Crystal structure and bonding characteristics of In-doped β-Zn4Sb3

Journal of Solid State Chemistry 193 (2012) 89–93

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Crystal structure and bonding characteristics of In-doped b-Zn4Sb3 Dingguo Tang a,b, Wenyu Zhao a, Sudan Cheng a, Ping Wei a, Jian Yu a, Qingjie Zhang a,n a b

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China Key Laboratory of Catalysis and Materials Science of the State Ethnic Affair Commission & Ministry of Education, South-Central University for Nationalities, Wuhan 430074, China

a r t i c l e i n f o

abstract

Available online 5 April 2012

The effects of indium impurity on the crystal structure and bonding characteristics of In-doped bZn4Sb3 were investigated by powder X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The XRD Rietveld refinement indicates that the indium impurity preferentially substitutes one of Sb atoms in Sb–Sb dimer at the 12c Sb(2) site and simultaneously leads to the increase of Zn occupancy. The observations of binding energy shift and a new valence state in Sb 3d core-level XPS spectra can be attributed to the charge transfer from In and Zn to Sb. As a result, more electropositive Zn atoms are needed to maintain the charge balance. The reduction of the lattice thermal conductivity is ascribed to the formation of the asymmetric Sb–In bond, resulting in much low lattice thermal conductivity of 0.49 W  1 K  1 of Zn4Sb2.96In0.04. & 2012 Elsevier Inc. All rights reserved.

Keywords: b-Zn4Sb3 Indium doping Rietveld refinement X-ray photoelectron spectroscopy

1. Introduction In the past decades, thermoelectric (TE) materials have attracted a great deal of attention for their possible applications in commercial energy conversion and power generation. Among them, b-Zn4Sb3 is considered as competitive candidate p-type TE material for high-efficient thermoelectric generators because it boasts of large thermoelectric dimensionless figure of merit ZT of 1.3 at 670 K [1–3]. The ZT, defined as ZT ¼ a2sT/k, is dependent for the conversion efficiency of thermoelectric generator, here a is the Seebeck coefficient, s the electrical conductivity, T the absolute temperature, k the thermal conductivity and a2s the power factor. The power factor of b-Zn4Sb3 is reasonably high and reaches 1.31 mW m  1 K  2 at 673 K [4], nonetheless what makes b-Zn4Sb3 a remarkable high ZT thermoelectric material is the extraordinary low thermal conductivity (e.g., 0.5 W m  1 K  1 at 673 K [5]). The thermal conductivity is the sum of the lattice thermal conductivity kL and the carrier thermal conductivity kC, and the kL is dominant for b-Zn4Sb3. The exceptionally low lattice thermal conductivity of b-Zn4Sb3 is essentially attributed to the strong phonon scattering induced by the complicated crystal structure. The core structure of b-Zn4Sb3 contains three distinct atom sites, namely 36f Zn(1), 18e Sb(1), and 12c Sb(2) in space group R-3c [6,7]. The three-interstitial model proposed by Snyder et al. indicates that Sb3  and Sb2  ions occupy the 18e Sb(1) and 12c Sb(2) sites, respectively. Whereas Zn2 þ ions may occur at the

n

Corresponding author. fax: þ 86 27 87867199. E-mail addresses: [email protected] (W. Zhao), [email protected] (Q. Zhang). 0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2012.03.059

interstitial site besides the 36f Zn(1) site [8]. The structure of Sb atoms provides a crystalline framework suitable for electron transfer, and the diffuse, disordered Zn atoms (including interstitial Zn atoms and Zn vacancies) scatter phonons and reduce the lattice thermal conductivity. Therefore, b-Zn4Sb3 is a kind of ‘‘phonon-glass electron-crystal’’ (PGEC) semiconductor which is very suitable for thermoelectric conversion [9]. The origin of the low lattice thermal conductivity of b-Zn4Sb3 is in debate. Snyder et al. reported that the diffuse, disordered Zn atoms resulted in the low lattice thermal conductivity of b-Zn4Sb3 [8,10]. Schweika et al. considered that the low thermal conductivity of b-Zn4Sb3 could originate from the dynamical disorder or so-called localized dumbbell vibration due to the anharmonic vibration of the Sb–Sb dimers in b-Zn4Sb3 on the basis of the inelastic neutron scattering spectra and the heat capacity ¨ measurements [11]. Haussermann et al. suggested that the remarkably low lattice thermal conductivity of b-Zn4Sb3 might result from the localized multicenter bonding in the electron-poor framework semiconductors [12]. In the paper, the influence of In substitution for Sb in b-Zn4Sb3 on the lattice thermal conductivity was discussed. We ensure that the asymmetric dumbbell vibration of Sb–In dimers induced by the In substitution for Sb at the 12c Sb(2) site plays important role in reducing the lattice thermal conductivity of the In-doped Zn4Sb3. This provides a new explanation for the remarkable low lattice thermal conductivity of b-Zn4Sb3. Powder X-ray diffraction (XRD) and Rietveld refinement method were used to investigate the lattice distortion of In-doped Zn4Sb3. X-ray photoelectron spectroscopy (XPS) analysis was carried out to reveal the effects of indium impurity on the binding characteristics and elemental chemical states of In-doped Zn4Sb3.

