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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Doping-Induced Polymorph and Carrier Polarity Changes in Thermoelectric Ag(Bi,Sb)Se2 Solid Solution Kenta Sudo,†,# Yosuke Goto,*,† Ryota Sogabe,† Kazuhisa Hoshi,† Akira Miura,‡ Chikako Moriyoshi,§ Yoshihiro Kuroiwa,§ and Yoshikazu Mizuguchi† †
Department of Physics, Tokyo Metropolitan University, Hachioji 192-0397, Japan Faculty of Engineering, Hokkaido University, Kita-13, Nishi-8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan § Department of Physical Science, Hiroshima University, 1-3-1 Kagamiyama, Higashihiroshima, Hiroshima 739-8526, Japan Downloaded via UNIV OF LOUISIANA AT LAFAYETTE on May 12, 2019 at 00:24:16 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: Silver bismuth diselenide (AgBiSe2) is an n-type thermoelectric material that exhibits a complex structural phase transition from the hexagonal to cubic phase, while silver antimony diselenide (AgSbSe2) is a p-type thermoelectric material that crystallizes in the cubic phase at all temperatures. Here, we investigate the crystal structure and thermoelectric properties of Ag(Bi,Sb)Se2 solid solution, employing AgBi0.9Sb0.1Se2 and AgBi0.7Sb0.3Se2 as representative samples. The carrier polarity of AgBi0.9Sb0.1Se2 is converted from the n-type to ptype by Pb doping, accompanied by a polymorphic change to the cubic phase. It is difficult to obtain highly conductive p-type hexagonal AgBiSe2-based materials, although first-principles calculations predict high-performance thermoelectric properties for these systems. We also demonstrate that cubic AgBi0.7Sb0.3Se2 undergoes a polymorphic change to the hexagonal phase upon Nb doping. The present study show that polymorphic changes inevitably occurred upon Pb/Nb doping to optimize thermoelectric properties of Ag(Bi,Sb)Se2 solid solution.
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INTRODUCTION Thermoelectric generators, which directly convert a temperature difference into electricity, are a feasible means of harvesting energy from waste heat. The maximum efficiency of a thermoelectric device is primarily governed by the material’s dimensionless figure of merit, ZT = S2Tρ−1κ−1, where T, S, ρ, and κ are the absolute temperature, Seebeck coefficient, electrical resistivity, and thermal conductivity, respectively. Achieving high ZT is often hindered due to contradicted relationship among these electrical and thermal transport properties. A promising approach is to identify materials with low intrinsic thermal conductivities, followed by optimization of the electrical power factor, PF = S2ρ−1, by tuning the carrier concentration and electronic structure in order to maximize the figure of merit.1−3 Silver pnictogen diselenides, AgBiSe2 and AgSbSe2, have been gaining interest for thermoelectrics due to their intrinsically low lattice thermal conductivity (less than 1 W m−1 K−1 at 300 K) and the natural abundance of the constituent elements.4−22 On one hand, AgBiSe2 crystallizes in the hexagonal Pm3̅1 space group (hexaphase) at 300 K and undergoes a structural phase transition to the rhombohedral R3̅m space group (rhombo-phase) at ∼500 K and to the cubic Fm3̅m space group (cubic-phase) at ∼580 K (see Figure 1). The thermoelectric properties of n-type AgBiSe2 have been optimized by electron doping via aliovalent ion substitution,8,9 leading to a dimensionless figure of merit close to unity. © XXXX American Chemical Society
Figure 1. Crystal structures of hexagonal, rhombohedral, and cubic AgBiSe2.
