracted increasing attentions. A variety of new thermoelectric materials with high zT have been reported, such as Cu2Se24 , β-Zn4Sb325, SnSe22 and Ag8GeTe626. Interestingly, the high zT in these materials share the similarity of an intrinsically low lattice thermal conductivity due to various mechanisms including liquid-like ions24, 25, complex crystal structures26, lattice anharmonicity22 and low sound velocity26. It is reasonable to believe that a material with intrinsically low lattice thermal conductivity has a great potential for high thermoelectric performance. Introduction of point defects, especially vacancies27 or interstitial28 atoms, has been demonstrated as an effective approach for scattering phonons to reduce the lattice thermal conductivity due to the mass and strain fluctuations between host and guest atoms. In more details, alloying InSb with In2Te3 introduces cation vacancies, has shown a reduction in the lattice thermal conductivity by more than an order of magnitude27. Interstitial Cu in SnTe-Cu2Te alloys has also led to the lowest lattice thermal conductivity28 in thermoelectric

SnTe-based materials. These results guide us to explore new thermoelectric materials with intrinsic vacancies. Cu2SnSe429, 30, a compound with environmentally friendly constituent elements crystalizes in a cubic structure (space group number 216), intrinsically comes with vacancies due to an occupancy of only 3/4 on the cation sites (4a). In more details, 2/4 and 1/4 of the cation sites are occupied by Cu and Sn, respectively, leaving 1/4 of the cation sites vacant. The detailed crystal structure can be seen in Fig. 1a. It is further known that the cation sites are randomly occupied by vacancies (1/4), Sn (1/4) and Cu (1/2) 29, 31. Therefore, one can reformulate this compound as Cu2Sn□Se4, where □ denotes the cation vacancy. Quaternary compounds with similar constituent elements to Cu2Sn□Se4, such as Cu2ZnSnSe432, 33 and Cu2CdSnSe434, 35, crystalize in a tetragonal structure (space group number 121) have attracted attentions for thermoelectric applications and a peak zT of ~0.65 has been reported in Cu2CdSnSe4 with the help of nanostructuring35. The similarity in crystal structure between Cu2SnSe4 and Cu2Zn(Cd)SnSe4 is the close packing of hexagonal array of selenide anions.

½Cu; ¼Sn; ¼Vacancy;

Se

Fig. 1 Crystal structure of Cu2SnSe4

According to the above mentioned strategy of vacancy scattering of phonons for low lattice thermal conductivity, the existence of intrinsic cation vacancies in Cu2SnSe4 is expected to have a low lattice thermal conductivity and therefore a high zT. The thermoelectric properties above room temperatures of this compound have so far been rarely reported29. In this work, a detailed investigation on the thermoelectric properties of Cu2SnSe4, with and without intrinsic doping by excess or deficiency of Cu, are carried out. Due to the high concentration of intrinsic cation vacancies, a lattice thermal

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The detailed crystal structure of the Cu2SnSe4 has been well investigated previously31. It crystalizes in a cubic structure as shown in Fig. 1. The XRD results of all samples obtained in this work are shown in Fig. 2. The XRD pattern of the Cu2+δSnSe4 (-0.01≤δ≤0.1) samples can be well indexed to the corresponding cubic structure of Cu2SnSe4. Fig. 3a shows the Rietveld refinement for Cu2SnSe4, which indicates the cubic crystal structure. Energy Dispersive Spectrometer (EDS) measurements show an average Cu:Sn:Se mole ratio of 29.2:15.6:55.2≈2:1:4, which is in good agreement with the stoichiometry of this compound. According to the optical measurements (Fig. 3b), the estimated band gap for the samples in this work is about 0.43 eV, which agrees well with that of reported for cubic Cu2SnSe4 (0.35 eV)29. While this value is significantly smaller than that of 0.84 eV reported for monoclinic Cu2SnSe339. All these results support the formation of cubic Cu2SnSe4 in this work. Temperature dependent lattice thermal conductivity (κL) and figure of merit (zT) for Cu2SnSe4, Cu2SnCdSe4 and Cu2SnZnSe4 is shown in Fig. 3c and 3d, respectively. Comparing to Cu2CdSnSe4 and Cu2ZnSnSe4, compounds having very similar constituent elements, the existence of vacancies in Cu2SnSe4 leads to a much lower lattice thermal conductivity (κL) and therefore a high zT as shown in Fig. 1c and 1d, respectively. In more details, the κL of Cu2SnSe4 is at least 30% lower than that of the other two in the entire temperature range. The κL for Cu2SnSe4 is as low as ~0.7 Wm-1K-1 at 700 K. This low κL further leads to a much higher zT. Similar effect on the phonon scattering by vacancies for a low κL has also been previously observed in thermoelectric solid solutions27. (a)