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2. Material and methods In-doped Zn4Sb3 with nominal composition Zn4Sb3  mInm (0rm r0.18) were synthesized by the combination of meltquenching (MQ) and spark plasma sintering (SPS) techniques. Highly pure powder metals of Zn (5N), Sb (4N), and In (4N) were used as raw materials. The mixture of these raw materials in stoichiometric ratio was sealed into a silica tube under vacuum. The tube was heated up and kept at 1023 K for 2 h, after that was cooled down quickly to room temperature in oil. The obtained ingots were then ground into fine powder and sintered into polycrystalline bulk material by SPS (Dr Sinter: SPS-1050) at 673 K for 3 min in an alloy die. The bulk materials were ground into uniform powders for X-ray diffraction (XRD) measurement. XRD data were acquired by BRUKER D8 Advance diffractometer with Cu Ka radiation. The accelerating voltage, applied current, scanning speed, step-by-step scanning size, DS-SS-RS slit, and scanning range were 40 kV, 40 mA, 4.26721 min  1, 0.0171, 1.01– 2.01 6.6 mm, and 101–1351, respectively. Rietveld refinements of the XRD data based on the three-interstitial model were carried out using GSAS [13] and EXPGUI [14] softwares. The Pseudo-Voigt function, a weighted sum of Gaussian and Lorentzian functions, was used as the peak profile function for the Rietveld refinement. The bulk materials were cut into circular sheets of 8 mm  2 mm (diameter  thickness) for X-ray photoelectron spectroscopy (XPS) analysis. The sheets were polished carefully to ensure flat surface, and washed in acetone by an ultrasonic cleaner. They remained in acetone during transfer into a N2-atomosphere glove box attached to the XPS spectrometer. The sheets were then taken out of the solution, flashed with high-pure N2 and dried in N2 stream before their insertion into the fast entry chamber of the apparatus, to avoid extensive oxidation of the surface of sample. XPS spectra were acquired by THERMO-VG Multilab 2000 spectrometer under a vacuum of 3  10  7 Pa. The radiation source is Mg Ka (hn ¼1253.6 eV) with a power of 400 W, namely, 15 kV voltage and 27.5 mA current. Prior to the first scanning, low-energy Ar þ ions sputtering was conducted to eliminate the contaminant on the sample surface. Survey scans were acquired at pass energy of 100 eV and step size of 1 eV, while for the core level scans pass energy and step size of 25 eV and 0.05 eV were adopted, respectively. Binding energies were reproducible to within 70.2 eV and C 1s photoelectron peak of the adventitious carbon at 284.6 eV was used as a reference for charge shift calibration. The curve-fittings of core level photoelectron spectra were proceeded with the software Avantage provided by Thermo Fisher-VG Corporation. A smart background subtraction was used for all the spectra. The decomposition of peak was made using mixed Gaussian–Lorentzian functions because the basic shape of XPS peak was Lorentzian and modified by instrumental and other factors with Gaussian contribution [15]. The values of full width at half maximum (FWHM) were restricted to be close for same core level photoelectron peak during the curve-fitting. Surface elemental composition was determined by integrating the Zn 2p, Sb 3d, In 3d and C 1s curves and converting these values to relative atomic ratios using theoretical sensitive factors provided by the manufacturer of XPS apparatus [16]. Thermal conductivity k was calculated using the equation k ¼ lrCp, where Cp is the specific heat capacity, r the bulk density of material and l the thermal diffusivity coefficient. The l was measured by a laser flash technique (Netzsch LFA 427) in a flowing Ar atmosphere. The Cp was measured using differential scanning calorimeter. The r was obtained by the Archimedes method. Lattice thermal conductivity kL was obtained by subtracting the carrier contribution from k using the equation kL ¼ k  kC. Here, the carrier thermal conductivity kC is expressed by the Wiedemann–Franz equation kC ¼ sLT, where L is the Lorenz number with a value of 2.45  10  8 V2 K  2 [12], and s is the

electrical conductivity. The s was measured with the standard four-probe method (Sinkuriko: ZEM-1) in Ar atmosphere.

