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The effect of Cu addition on the thermoelectric properties of Cu2CdGeSe4
Intermetallics 57 (2015) 156e162
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The effect of Cu addition on the thermoelectric properties of Cu2CdGeSe4 Raju Chetty a, Jayaram Dadda b, Johannes de Boor b, Eckhard Müller b, c, Ramesh Chandra Mallik a, * a b c
Thermoelectric Materials and Devices Laboratory, Department of Physics, Indian Institute of Science, Bangalore 560012, India €ln, Germany Institute of Materials Research, German Aerospace Center (DLR), D-51170 Ko Justus Liebig University Giessen, Institute of Inorganic and Analytical Chemistry, Heinrich-Buff-Ring 58, D-35392 Giessen, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 June 2014 Received in revised form 20 October 2014 Accepted 23 October 2014 Available online
Recently, research in copper based quaternary chalcogenide materials has focused on the study of thermoelectric properties due to the complexity in the crystal structure. In the present work, stoichiometric quaternary chalcogenide compounds Cu2þxCd1xGeSe4 (x ¼ 0, 0.025, 0.05, 0.075, 0.1, 0.125) were prepared by solid state synthesis. The powder X-ray diffraction patterns of all the samples showed a tetragonal crystal structure with the space group I-42m of the main phase, whereas the samples with x ¼ 0 and x ¼ 0.025 revealed the presence of an orthorhombic phase in addition to the main phase as confirmed by Rietveld analysis. The elemental composition of all the samples characterized by Electron Probe Micro Analyzer showed a slight deviation from the nominal composition. The transport properties were measured in the temperature range of 300 Ke723 K. The electrical conductivity of all the samples increased with increasing Cu content due to the enhancement of the hole concentration caused by the substitution of Cd (divalent) by Cu (monovalent). The positive Seebeck coefficient of all the samples in the entire temperature ranges indicates that holes are the majority carriers. The Seebeck coefficient of all the samples decreased with increasing Cu content and showed a reverse trend to the electrical conductivity. The total thermal conductivity of all the samples decreased with increasing temperature which was dominated by the lattice contribution. The maximum figure of merit ZT ¼ 0.42 at 723 K was obtained for the compound Cu2.1Cd0.9GeSe4. © 2014 Elsevier Ltd. All rights reserved.
Keywords: B. Annealing B. Thermoelectric properties D. Microstructure F. X-ray diffraction F. Scanning electron microscopy
1. Introduction In recent years, there has been a focus on finding new compound semiconductors in the field of thermoelectrics, which convert thermal energy into electrical energy and vice-versa. The performance of thermoelectric (TE) materials depends on the dimensionless figure of merit, defined by ZT ¼ (S2s/lT) T where S, s, T and lT represent the Seebeck coefficient, electrical conductivity, absolute temperature and total thermal conductivity, respectively. The latter is composed of the carrier (lC) and the lattice contribution (lL). Recently, Cu-based quaternary chalcogenide compounds with formula the Cu2MTQ4 (with M ¼ Zn, Cd, Hg; T ¼ Sn, Ge and Q ¼ S, Se) have been found interesting in the study of thermoelectric properties due to their complex crystal structures [1e7].
* Corresponding author. Tel.: þ91 80 2293 2450; fax: þ91 80 2360 2602. E-mail address: [email protected] (R.C. Mallik). http://dx.doi.org/10.1016/j.intermet.2014.10.015 0966-9795/© 2014 Elsevier Ltd. All rights reserved.
These compounds follow the concept of two structural functional units: a Cu2Q4 (e.g. Cu2Se4) tetrahedral array acts as an electrical conducting unit and the other MTQ4 (e.g. CdSnSe4) tetrahedral array acts as an insulating unit [2]. This complexity in crystal structure leads to the suppression of thermal conductivity [1,2]. These quaternary compounds are disordered structures, which are derived from sphalerite and wurtzite cells by ordering metals on the cation sites and result in tetragonal and orthorhombic crystal structures [8]. The electrical conductivity which is one of the influencing parameters in the improvement of ZT is too low for these compounds because of their wide band gap (Eg~1.20 eV) [9,10]. Therefore, the electrical conductivity can be enhanced by tuning the carrier concentration via doping and/or altering the stoichiometry, while the Seebeck coefficient reduces. However, this leads to the improvement of ZT values due to the reasonable values of the power factor (S2s), and low thermal conductivity [1e3,5]. Various attempts have been made by several researchers in the improvement of ZT such as (a) tuning the carrier concentration by
R. Chetty et al. / Intermetallics 57 (2015) 156e162