Furthermore, a remarkably high thermoelectric performance in p-type hexagonal AgBiSe2 has been predicted using firstprinciples calculations, due to its anisotropic hole pocket in the valence band.10 Although p-type hexagonal AgBiSe2 has been prepared by solution synthesis,5,6 its thermoelectric properties were yet to be optimized. On the other hand, AgSbSe2 is known as a p-type semiconductor that crystallizes in the cubic Fm3̅m space group at all temperatures. The thermoelectric Received: April 10, 2019
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DOI: 10.1021/acs.inorgchem.9b01038 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 2. SPXRD patterns, chemical composition, and SEM images of Ag(Bi,Sb)Se2. (a, d) SPXRD patterns of nondoped hexagonal AgBi0.9Sb0.1Se2 (a) and cubic AgBi0.7Sb0.3Se2 (d). The wavelength of the radiation beam was determined to be 0.496345(1) Å. The circles and solid curve represent the observed and calculated patterns, respectively, and the difference between the two results is shown at the bottom. The vertical marks indicate the Bragg diffraction positions for the hexagonal and cubic phases. For part a, Bragg diffraction positions for the rhombohedral phase are also denoted at the bottom. The inset shows expanded profiles between 13° and 15°. (b, e) Relationship between analyzed and nominal content of Pb (b) and Nb (e) in AgBi0.9Sb0.1Se2 and AgBi0.7Sb0.3Se2. Chemical compositions of the samples were calculated assuming Se content of 2. The black dashed line denotes the ideal result calculated by assuming that the nominal content is equal to the analyzed content. (c, f) SEM images of AgBi0.7Sb0.3Se2 doped with Pb (c) and Nb (f). For Nb-doped AgBi0.7Sb0.3Se2, an amorphous-like Nb-rich secondary phase is denoted by yellow circles.
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properties of AgSbSe2 were optimized by substituting aliovalent ions or by introducing an Sb deficiency, leading to a ZT value of unity at ∼600 K.14−22 Considering the p-type polarity of AgSbSe2, the use of solid solutions of Ag(Bi,Sb)Se2 is a promising approach for achieving p-type hexagonal AgBiSe2-based materials. The phase diagram of Ag(Bi,Sb)Se2 solid solution shows that the cubic phase is stable at Sb contents of >20% at 300 K.23 However, detailed thermoelectric properties have not yet been reported. In addition, the thermoelectric properties in the vicinity of structural phase transitions from high-symmetry structures are also of interest. Inorganic materials with high symmetry are promising compounds for thermoelectrics because of the degeneracy of their electronic band structure, which leads to a high carrier mobility and a large density-ofstates effective mass.24−31 However, recent studies on GeTe highlighted that slight symmetry reductions in the vicinity of high-symmetry structures are also promising because of the cooccurrence of band convergence and reduced thermal conductivity.32,33 To study the crystal structure and thermoelectric properties of Ag(Bi,Sb)Se2 solid solutions, we begin by investigating AgBi0.9Sb0.1Se2 and AgBi0.7Sb0.3Se2 as representative samples. We report that the carrier polarity of AgBi0.9Sb0.1Se2 is converted from n-type to p-type upon Pb doping, accompanied by a polymorphic change to the cubic phase. Alternatively, we also demonstrate that cubic AgBi0.7Sb0.3Se2 undergoes a polymorphic change to the hexagonal phase upon Nb doping. The present study show that polymorphic changes inevitably occurred upon Pb/Nb doping to optimize thermoelectric properties of Ag(Bi,Sb)Se2 solid solution.