3. Results and discussion A phase of rhombohedral structure b-Zn4Sb3 with space group R-3c is identified by XRD for all resultant samples with nominal composition Zn4Sb3  mInm (0 rmr0.18) Some weak peaks of InSb have been detected when m is more than 0.10, indicating that the upper limit of m is close to 0.10. The refinement factors of pure b-Zn4Sb3 are close to those in the literatures as shown in Table 1, showing the Rietveld refinement strategies used in the paper are reasonable. To determine the location of the indium impurity, Rietveld refinements of XRD data for the single-phase Zn4Sb3  mInm (0.02rmr0.10) are carried out based on the threeinterstitial model, assuming that the indium impurity individually occupies the 18e Sb(1) site or 12c Sb(2) site in b-Zn4Sb3. It is found that the refinement reliability factors w2, Rwp and Rp, based on the assumption that the indium impurity occupies the 12c Sb(2) site, are always smaller than those based on the case at the 18e Sb(1) site, as shown in Table 1. Therefore, it can be concluded that the indium impurity preferentially occupies the 12c Sb(2) site in Zn4Sb3  mInm. The Rietveld refinements of Zn4Sb3 mInm (m¼0,0.02,0.04,0.10) based on the three-interstitial model and the assumption of the In substitution for Sb at the 12c Sb(2) site are shown in Fig. 1. The differences between the measured data and the calculated data are shown at the bottoms of the plots. The good reliabilities of the Rietveld refinement method are achieved for both pure and In-doped Zn4Sb3 compounds. The lattice parameters a, c and V of Zn4Sb3 mInm from the Rietveld refinement are listed in Table 2. It can be seen that a increases and c decreases with increasing the m in the range of 0–0.10, and as a result, cell volume becomes larger by the In doping, indicating that the In dopant has entered into the lattice and makes the unit cell expand. Fig. 2(a) shows the occupancies of Zn atoms at the 36f Zn(1) and interstitial sites in Zn4Sb3  mInm (0 rmr0.10). The total occupancy of Zn atom is calculated and labeled as sum. It can be seen that the occupancy of Zn atom at the Zn(1) site increases slightly from 0.9035 to 0.9168, while the occupancy of the interstitial Zn atom increases significantly from 0.1744 to 0.2143 with increasing the m in the range of 0–0.10. The atomic occupancies of Zn4Sb3  mInm are listed in Table SI as supplementary information, and the uncertainties are indicated in the parentheses after the occupancies. The total occupancy of Zn

Table 1 Refinement reliability factors w2, Rp and Rwp of Zn4Sb3  mInm on the assumption that the indium impurity occupies the 18e Sb(1) site or 12c Sb(2) site individually. Zn4Sb3  mInm m¼ 0.02 m¼ 0.04 m¼ 0.06 m¼0.08 m ¼0.10 b-Zn4Sb3 Our work

Ref.n

4.65 [10]

w2

18e

3.20

3.50

3.80

4.92

5.27

2.55

Rp/(%)

12c 18e

2.75 3.78

3.12 3.75

2.99 3.73

4.39 4.28

4.79 4.35

– 3.36

12c Rwp/(%) 18e

3.50 5.23

3.54 5.34

3.47 5.63

4.02 6.31

4.09 6.60

– 4.54

12c

4.85

5.05

5.00

5.96

6.30

–

2.03 [17] – 2.68 [17] –

n Refinement reliability factors w2, Rp and Rwp of Zn4Sb3 from the reference [10] and [17] are shown for comparison.

D. Tang et al. / Journal of Solid State Chemistry 193 (2012) 89–93

91

Fig. 1. Rietveld refinement of powder X-ray diffraction data of Zn4Sb3  mInm (m¼ 0,0.02,0.04,0.10) based on the three-interstitial model, assuming that In substitutes Sb at the 12c Sb(2) site. The difference between the measured data and the calculated data is shown at the bottom of the plot.