doping and/or variation in the stoichiometry (b) reducing thermal conductivity by synthesizing nano-structured materials and (c) alloying of solid solution compounds. The maximum value of ZT obtained using approach (a) for bulk materials such as Cu2.1Zn0.9SnSe4, Cu2.1Cd0.9SnSe4, Cu2ZnSn0.9In0.1Se4 and Cu2.075Zn0.925GeSe4 reached 0.91 at 860 K [1], 0.65 at 700 K [2], 0.95 at 850 K [3] and 0.45 at 670 K [11], respectively. Using approaches (a) and/or (b), the peak values of ZT were 0.65 at 723 K for Cu2CdSnSe4 [12], 0.71 at 685 K for Cu2.15Cd0.85SnSe3.9 [13], 0.55 at 723 K for Cu2.15Zn0.85GeSe3.9 [14]. Using the approach (c), the maximum of ZT ¼ 0.46 was obtained at 800 K for Cu2Zn0.4Fe0.6SnSe4 [15] and 0.6 at 575 K for Cu2HgSnSe2Te2 [16]. In this work, we have chosen the compound Cu2CdGeSe4 which belongs to the family of Cu-based quaternary chalcogenides because of its beneficial thermoelectric properties to achieve good ZT. The physical properties of Cu2CdGeSe4 have already been studied. It has very low electrical conductivity and a large Seebeck coefficient [10]. As mentioned above, this class of compounds shows low thermal conductivity due to their complex crystal structures. Therefore, we have attempted to increase the electrical conductivity by varying the stoichiometry of the compound Cu2CdGeSe4 through the addition of Cu on a Cd site to optimize thermoelectric properties. 2. Experimental details The quaternary compounds Cu2þxCd1xGeSe4(x ¼ 0, 0.025, 0.05, 0.075, 0.1, 0.125) were prepared by a standard solid state synthesis method using high purity (5N) elements. The stoichiometric ratio of the elements were taken in quartz tubes and sealed under a vacuum (~104 mbar). The samples were slowly heated to 1173 K and kept there for 48 h and quenched immediately in water. The quenched samples were annealed at 773 K for 96 h, and the furnace was then switched off. The resulting ingots were powdered using the mortar and pestle. Then powders were transferred into graphite dies and hot pressed under a dynamic vacuum at 823 K for 30 min and ~30 MPa. The hot pressed samples were kept annealing for 96 h at 823 K for homogenization. The relative densities of the hot-pressed samples were determined as ~85% for all the samples using Archimedes' principle and also from sample dimensions. The powder X-Ray Diffraction (XRD) patterns of the hot pressed (HP) samples were collected by a Bruker D8 Advance diffractometer using Cu-Ka radiation. Rietveld refinement was carried out for a crystallographic phase identification using Fullprof software [17]. Electron probe microanalysis (EPMA) (JEOL JXA-8530F wavelengthdispersive spectrometry (WDS)) was carried out for the hotpressed samples using internal standards. The Seebeck coefficient and electrical conductivity were measured simultaneously on an in-house developed setup [18]. The thermal diffusivity (D) was measured on Netzsch LFA 427 (Netzsch, Germany), while specific heat (Cp) was measured on Netzsch DSC 404. The thermal conductivity was then calculated using the relation l ¼ D Cp d. The uncertainty in the measurements is 5% for resistivity and Seebeck [18], and 8% for thermal conductivity.
157
temperature phase with a tetragonal crystal structure at ~878 K for Cu2CdGeSe4 [20,21]. Figs. 1 and 2 show the powder XRD pattern of all the samples and the Rietveld refinement for the compound Cu2CdGeSe4. The Rietveld powder XRD pattern of all the samples after hot pressing reveals the tetragonal crystal structure as the main phase, whereas the samples Cu2þxCd1xGeSe4 (x ¼ 0, 0.025) show a mixture of tetragonal and orthorhombic crystal structures. This may be due to the insufficient annealing and/or deviation from the stoichiometry. Gulay et al. reported that the XRD pattern of the compound Cu2CdGeSe4 after the heat treatment (i.e. Differential Thermal Analysis measurement) revealed a mixture phase of tetragonal and orthorhombic phases, which may be due to the insufficient time given for the structural transformation [19]. Piskach et al. also reported that the different modifications of crystal structures when the deviation from the composition CdSe (50 mol %) -Cu2GeSe3 (50 mol %) in the binary phase diagram of the Cu2GeSe3-CdSe system was observed [22]. Lattice parameters a, c of all the samples calculated using the Si as an internal standard are shown in Fig. 3. The lattice parameter a is slightly decreased with increase of Cu content up to x ¼ 0.075 in Cu2þxCd1xGeSe4 and increased for the compound with the highest nominal Cu content. The lattice parameter c is increased from the sample with x ¼ 0 to x ¼ 0.025 in Cu2þxCd1xGeSe4 which may be due to the presence of high temperature phase of Cu2CdGeSe4. Further lattice parameter c is decreased with Cu content up to x ¼ 0.075 in Cu2þxCd1xGeSe4 and increased for the compound with the highest nominal Cu content. Therefore, the improper variation in the lattice parameters may be due to a deviation from the nominal composition, which was confirmed by EPMA (See Table 1). This irregular behavior of lattice parameters and the EPMA results with nominal Cu doping contents may be due to the presence of a small amount of impurity phases which are not in the detectable range of the equipments. Fig. 4 shows the fractured surface images with 2000 magnification of all the hot pressed samples. A rough surface with no specific morphology was observed for all the hot pressed samples. The surface morphology of all the samples showed that grain growth was not continuous with a rough surface, which indicates a low compaction of the hot pressed samples. The phase purity and elemental composition of all the samples were analyzed by EPMA. Fig. 5 shows the back-scattered electron micrographs with a magnification of 1000. The elemental composition of all the samples which deviates from the nominal composition is shown in Table 1.