10

(115)

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Fig. 2 XRD patterns for Cu2+δSnSe4 (-0.01≤δ≤0.1)

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hω (eV)

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3. Results and Discussion

(b)

Experimental Fitted Difference (hkl) lines

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κ L (W/m-K)

2. Experimental Ingots of polycrystalline Cu2+δSnSe4 (-0.02≤δ≤0.1) were synthesized in evacuated quartz ampoule by melting the stoichiometric ratio of high purity Cu, Sn and Se (>99.99%) at 1127 K for 6 hours, quenching in cold water and then annealing at 773 K for 3 days. The obtained ingots were hand ground into fine powder for the X-ray diffraction (XRD) to identify the phase composition and hot press. Pellet samples were obtained by an induction heating hot press system36 at 723 K for 20 min under a uniaxial pressure of ~80 Mpa. The resulting pellets, with a density higher than 95% of the theoretical density value, were ~12 mm in diameter and ~1.5 mm in thickness. The electrical transport properties including Seebeck coefficient, Hall coefficient (RH) and resistivity (ρ=1/σ) of the pellet samples were measured simultaneously under vacuum in the temperature range of 300–700 K, during both heating and cooling process. The Hall coefficient and resistivity were measured using the van der Pauw technique. Additionally, the Hall coefficient measurement was carried out under a reversible magnetic field of 1.5 T. The Seebeck coefficient was obtained from the slope of the thermopower vs. temperature gradients of 0~5 K37. Thermal diffusivity (D) measured by a laser flash technique with the Netzsch LFA457 system was used to calculate the thermal conductivity via κ=dCpD, where d is the density measured by a mass/volume method using the mass and geometric volume of the pellet, Cp is the heat capacity determined by the Dulong-Petit limit which is assumed to be temperature independent. Sound velocities (longitudinal and transverse branches) were measured on the pellet samples, using an ultrasonic pulse-receiver (Olympus-NDT) equipped with an oscilloscope (Keysight) at room temperature. The measure uncertainty for each transport property (S, σ and κ) is 5% approximately. The optical reflectance coefficient was measured using diffuse reflectance spectroscopy (Bruker Tensor 2 equipped with a diffuse reflectance attachment). The band gap of the samples are estimated based on these optical reflectance measurements and the measurement details have been given elsewhere38.

Intensity (a.u.)

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conductivity as low as 0.6 Wm-1K-1 is obtained by nature. This further leads to a peak zT as high as 0.6. Moreover, the underlying physics of this material including the scattering mechanism, deformation potential coefficient, effective mass and optimal carrier concentration, are discussed. This work demonstrates the potential of compounds with intrinsic vacancies for thermoelectric applications.

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0.05 300

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Fig. 3 (a) XRD pattern with a Rietveld refinement for Cu2SnSe4 and (b) Normalized optical absorption versus photon energy at room temperature for Cu2+δSnSe4 (-0.01≤δ≤0.1). Temperature dependent (c) lattice thermal conductivity (κL) and (d) figure of merit (zT) for Cu2SnSe4, Cu2SnCdSe4 and Cu2SnZnSe4.

It is normally required to optimize the carrier concentration40 for realizing the potentially high thermoelectric figure of merit, although the lattice thermal conductivity is low. This is also important to understand the fundamental physics of the material. Here, either an excess or a deficiency of Cu is intentionally used to tune the carrier concentration of Cu2SnSe4 for optimizing the electronic performance. This enables a hole concentration ranging from


2

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Fig. 4. Temperature dependent (a) Hall coefficient (RH) and (b) Hall mobility (µH) for Cu2+δSnSe4 (-0.01≤δ≤0.03), indicating the scattering of carriers dominated by acoustic phonons. (c) Hall carrier concentration (nH) dependent Seebeck coefficient at 300 K and 500 K for Cu2+δSnSe4 (-0.01≤δ≤0.1), showing consistence with the single Kane band model with acoustic scattering. (d) Temperature dependent density of state effective mass (m*) and deformation potential coefficient (Edef) for Cu2+δSnSe4 (-0.01≤δ≤0.1).