Table 2 The lattice parameters a, c, V and the lattice thermal conductivity (kL) of Zn4Sb3  mInm at room temperature. Zn4Sb3  mInm

m¼ 0

m ¼0.02

m¼0.04

m¼ 0.06

m ¼0.08

m¼ 0.10

˚ a (A) ˚ c (A)

12.2298(8)

12.2312(1)

12.2326(6)

12.2334(3)

12.2342(5)

12.2352(2)

12.4201(1)

12.4197(7)

12.4195(4)

12.4193(3)

12.4191(6)

12.4189(5)

V (A˚ 3) kL (W m  1 K  1)

1608.801(9)

1609.105(6)

1609.460(5)

1609.636(4)

1609.828(7)

1610.046(9)

0.96

0.50

0.49

0.63

0.59

0.56

Fig. 2. (a) The occupancy of Zn atom at the 36f Zn(1) site and the total occupancy of Zn atom at the interstitial sites. The sum refers to the total occupancy of Zn atom. (b) The occupancies of Sb atoms at the 18e Sb(1) and 12c Sb(2) sites and the occupancy of In atom at the 12c Sb(2) site. 12c-sum refers to the total occupancy at the 12c Sb(2) site.

atom is 1.081 when m ¼0, consistent with the value of 1.077 reported by Snyder et al. [10], and increases monotonously with increasing the m. This implies that more Zn atoms are needed in the lattice of Zn4Sb3  mInm due to the In substitution for Sb at the 12c Sb(2) site. Fig. 2(b) shows the occupancies of Sb atoms at the 18e Sb(1) and 12c Sb(2) sites and that of In atom at the 12c Sb(2) site in Zn4Sb3  mInm (0 rmr0.10), labeled as 18e Sb, 12c Sb

and 12c In, respectively. The total occupancy at the 12c Sb(2) site is calculated as the sum of the occupancies of Sb and In atoms at the 12c Sb(2) site and labeled as 12c-sum. It can be seen that the occupancy of Sb atom at the 18e Sb site keeps constant in Zn4Sb3  mInm. The occupancy of Sb atom and that of In atom at the 12c Sb(2) site gradually decreases and increases with increasing the m in the range of 0–0.10, respectively. However, the

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total occupancy at the 12c Sb(2) site almost keeps invariable. The correlative relationship of the occupancies of Sb and In atoms at the 12c Sb(2) site further provides a direct evidence for the In substitution for Sb at the 12c Sb(2) site in Zn4Sb3  mInm. The XPS results indicate that the In substitution for Sb at the 12c Sb(2) site in b-Zn4Sb3 simultaneously changed the valences of Sb and Zn because the electronegativities of Zn (1.65), Sb (2.05) and In (1.78) are different. The binding energies of Sb 3d core level of Zn4Sb3  mInm (0rm r0.12) are shown in Fig. 3(a). It can be seen that the binding energies of Sb 3d5/2 move to lower values with increasing the m, which indicates that the chemical state of Sb element becomes more negative due to the In substitution for Sb. It implies that the In atom acts as an electron donor and the negative charge is transferred from In to Sb. As discussed above that the In substitution for Sb preferentially occurs at the 12c Sb(2) site, the negative charge of In is mainly transferred to the nearest Sb atom at the 12c Sb(2) site. Fig. 3(b) shows the curve fitting of Sb 3d5/2 photoelectron peak for Zn4Sb3. According to the three-interstitial model of b-Zn4Sb3, the valence of Zn atom is þ2 and those of Sb atoms at the 18e and 12c sites are 3 and  2, respectively [8]. The curving fitting reveals that two peaks at the binding energies (B.E.) of 526.9 and 527.6 eV are ascribed to  3 and 2 valences of Sb element, respectively. The area ratio of two peaks is calculated to be 1.5, which is consistent with the theoretical atomic ratio of Sb in b-Zn4Sb3. Fig. 3(c) shows the curve fitting of Sb 3d5/2 photoelectron peak for Zn4Sb2.91In0.09. Besides the two peaks attributed to 3 and  2 valences of Sb