3. Results and discussion 3.1. Structural and phase characterization The existence of two crystal structure modifications for the compound Cu2CdGeSe4 such as a tetragonal structure with a I-42m space group and an orthorhombic structure with a Pmn21 space group is reported [19]. The high temperature phase with the orthorhombic crystal structure can be transformed into a low
Fig. 1. (a) Powder XRD patterns for the compounds Cu2þxCd1xGeSe4(x ¼ 0, 0.025, 0.05, 0.075, 0.1).
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concentration. The creation of more holes may not only be due to the substitution of monovalent Cu on the divalent Cd site because the EPMA revealed the deviation from the nominal composition. Other possible antisite defects such as the substitution of Cu on the Ge site (CuGe) and Cd on the Ge site (CdGe) might have been influenced by the increase of carrier concentration. The similar behavior of temperature dependent conductivity for all the substituted samples showed a downturn in the data like an unsubstituted compound, whereas the samples Cu2þxCd1xGeSe4(x ¼ 0.025, 0.05) showed the onset of intrinsic conduction at the temperature ~600 K for Cu2.025Cd0.975GeSe4 and ~670 K for Cu2.05Cd0.95GeSe4. The downturn and onset of intrinsic conduction which moved towards higher temperatures with an increase of nominal content may be due to the existence of the deviation from the nominal composition, which was observed by EPMA. The energy gap of a semiconductor can be estimated in the intrinsic conduction region using the expression.
s ¼ s0 eEg =2kB T Fig. 2. Rietveld refinement powder XRD pattern for Cu2CdGeSe4.
4. Transport properties 4.1. Electrical conductivity The transport properties were measured in the temperature range 336 Ke723 K. The temperature dependence of electrical conductivity for all the samples is shown in Fig. 6a. The compound Cu2CdGeSe4 showed a low electrical conductivity of 2.85 S/cm at 336 K, which is in agreement with the reported value of s which is 2.10 S/cm at 300 K [10]. The electrical conductivity increased with an increase of temperature up to 400 K, which may be due to an increase in carrier concentration. Further, the s value decreased up to 520 K, which may be attributed to the dominance of reduction in carrier mobility. Again, s increased above 520 K due to the onset of an intrinsic conduction. A similar type of electrical conductivity with two turns in the data has already been reported by Mkrtchyan et al. for the compound Cu2CdGeSe4, which states that conductivity increases up to 500 K and decreases to 770 K, and increases further due to the onset of intrinsic electrical conductivity [10]. The electrical conductivity of all the samples increased with an increase in the nominal Cu content, which is due to the increase in carrier
(1)
where s0 is a pre-exponential constant, Eg is the energy gap, and kB is the Boltzmann constant. The energy gap of the compound Cu2CdGeSe4 was estimated by plotting the logarithmic of electrical conductivity as a function of the reciprocal of temperature as shown in the inset of Fig. 6a. The band gap of the compound Cu2CdGeSe4 was determined to be Eg ¼ 0.59 eV, which was comparable with the literature value ~0.46 eV (estimated from the intrinsic conduction region electrical conductivity data from Ref. [10]). The electrical conductivity increased with increase of temperature up to the onset of intrinsic conduction for all the Cu added samples indicates the non degenerate semiconducting like behavior. The maximum electrical conductivity (72 S/cm at 723 K) was observed for the highest Cu content sample Cu2.1Cd0.9GeSe4. Therefore, an excess of Cu adding sample with composition Cu2.125Cd0.875GeSe4 was prepared and the transport properties were studied. The electrical conductivity of sample Cu2.125Cd0.875GeSe4 is increased and it decreased with increase of temperature which indicates the degenerate semiconductor behavior. The carrier concentration data is helpful in explaining the electrical conductivity behavior with increase of doping content and it is explained in the following section.
4.2. Seebeck coefficient The temperature dependent Seebeck coefficient for all the samples is displayed in Fig. 6b. The Seebeck coefficients of all the samples systematically decrease with an increase in Cu content. This is due to the increase in carrier concentration, which is evidenced by the increase in electrical conductivity. To have clear idea clear idea about the trend of electrical conductivity with doping, carrier concentration was roughly estimated using the Mott's formula from the room temperature Seebeck coefficient value.