METHODS
Polycrystalline samples of Ag(Bi 1 − x Pb x ) 0 . 9 Sb 0 . 1 Se 2 , Ag(Bi1−xPbx)0.7Sb0.3Se2, Ag1−xNbxBi0.9Sb0.1Se2, and Ag1−xNbxBi0.7Sb0.3Se2 (x = 0, 0.05, 0.10, 0.15, 0.20) were prepared using elemental Ag (99.9%), Bi (99.999%), Pb (99.9%), Sb (99.9%), Se (99.999%), and Nb (99.9%) as starting materials. A stoichiometric ratio of these starting materials was mixed, pressed into a pellet, and then heated at 500 °C for 15 h in an evacuated quartz tube. To obtain dense samples, the samples were hot pressed at ∼500 °C at 50 MPa in an Ar atmosphere (S. S. Alloy, PLASMAN CSP-KIT-02121). The relative density of the obtained samples was calculated as >95%. Synchrotron powder X-ray diffraction (SPXRD) measurements were acquired at room temperature using the BL02B2 beamline of SPring-8 (Proposal Number 2018B1246). Diffraction data were collected using a high-resolution one-dimensional semiconductor detector, the multiple MYTHEN system.34 The wavelength of the radiation beam was determined to be 0.496345(1) Å using a CeO2 standard. The crystal structure parameters were refined via the Rietveld method using RIETAN-FP software,35 and the crystal structure was visualized using VESTA software.36 The sample morphology and chemical composition were examined using a scanning electron microscope (SEM; Hitachi, TM3030) equipped with an energy dispersive X-ray spectrometer (EDX; Oxford, SwiftED3000). Chemical compositions of the samples were calculated assuming a Se molar ratio of two. The electrical resistivity ρ and the Seebeck coefficient S were measured using the conventional four-probe geometry (Advance Riko, ZEM-3) in a He atmosphere. The thermal conductivity was calculated using κ = DCpds, where D, Cp, and ds are the thermal diffusivity, specific heat, and sample density, respectively. The thermal diffusivity was measured by a laser flash method (Advance Riko, TC1200-RH), and the specific heat was calculated from the Dulong− Petit relationship to avoid uncertainties in the specific heat measurement. B
DOI: 10.1021/acs.inorgchem.9b01038 Inorg. Chem. XXXX, XXX, XXX−XXX
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RESULTS AND DISCUSSION Chemical Composition and Crystal Structure. Figure 2a shows SPXRD patterns for nondoped AgBi0.9Sb0.1Se2. Almost all diffraction peaks can be assigned to those of the hexaphase. The proportion of the rhombo-phase was found to be 13.2 wt %, which is in reasonable agreement with previously reported crystal structure analyses for AgBiSe2.11,13 Refinement results including structural parameters and reliability factors are listed in Table S1 in the Supporting Information. Figure 2b shows the Pb content determined by EDX. As the nominal Pb content increased, the analyzed Pb content monotonically increased. The SEM image shown in Figure 2c displays the homogeneity of the 10%-Pb-doped sample at the micrometer scale. SPXRD patterns for Pb-doped AgBi0.9Sb0.1Se2 are shown in Figure 3a. It is evident that Pb doping into hexagonal
revealed that an Nb-rich secondary phase was precipitated in the Nb-doped samples, as presented in Figure 2f. Because Bragg diffraction due to such impurities was not observed by SPXRD, as shown in Figure 3b, this Nb-rich secondary phase is likely attributable to an amorphous-like phase. In contrast to Pb doping, which stabilizes the cubic phase, Nb doping stabilizes the hexaphase. Namely, 5%-Nb doping into cubic AgBi0.7Sb0.3Se2 results in a polymorphic change to the hexaphase. The lattice parameter a (c) tends to decrease (increase) with increasing nominal Nb content, as shown in Figure 3d,e. This behavior is slightly different from a previous report on an Ag1−xNbxBiSe2 system, where both a and c decrease with increasing Nb content.7 It is difficult to determine the Nb content using the Rietveld refinement, most likely due to small amount of Nb and similar electron number of Ag and Nb. Hereafter, we use the nominal content for simplicity. Thermoelectric Properties. Figure 4a,b shows the temperature dependence of the electrical resistivity and
Figure 3. Doping dependence of SPXRD patterns (a, b) and lattice parameters (c−e) for AgBi0.9Sb0.1Se2 and AgBi0.7Sb0.3Se2.