element, there is another small peak at B.E.¼527.4 eV to satisfy the curve fitting. The peak is identified as  (2þ d) (0o d o1) valence, which means that a partially electron transfer from In to Sb takes place due to the In substitution for Sb at the 12c Sb(2) site. The areas of fitting peaks are calculated to obtain the atomic ratios between the Sb atoms with different valences, as shown in Fig. 3(d). It can be seen that the amount of Sb2  decreases and that of Sb(2 þ d)  increases with increasing the m, which is consistent with the decrement of the binding energy of Sb in Zn4Sb3  mInm. Fig. 4(a) shows the XPS of Zn 2p core level of Zn4Sb3  mInm. It can be seen that the binding energies shift to high values with increasing the m in Zn4Sb3  mInm. This indicates that the charge transfer from Zn to Sb takes place in Zn4Sb3 mInm induced by the In substitution for Sb. As discussed above, the In substitution for Sb results in negative charge aggregation at the 12c Sb(2) site. To maintain the charge balance and stabilize the crystal structure, more Zn2 þ cations are required in Zn4Sb3 mInm [17]. This may well explain why the Zn occupancy in Zn4Sb3  mInm gradually increases with increasing the m, as shown in Fig. 2(a). Fig. 4(b) shows the binding energies of In 3d core level of Zn4Sb3  mInm. It can be seen that the peak of In 3d becomes clear with increasing the m in Zn4Sb3 mInm, and the binding energies of In 3d are nearly same. This means that the In impurity keeps a stable chemical state in spite of different doping levels in the Zn4Sb3 mInm. The thermal measurements show that a much lower lattice thermal conductivity of 0.49 W m  1 K  1 of Zn4Sb2.96In0.04

Fig. 3. (a) X-ray photoelectron spectra of Sb 3d core level of Zn4Sb3  mInm. (b) Curve-fitting of Sb 3d5/2 photoelectron peak of Zn4Sb3, including  3 and  2 valences related to the18e Sb(1) and 12c Sb(2) sites, respectively. (c) Curve-fitting of Sb 3d5/2 photoelectron peak of Zn4Sb2.91In0.09, showing another valence of  (2þ d) besides  3 and  2 valences of Sb element. (d) Relative atomic ratios of Sb atoms with different valences, calculated from the curve-fittings of Sb 3d5/2 photoelectron peaks.

D. Tang et al. / Journal of Solid State Chemistry 193 (2012) 89–93

93

Fig. 4. X-ray photoelectron spectra of (a) Zn 2p and (b) In 3d core levels of Zn4Sb3  mInm.

is obtained, comparing to 0.96 W m  1 K  1 of pure b-Zn4Sb3 (see Table 2). The remarkable reduction in the lattice thermal conductivity is attributed to the In substitution for Sb at the 12c Sb(2) site in b-Zn4Sb3. The asymmetric Sb–In ionic bond, formed by the In substitution for Sb at the 12c Sb(2) site, changes the vibration behavior of the Sb–Sb dimer and leads to more dynamical disorders of the localized dumbbell vibrations in b-Zn4Sb3. Schweika et al. suggested that the low kL of b-Zn4Sb3 mainly originated from the dynamical disorder produced by the dumbbell vibration of Sb–Sb dimers [11]. According to the opinion, the Sb–In ionic bonds in Zn4Sb3  mInm may cause more dynamical disorders due to the symmetry destruction of the chemical bond compared to the Sb–Sb covalent bonds. This provides a new explanation for the remarkable reduction in the lattice thermal conductivity of Zn4Sb3  mInm.

Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 50930004, 50972114, 10832008), the National Basic Research Program of China (973-program) under Project No. 2007CB607506, the Program for New Century Excellent Talents in University (NCET-09-0627), and State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology) under project No. 2012-KF-11.

Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jssc.2012.03.059.

References 4. Conclusions In-doped b-Zn4Sb3 compounds with nominal composition Zn4Sb3 mInm (0rmr0.18) were investigated by XRD and XPS in the paper. The Rietveld refinement reveals that the indium impurity preferentially occupies the 12c Sb(2) site in Zn4Sb3 mInm. The binding energies of Sb 3d decrease with increasing the m in Zn4Sb3 mInm, indicating that the charge transfer from In to Sb atoms takes place. Curve-fittings of Sb 3d photoelectron spectra reveal that the Sb atom at the 12c Sb(2) site is charged to more negative. It is found that the additional Zn2 þ ions are needed to maintain the charge balance near the 12c Sb(2) site, which is consistent with the increase of the occupancy of Zn in Zn4Sb3 mInm with increasing the m. The In substitution for Sb at the 12c Sb(2) site resulted in the formation of the asymmetric Sb–In ionic bond and changed the vibration behavior of Sb–Sb dimer. As a result, it caused more dynamical disorders of the localized dumbbell vibrations and enhanced phonon scattering in Zn4Sb3 mInm. The work provides a new explanation for the remarkable reduction in the lattice thermal conductivity of Zn4Sb3 mInm.

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