Sd ðT > qD Þ ¼
Fig. 3. Variation lattice parameter as a function of excess nominal Cu content in Cu2þxCd1xGeSe4 (x ¼ 0, 0.025, 0.05, 0.075, 0.1).
p2 k2B 2me 2=3 T eZ2 3np2
(2)
with qD Debye temperature, me being the carrier mass, e the carrier charge, kB the Boltzmann constant, ħ the Planck's constant, T the temperature, and n the charge carrier density, respectively. The carrier concentration increased with increase of Cu content from the 6.35 1019 cm3 for Cu2CdGeSe4 to 1.15 1021 cm3 for Cu2.125Cd0.875GeSe4. This evidences the behavior of Seebeck coefficient and electrical conductivity with increase of doping content. The positive Seebeck coefficient of all the samples indicates the
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Table 1 Nominal composition, EPMA composition, relative density, reliable factors of refinement (Rwp, Rexp), s, S, l, ZT for Cu2þxCd1xGeSe4 (x ¼ 0, 0.025, 0.05, 0.075, 0.1, 0.125). Nominal composition
EPMA, composition
Reliable factors of refinement Rwp (%) Rexp (%)
Relative density (%)
s723 K (S/cm)
S723
Cu2CdGeSe4 Cu2.025Cd0.975GeSe4 Cu2.05Cd0.95GeSe4 Cu2.075Cd0.925GeSe4 Cu2.1Cd0.9GeSe4 Cu2.125Cd0.875GeSe4
Cu2.086Cd0.950Ge0.906Se4.058 Cu2.123Cd0.935Ge0.901Se4.041 Cu2.105Cd0.892Ge0.909Se4.094 Cu2.103Cd0.915Ge0.911Se4.071 Cu2.147Cd0.900Ge0.903Se4.050
5.77 3.90 4.70 3.95 3.37
85 85 83 88 86 84
8 17 23 57 72 42
389 336 314 252 226 167
2.86 3.76 3.04 2.77 2.77
dominant carriers are the holes. The behavior of Seebeck coefficient with temperature for Cu2CdGeSe4 is observed in the similar way like in the electrical conductivity. The Seebeck coefficient increases with an increase of temperature up to ~500 K for the compound Cu2CdGeSe4, ~620 K for Cu2.025Cd0.975GeSe4 and ~700 K for Cu2.05Cd0.95GeSe4. Above these temperatures the Seebeck coefficient started decreasing, when the onset of intrinsic conduction appeared. This reduction of the Seebeck coefficient is due to the bipolar effect, which can be explained by the equation (3) [23]. The average Seebeck coefficient (S) of a compound consists of two types of carriers and can be expressed as
S¼
Sn sn þ Sp sp sn þ sp
(3)
where Sn is the Seebeck coefficient by electrons, Sp is the Seebeck coefficient by holes, sn is the electrical conductivity by electrons, and sp is the electrical conductivity by holes. The onset of the intrinsic conduction regime in Seebeck coefficient data for the samples Cu2þxCd1xGeSe4(x ¼ 0, 0.025, 0.05) is almost consistent with the electrical conductivity data. The maximum value of the Seebeck coefficient 477 mV/K at 500 K was observed for the compound Cu2CdGeSe4. The energy gap of a semiconductor can be estimated from the maximum value of the Seebeck coefficient using the formula
Eg ¼ 2eSmax Tmax
K
(mV/K)
l723 K (W/m-K)
ZT (723 K)
0.53 0.64 0.61 0.71 0.64 0.59
0.16 0.22 0.27 0.37 0.42 0.14
(4)
where e is the elementary charge, Smax is the maximum value of the Seebeck coefficient and Tmax is the absolute value of the temperature at which the Smax occurs [23]. The energy gap of the compound Cu2CdGeSe4 was estimated as Eg ¼ 0.48 eV using eq (2), which is consistent with the reported value ~ 0.42 eV (estimated from Smax) [10]. The energy gap of Cu2CdGeSe4 estimated from the Seebeck coefficient and electrical conductivity data are in agreement. The temperature dependent power factor (S2s) was evaluated for all the compounds from the Seebeck coefficient and electrical conductivity. The maximum S2s ¼ 3.73 mW/cm-K2 at 723 K was obtained for the compound Cu2.1Cd0.9GeSe4. The power factor of excess Cu added sample Cu2.125Cd0.875Se4 is decreased drastically to a value of 1.14 mW/cm-K2 at 710 K and this is mainly influenced by the increase of carrier concentration leads to dropping of Seebeck coefficient. Still, the S2s values are lower because there is not much improvement in the electrical conductivity with an increase in nominal Cu content as compared with other quaternary compounds [1e3]. The existence of lower electrical conductivity values may be due to the difference between the nominal and actual composition determined by EPMA (see Table 1). This could possibly indicate that the holes created by a Cu addition on a Cd site might have been compensated for by the electrons created by the defects
Fig. 4. (aed) Secondary electron SEM images for Cu2þxCd1xGeSe4(x ¼ 0, 0.025, 0.05, 0.1).
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Fig. 5. (aed) Backscattered electron micrographs for Cu2þxCd1xGeSe4(x ¼ 0, 0.025, 0.05, 0.1).