the nH-dependent Seebeck coefficient at these temperatures. Based on the SKB model, the temperature dependent density of state effective mass (m*) and the strength of the carrier scattering by acoustic phonons (deformation potential coefficient, Edef44, 45) for Cu2+δSnSe4 (-0.01≤δ≤0.06) are estimated42 and shown in Fig. 4d. A m*~0.89 me and a Edef ~21 eV are obtained for Cu2SnSe4 at room temperature, which is in good agreement with the literature (m*=0.8 me)29. It is found that both m* and Edef is nearly independent of the carrier concentration, indicating a rigid band behavior for Cu2+δSnSe4. It is normally believed that a wide band gap semiconductor tends to have a heavy effective mass because of its weak interaction between the conduction and valence bands. The increased effective mass at high temperature observed in this work is probably due to the increase of band gap resulting from thermal expansion. Similar temperature dependence on the effective mass has been observed in lead chalcogenides41, 46-48 and CuGaTe249. The deformation potential coefficient (Edef) characterizes the energy fluctuation of the band edge per unit change of the volume caused by the lattice vibrations. Since thermal expansion usually have complicated effects on both the band structure and the modes of the lattice vibrations, the temperature dependence of deformation potential coefficient has been rarely discussed in the literature. The decrease in Edef with increasing temperature has also been observed in CuGaTe249. 2 (b)10

(a) 400

300

Ω cm) ρ (mΩ

3.3×1018 to 9.2×1020 cm-3 at room temperature, as determined by the Hall coefficient measurements. The Hall carrier concentration (nH=1/eRH, e is the electron charge) for Cu2+δSnSe4 (-0.01≤δ≤0.03) increases with increasing Cu content. As for the samples with δ≥0.06, the extremely high carrier concentrations lead to a Hall voltage too small to be measured accurately at high temperatures, and therefore the discussion on the transport properties are focused on the samples with δ5% for all the samples with different compositions. The lowest κL for Cu2+δSnSe4 here is about 0.6 W/m-K, which approaches the κLmin, and therefore there is small room for the enhancement of thermoelectric performance through reducing κL. Temperature dependent zT for Cu2+δSnSe4 (-0.02≤δ≤0.1) is shown in Fig. 6. The zT of all the samples increases with increasing temperature, reaching a peak value of ~0.6. The high zT mainly comes from the low κL due to the strong phonon scattering by intrinsic cation vacancies. This zT is actually comparable with the reported highest one of the

quaternary analogue compound Cu2CdSnSe4, but with the help of nanostructuring35. 0.6

Cu2+δSnSe4 δ = -0.02 δ = -0.01 δ = -0.006 δ=0 δ = 0.03 δ = 0.06 δ = 0.1

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0.3 0.2 0.1 0.0

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T (K)

Fig. 6 Temperature dependent zT for Cu2+δSnSe4 (-0.02≤δ≤0.1).

Summary Guided by the strategy of vacancy scattering for low lattice thermal conductivity, a new cheap thermoelectric compound, cubic Cu2SnSe4, with intrinsic cation vacancy is demonstrated to be promising for thermoelectric applications. Comparing to the other analogue materials without intrinsic vacancies, Cu2SnSe4 indeed shows the lowest lattice thermal conductivity and therefore the highest zT in the entire temperature range, due to existence of 1/4 vacant cation sites. A further carrier concentration optimization realizes a peak zT of ~0.6, indicating its potential for thermoelectric applications. Utilization of other independent approaches such as band engineering should in principle lead to a further thermoelectric performance improvement. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 51422208, 11474219 ， 51401147), the national Recruitment Program of Global Youth Experts (1000 Plan), the fundamental research funds for the central universities.


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Table of Contents 0.6

Cu2+δSnSe4 δ = -0.02 δ = -0.01 δ = -0.006 δ=0 δ = 0.03 δ = 0.06 δ = 0.1

0.5 0.4

zT

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

0.3 0.2 0.1 0.0

½Cu; ¼Sn; ¼Vacancy;

Se

300

400

500

600

T (K)


700

Thermoelectric Properties of Cu2SnSe4 with Intrinsic Vacancy

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