AgBi0.9Sb0.1Se2 induces a polymorphic change to the cubic phase. As shown in Figure 2e, the lattice parameter a increases with increasing Pb content, indicating that Pb ions were incorporated in the AgBi0.9Sb0.1Se2 phase. Unlike AgBi0.9Sb0.1Se2, the SPXRD pattern of nondoped AgBi0.7Sb0.3Se2 is indexed as a cubic phase, as shown in Figure 2d. The analyzed Pb content, SPXRD patterns, and lattice parameters for Pb-doped AgBi0.7Sb0.3Se2 are shown in Figures 2b and 3a,c, respectively. In general, these characterizations indicate a similar sample quality for the Pb-doped AgBi0.7Sb0.3Se2 and AgBi0.9Sb0.1Se2. Nb content for Ag1−xNbxBi0.9Sb0.1Se2 with x = 0.05 is evaluated to be 0.03 using EDX, indicating that several Nb are actually incorporated in the AgBi0.9Sb0.1Se2-based phase. However, the analyzed Nb content in Nb-doped samples deviates from the nominal content and remains almost constant, as shown in Figure 2e. SEM and EDX analyses
Figure 4. Temperature dependence of thermoelectric carrier transport properties. Electrical resistivity (ρ) and Seebeck coefficient (S) of Ag(Bi1−xPbx)0.9Sb0.1Se2 (a, b), Ag(Bi1−xPbx)0.7Sb0.3Se2 (c, d), Ag1−xNbxBi0.9Sb0.1Se2 (e, f), and Ag1−xNbxBi0.7Sb0.3Se2 (g, h).
Seebeck coefficient for Pb-doped AgBi0.9Sb0.1Se2. We present measurement results for Pb-doped samples up to ∼600 K because the samples appear to be unstable at higher temperatures probably due to Se volatilization. The sign of the Seebeck coefficient for nondoped AgBi0.9Sb0.1Se2 is negative over the entire measured temperature range, which is indicative of n-type polarity. The carrier polarity switches to C
DOI: 10.1021/acs.inorgchem.9b01038 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 5. Temperature dependence of the total thermal conductivity (κtotal) (a) and lattice thermal conductivity (κlattice) (b) of AgBi0.9Sb0.1Se2 and AgBi0.7Sb0.3Se2 doped with Pb and Nb. The doping content x is not shown for simplicity. The thermal conductivity as a function of x at 300 K is shown in Figure S1.
Figure 6. Temperature dependence of the dimensionless figure of merit (ZT) for AgBi0.9Sb0.1Se2 and AgBi0.7Sb0.3Se2 doped with Pb (a, b) and Nb (c, d).
using Pb is compensated by defect formation. However, detailed study on defect chemistry for Ag(Bi,Sb)Se2 is beyond the scope of the present study. To enhance p-type conductivity in Pb-doped samples, we examined Pb-doped AgBi0.7Sb0.3Se2 because AgSbSe2 is known as a p-type semiconductor. As shown in Figure 4c,d, Pb doping changes the carrier polarity of AgBi0.7Sb0.3Se2 from n-type to ptype, as in AgBi0.9Sb0.1Se2. However, the Seebeck coefficient remains as high as >200 μV K−1 for Pb-doped AgBi0.7Sb0.3Se2, and the electrical resistivity of these samples is ∼10−3 Ω m. It is possible that the band structure of AgBiSe2 is tuned by alloying with AgSbSe2 in terms of band convergence. To examine this possibility, the thermoelectric properties of Nb-
p-type upon Pb doping. The room-temperature Seebeck coefficient of 20%-Pb-doped AgBi0.9Sb0.1Se2 decreased to +150 μV K−1, which is comparable to that of typical thermoelectric materials. However, the electrical resistivity remains high, at approximately ∼10−3 Ω m. The power factor S2ρ−1 was evaluated to be less than 0.2 mW m−1 K−2. These results show that it is still difficult to reduce the electrical resistivity of p-type hexagonal AgBi0.9Sb0.1Se2 by Pb doping, despite theoretical predictions of high thermoelectric performance for p-type hexagonal AgBiSe2-based materials.10 Namely, polymorphic changes inevitably occur before the electrical resistivity decreases to less than 10−4 Ω m, a typical value for thermoelectric materials. We deduce that aliovalent ion doping D
DOI: 10.1021/acs.inorgchem.9b01038 Inorg. Chem. XXXX, XXX, XXX−XXX
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doping. The present study show that polymorphic changes inevitably occurred upon Pb/Nb doping to optimize thermoelectric properties of Ag(Bi,Sb)Se2 solid solution.