Fig. 6. (a) Temperature dependent electrical conductivity for Cu2þxCd1xGeSe4(x ¼ 0, 0.025, 0.05, 0.075, 0.1, 0.125). Inset figure corresponds to the ln (s) versus 1/T. (b) Seebeck coefficient as a function of temperature for Cu2þxCd1xGeSe4(x ¼ 0, 0.025, 0.05, 0.075, 0.1, 0.125). (c) Temperature dependent total thermal conductivity for Cu2þxCd1xGeSe4(x ¼ 0, 0.025, 0.05, 0.075, 0.1, 0.125). (d) Temperature dependent lattice thermal conductivity for Cu2þxCd1xGeSe4(x ¼ 0, 0.025, 0.05, 0.075, 0.1, 0.125). Inset Figure shows the lL as a function of 1/T.
R. Chetty et al. / Intermetallics 57 (2015) 156e162
161
[such as Ge on Cd (GeCd) and Ge on Cu (GeCu)] or may be due to a low sample density. 4.3. Thermal conductivity Fig. 6c displays the temperature dependent total thermal conductivity of all the samples and exhibit below 1.8 W/m-K in the entire range of temperature measured. The total thermal conductivity of all the samples decreased with an increase in temperature and reached values below 0.7 W/m-K at 723 K. The total thermal conductivity is dominated by the lattice part of thermal conductivity due to the insignificant contribution from the carrier part, lC, which can be calculated from the WeidemanneFranz relation lC ¼ LT/r, where L is the Lorenz number. The value of the Lorenz number can be obtained by using the following formula [24]
L¼
kB e
2
" #2 ! ðr þ 7=2ÞFrþ5=2 ðhÞ ðr þ 5=2ÞFrþ3=2 ðhÞ ðr þ 3=2ÞFrþ1=2 ðhÞ ðr þ 3=2ÞFrþ1=2 ðhÞ
(5)
To find the correct L value, the reduced Fermi energy h has to be calculated by using the measured Seebeck coefficient values from the following relation.
k S¼± B e
! ðr þ 5=2ÞFrþ3=2 ðhÞ h ðr þ 3=2ÞFrþ1=2 ðhÞ
where
Fn(h)
(6)
is the nth order Fermi integral, cn EF dc, h ¼ k T , kB is the Boltzmann constant, e the Fn ðhÞ ¼ ch B 0 1þe electron charge and EF the Fermi energy. By assuming the acoustic phonon scattering (r ¼ 1/2) as the main carrier scattering mechanism, the Lorenz number can be calculated by substituting the values of h and r into the equation (5). Fig. 6d shows the lattice thermal conductivity of all the samples as a function of temperature. The lattice thermal conductivity of all the samples decreased with increasing temperature and showed a 1/T dependence up to 650 K (See inset of the Fig. 6d for the Cu2CdGeSe4), which indicates the dominance of phononephonon scattering. Above 650 K, the thermal conductivity deviated from the straight line of 1/T dependence, which may be due to the presence of a bipolar contribution. The value of lL ~1.37 W/m-K at 336 K was obtained for the unsubstituted compound Cu2CdGeSe4, which is smaller than for binary compounds such as ZnSe [19 W/m-K at 300 K] [25],and CdSe [~9 W/m-K at 300 K] [25]. The suppression of thermal conductivity in the quaternary compounds as compared to binary compounds is due to their distorted crystal structure, which leads to an increase in phonon scattering as reported earlier [1,2,12,14]. The lattice thermal conductivity of all the samples was expected to decrease with an increase in nominal Cu content due to pointdefect scattering, but it did not follow any particular trend. This may be due to the variation in the porosity caused by the low density of the samples, which was clearly observed by BSE micrographs. The relative density of all the samples is listed in Table 1 and it is observed that these values are varied from sample to sample may results improper trend in the behavior thermal conductivity with doping. It may also be due to the variation in the observed composition of all the elements (Cu, Cd, Ge, Se) present in the compounds which did not follow systematically, in comparison with nominal composition, which was confirmed by EPMA. Fig. 7 displays the temperature dependent thermoelectric figure of merit (ZT), which was evaluated from the S, s and l values. The ZT values increased with increasing temperature for all the samples. The peak ZT ¼ 0.42 at 723 K was obtained for the compound Cu2.1Cd0.9GeSe4 due to the maximum power factor and the Z
∞
Fig. 7. Thermoelectric figure of merit (ZT) as function of temperature for Cu2þxCd1xGeSe4(x ¼ 0, 0.025, 0.05, 0.075, 0.1, 0.125).
reasonable value of thermal conductivity. This value is slightly lower than that of the other quaternary compounds and is mainly affected by the low values of the power factor as compared to the reported values. A further improvement in ZT can be achieved by enhancing the power factor and suppressing thermal conductivity. 5. Conclusions Quaternary chalcogenide compounds with the chemical formula Cu2þxCd1xGeSe4(x ¼ 0, 0.025, 0.05, 0.075, 0.1, 0.125) were prepared by melting and annealing. The powder XRD pattern of the compounds Cu2þxCd1xGeSe4(x ¼ 0, 0.025) revealed the mixture of low (tetragonal) and high (orthorhombic) temperature modifications, whereas a compound Cu2þxCd1xGeSe4 (x ¼ 0.05, 0.075, 0.1) showed a tetragonal crystal structure confirmed by a Rietveld analysis. The increase in electrical conductivity with an increase of nominal Cu content is not only due to the excess of Cu addition by replacing Cd. Other defects such as Cu on Ge and Cd on Ge might have influenced the conductivity where the deviation from the nominal composition was observed by EPMA. The positive Seebeck coefficients for all the samples indicate that the majority carriers are the holes. The Seebeck coefficient decreased with an increase in Cu content and it followed the reverse trend for electrical conductivity. The total thermal conductivity of all the samples is dominated by the lattice contribution, due to the negligible contribution from the carriers. Lattice thermal conductivity could not follow a particular trend with an increase in Cu content, and may be influenced by the variation in the porosity in all the samples. The maximum ZT ¼ 0.42 at 723 was obtained for Cu2.1Cd0.9GeSe4. Acknowledgments The authors would like to thank Prof. Satyam Suwas for providing the hot press facility and the Department of Science & Technology (DST), India for financial support through Grant No. INT/FRG/DAAD/P-222/2012. References [1] Liu ML, Huang FQ, Chen IW, Chen LD. Appl Phys Lett 2009;94:202103. [2] Liu ML, Chen IW, Huang FQ, Chen LD. Adv Mater 2009;21:3808. [3] Shi XY, Huang FQ, Liu ML, Chen LD. Appl Phys Lett 2009;94:122103.
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Intermetallics journal homepage: www.elsevier.com/locate/intermet
The effect of Cu addition on the thermoelectric properties of Cu2CdGeSe4 Raju Chetty a, Jayaram Dadda b, Johannes de Boor b, Eckhard Müller b, c, Ramesh Chandra Mallik a, * a b c
Thermoelectric Materials and Devices Laboratory, Department of Physics, Indian Institute of Science, Bangalore 560012, India €ln, Germany Institute of Materials Research, German Aerospace Center (DLR), D-51170 Ko Justus Liebig University Giessen, Institute of Inorganic and Analytical Chemistry, Heinrich-Buff-Ring 58, D-35392 Giessen, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 June 2014 Received in revised form 20 October 2014 Accepted 23 October 2014 Available online
Recently, research in copper based quaternary chalcogenide materials has focused on the study of thermoelectric properties due to the complexity in the crystal structure. In the present work, stoichiometric quaternary chalcogenide compounds Cu2þxCd1xGeSe4 (x ¼ 0, 0.025, 0.05, 0.075, 0.1, 0.125) were prepared by solid state synthesis. The powder X-ray diffraction patterns of all the samples showed a tetragonal crystal structure with the space group I-42m of the main phase, whereas the samples with x ¼ 0 and x ¼ 0.025 revealed the presence of an orthorhombic phase in addition to the main phase as confirmed by Rietveld analysis. The elemental composition of all the samples characterized by Electron Probe Micro Analyzer showed a slight deviation from the nominal composition. The transport properties were measured in the temperature range of 300 Ke723 K. The electrical conductivity of all the samples increased with increasing Cu content due to the enhancement of the hole concentration caused by the substitution of Cd (divalent) by Cu (monovalent). The positive Seebeck coefficient of all the samples in the entire temperature ranges indicates that holes are the majority carriers. The Seebeck coefficient of all the samples decreased with increasing Cu content and showed a reverse trend to the electrical conductivity. The total thermal conductivity of all the samples decreased with increasing temperature which was dominated by the lattice contribution. The maximum figure of merit ZT ¼ 0.42 at 723 K was obtained for the compound Cu2.1Cd0.9GeSe4. © 2014 Elsevier Ltd. All rights reserved.
Keywords: B. Annealing B. Thermoelectric properties D. Microstructure F. X-ray diffraction F. Scanning electron microscopy
1. Introduction In recent years, there has been a focus on finding new compound semiconductors in the field of thermoelectrics, which convert thermal energy into electrical energy and vice-versa. The performance of thermoelectric (TE) materials depends on the dimensionless figure of merit, defined by ZT ¼ (S2s/lT) T where S, s, T and lT represent the Seebeck coefficient, electrical conductivity, absolute temperature and total thermal conductivity, respectively. The latter is composed of the carrier (lC) and the lattice contribution (lL). Recently, Cu-based quaternary chalcogenide compounds with formula the Cu2MTQ4 (with M ¼ Zn, Cd, Hg; T ¼ Sn, Ge and Q ¼ S, Se) have been found interesting in the study of thermoelectric properties due to their complex crystal structures [1e7].
* Corresponding author. Tel.: þ91 80 2293 2450; fax: þ91 80 2360 2602. E-mail address: [email protected] (R.C. Mallik). http://dx.doi.org/10.1016/j.intermet.2014.10.015 0966-9795/© 2014 Elsevier Ltd. All rights reserved.