doped n-type AgBi0.9Sb0.1Se2 and AgBi0.7Sb0.3 Se2 were investigated, as shown in Figure 4e−h. The electrical resistivity decreased upon Nb doping, as previously reported for AgBiSe2, and the absolute value of the Seebeck coefficient was reduced. The calculated power factor reaches approximately 0.5−0.6 mW m−1 K−2, which is comparable to that of Nb-doped AgBiSe2, suggesting that the conduction band structure was not significantly affected by alloying with AgSbSe2. Notably, the carrier transport of the Nb-doped samples is almost independent of the nominal Nb content because the analyzed Nb content of these samples remained nearly constant, ∼0.05, as shown in Figure 2e. Figure 5a presents the temperature dependence of the thermal conductivity. The thermal conductivity of the examined samples is 0.4−0.8 W m−1 K−1 over the measured temperature range. It should be noted that the doping content was not denoted for simplicity. The thermal conductivity as a function of x at 300 K is shown in Figure S1. The thermal conductivity is primarily determined by lattice and electrical contributions. The electrical thermal conductivity can be evaluated using the Wiedemann−Franz relationship, κel = Lρ−1T, where L is the Lorentz number. Here, we used L = 1.5 × 10−8 V2 K−2 to evaluate κel. The lattice thermal conductivity was obtained by subtracting the electronic component from the total thermal conductivity. Figure 5b shows the lattice thermal conductivity as a function of temperature. Previous studies on I−V−VI2 compounds have reported that the cubic phase has a lower lattice thermal conductivity than other crystal structures due to amplified anharmonicity arising from the presence of lone-pair electrons.8 Furthermore, very recent studies show the local structural distortion by the lone-pair experimentally.37,38 However, the present results indicate that the hexagonal and cubic structures show comparably low lattice thermal conductivities, as shown in Figure S1 in Supporting Information, most likely because of conventional point defect scattering in the Ag(Bi,Sb)Se2 solid solution, the so-called alloy effect.13 Figure 6 shows the dimensionless figure of merit ZT. p-type polarity was obtained by Pb doping in AgBi0.9Sb0.1Se2 and AgBi0.7Sb0.3Se2. However, the thermoelectric performance is very poor because of the high electrical resistivity. For Ag1−xNbxBi0.9Sb0.1Se2, ZT reaches 0.7 at approximately 700 K, which is comparable to that of Nb-doped n-type AgBiSe2. In general, first-principles calculations provide a powerful means for evaluating thermodynamically stable phases. For example, the stability of various I−V−VI2 compounds has been discussed in terms of amplified anharmonicity due to the presence of lone-pair electrons.8,37,38 However, because the present samples are solid solutions, first-principles calculations in which the crystal structure is mimicked are computationally demanding. Therefore, an investigation of the polymorphic changes induced by doping using first-principles calculations is beyond the scope of the present study.
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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.9b01038.
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Refinement results including structural parameters and reliability factors and thermal conductivity as a function of x at 300 K (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yosuke Goto: 0000-0002-2913-2897 Akira Miura: 0000-0003-0388-9696 Present Address #
K.S.: High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by JST-CREST (Grant No. JPMJCR16Q6), a Grant-in-Aid for Scientific Research (Grant Nos. 15H05886, 16H04493, and 19K15291), and the Iketani Science and Technology Foundation (Grant No. 0301042-A), Japan.
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REFERENCES
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CONCLUSIONS In summary, we have investigated the crystal structure and thermoelectric properties of AgBi0.9Sb0.1Se2 and AgBi0.7Sb0.3Se2 as representatives of Ag(Bi,Sb)Se2 solid solution. The carrier polarity of AgBi0.9Sb0.1Se2 is converted from n-type to p-type upon Pb doping, accompanied by a polymorphic change to the cubic phase. It is difficult to obtain highly conductive p-type hexagonal AgBiSe2-based materials; however, AgBi0.7Sb0.3Se2 undergoes a polymorphic change to the hexaphase by Nb E
DOI: 10.1021/acs.inorgchem.9b01038 Inorg. Chem. XXXX, XXX, XXX−XXX
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