These compounds follow the concept of two structural functional units: a Cu2Q4 (e.g. Cu2Se4) tetrahedral array acts as an electrical conducting unit and the other MTQ4 (e.g. CdSnSe4) tetrahedral array acts as an insulating unit [2]. This complexity in crystal structure leads to the suppression of thermal conductivity [1,2]. These quaternary compounds are disordered structures, which are derived from sphalerite and wurtzite cells by ordering metals on the cation sites and result in tetragonal and orthorhombic crystal structures [8]. The electrical conductivity which is one of the influencing parameters in the improvement of ZT is too low for these compounds because of their wide band gap (Eg~1.20 eV) [9,10]. Therefore, the electrical conductivity can be enhanced by tuning the carrier concentration via doping and/or altering the stoichiometry, while the Seebeck coefficient reduces. However, this leads to the improvement of ZT values due to the reasonable values of the power factor (S2s), and low thermal conductivity [1e3,5]. Various attempts have been made by several researchers in the improvement of ZT such as (a) tuning the carrier concentration by
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doping and/or variation in the stoichiometry (b) reducing thermal conductivity by synthesizing nano-structured materials and (c) alloying of solid solution compounds. The maximum value of ZT obtained using approach (a) for bulk materials such as Cu2.1Zn0.9SnSe4, Cu2.1Cd0.9SnSe4, Cu2ZnSn0.9In0.1Se4 and Cu2.075Zn0.925GeSe4 reached 0.91 at 860 K [1], 0.65 at 700 K [2], 0.95 at 850 K [3] and 0.45 at 670 K [11], respectively. Using approaches (a) and/or (b), the peak values of ZT were 0.65 at 723 K for Cu2CdSnSe4 [12], 0.71 at 685 K for Cu2.15Cd0.85SnSe3.9 [13], 0.55 at 723 K for Cu2.15Zn0.85GeSe3.9 [14]. Using the approach (c), the maximum of ZT ¼ 0.46 was obtained at 800 K for Cu2Zn0.4Fe0.6SnSe4 [15] and 0.6 at 575 K for Cu2HgSnSe2Te2 [16]. In this work, we have chosen the compound Cu2CdGeSe4 which belongs to the family of Cu-based quaternary chalcogenides because of its beneficial thermoelectric properties to achieve good ZT. The physical properties of Cu2CdGeSe4 have already been studied. It has very low electrical conductivity and a large Seebeck coefficient [10]. As mentioned above, this class of compounds shows low thermal conductivity due to their complex crystal structures. Therefore, we have attempted to increase the electrical conductivity by varying the stoichiometry of the compound Cu2CdGeSe4 through the addition of Cu on a Cd site to optimize thermoelectric properties. 2. Experimental details The quaternary compounds Cu2þxCd1xGeSe4(x ¼ 0, 0.025, 0.05, 0.075, 0.1, 0.125) were prepared by a standard solid state synthesis method using high purity (5N) elements. The stoichiometric ratio of the elements were taken in quartz tubes and sealed under a vacuum (~104 mbar). The samples were slowly heated to 1173 K and kept there for 48 h and quenched immediately in water. The quenched samples were annealed at 773 K for 96 h, and the furnace was then switched off. The resulting ingots were powdered using the mortar and pestle. Then powders were transferred into graphite dies and hot pressed under a dynamic vacuum at 823 K for 30 min and ~30 MPa. The hot pressed samples were kept annealing for 96 h at 823 K for homogenization. The relative densities of the hot-pressed samples were determined as ~85% for all the samples using Archimedes' principle and also from sample dimensions. The powder X-Ray Diffraction (XRD) patterns of the hot pressed (HP) samples were collected by a Bruker D8 Advance diffractometer using Cu-Ka radiation. Rietveld refinement was carried out for a crystallographic phase identification using Fullprof software [17]. Electron probe microanalysis (EPMA) (JEOL JXA-8530F wavelengthdispersive spectrometry (WDS)) was carried out for the hotpressed samples using internal standards. The Seebeck coefficient and electrical conductivity were measured simultaneously on an in-house developed setup [18]. The thermal diffusivity (D) was measured on Netzsch LFA 427 (Netzsch, Germany), while specific heat (Cp) was measured on Netzsch DSC 404. The thermal conductivity was then calculated using the relation l ¼ D Cp d. The uncertainty in the measurements is 5% for resistivity and Seebeck [18], and 8% for thermal conductivity.
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temperature phase with a tetragonal crystal structure at ~878 K for Cu2CdGeSe4 [20,21]. Figs. 1 and 2 show the powder XRD pattern of all the samples and the Rietveld refinement for the compound Cu2CdGeSe4. The Rietveld powder XRD pattern of all the samples after hot pressing reveals the tetragonal crystal structure as the main phase, whereas the samples Cu2þxCd1xGeSe4 (x ¼ 0, 0.025) show a mixture of tetragonal and orthorhombic crystal structures. This may be due to the insufficient annealing and/or deviation from the stoichiometry. Gulay et al. reported that the XRD pattern of the compound Cu2CdGeSe4 after the heat treatment (i.e. Differential Thermal Analysis measurement) revealed a mixture phase of tetragonal and orthorhombic phases, which may be due to the insufficient time given for the structural transformation [19]. Piskach et al. also reported that the different modifications of crystal structures when the deviation from the composition CdSe (50 mol %) -Cu2GeSe3 (50 mol %) in the binary phase diagram of the Cu2GeSe3-CdSe system was observed [22]. Lattice parameters a, c of all the samples calculated using the Si as an internal standard are shown in Fig. 3. The lattice parameter a is slightly decreased with increase of Cu content up to x ¼ 0.075 in Cu2þxCd1xGeSe4 and increased for the compound with the highest nominal Cu content. The lattice parameter c is increased from the sample with x ¼ 0 to x ¼ 0.025 in Cu2þxCd1xGeSe4 which may be due to the presence of high temperature phase of Cu2CdGeSe4. Further lattice parameter c is decreased with Cu content up to x ¼ 0.075 in Cu2þxCd1xGeSe4 and increased for the compound with the highest nominal Cu content. Therefore, the improper variation in the lattice parameters may be due to a deviation from the nominal composition, which was confirmed by EPMA (See Table 1). This irregular behavior of lattice parameters and the EPMA results with nominal Cu doping contents may be due to the presence of a small amount of impurity phases which are not in the detectable range of the equipments. Fig. 4 shows the fractured surface images with 2000 magnification of all the hot pressed samples. A rough surface with no specific morphology was observed for all the hot pressed samples. The surface morphology of all the samples showed that grain growth was not continuous with a rough surface, which indicates a low compaction of the hot pressed samples. The phase purity and elemental composition of all the samples were analyzed by EPMA. Fig. 5 shows the back-scattered electron micrographs with a magnification of 1000. The elemental composition of all the samples which deviates from the nominal composition is shown in Table 1.
3. Results and discussion 3.1. Structural and phase characterization The existence of two crystal structure modifications for the compound Cu2CdGeSe4 such as a tetragonal structure with a I-42m space group and an orthorhombic structure with a Pmn21 space group is reported [19]. The high temperature phase with the orthorhombic crystal structure can be transformed into a low
Fig. 1. (a) Powder XRD patterns for the compounds Cu2þxCd1xGeSe4(x ¼ 0, 0.025, 0.05, 0.075, 0.1).
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concentration. The creation of more holes may not only be due to the substitution of monovalent Cu on the divalent Cd site because the EPMA revealed the deviation from the nominal composition. Other possible antisite defects such as the substitution of Cu on the Ge site (CuGe) and Cd on the Ge site (CdGe) might have been influenced by the increase of carrier concentration. The similar behavior of temperature dependent conductivity for all the substituted samples showed a downturn in the data like an unsubstituted compound, whereas the samples Cu2þxCd1xGeSe4(x ¼ 0.025, 0.05) showed the onset of intrinsic conduction at the temperature ~600 K for Cu2.025Cd0.975GeSe4 and ~670 K for Cu2.05Cd0.95GeSe4. The downturn and onset of intrinsic conduction which moved towards higher temperatures with an increase of nominal content may be due to the existence of the deviation from the nominal composition, which was observed by EPMA. The energy gap of a semiconductor can be estimated in the intrinsic conduction region using the expression.
s ¼ s0 eEg =2kB T Fig. 2. Rietveld refinement powder XRD pattern for Cu2CdGeSe4.
4. Transport properties 4.1. Electrical conductivity The transport properties were measured in the temperature range 336 Ke723 K. The temperature dependence of electrical conductivity for all the samples is shown in Fig. 6a. The compound Cu2CdGeSe4 showed a low electrical conductivity of 2.85 S/cm at 336 K, which is in agreement with the reported value of s which is 2.10 S/cm at 300 K [10]. The electrical conductivity increased with an increase of temperature up to 400 K, which may be due to an increase in carrier concentration. Further, the s value decreased up to 520 K, which may be attributed to the dominance of reduction in carrier mobility. Again, s increased above 520 K due to the onset of an intrinsic conduction. A similar type of electrical conductivity with two turns in the data has already been reported by Mkrtchyan et al. for the compound Cu2CdGeSe4, which states that conductivity increases up to 500 K and decreases to 770 K, and increases further due to the onset of intrinsic electrical conductivity [10]. The electrical conductivity of all the samples increased with an increase in the nominal Cu content, which is due to the increase in carrier
(1)
where s0 is a pre-exponential constant, Eg is the energy gap, and kB is the Boltzmann constant. The energy gap of the compound Cu2CdGeSe4 was estimated by plotting the logarithmic of electrical conductivity as a function of the reciprocal of temperature as shown in the inset of Fig. 6a. The band gap of the compound Cu2CdGeSe4 was determined to be Eg ¼ 0.59 eV, which was comparable with the literature value ~0.46 eV (estimated from the intrinsic conduction region electrical conductivity data from Ref. [10]). The electrical conductivity increased with increase of temperature up to the onset of intrinsic conduction for all the Cu added samples indicates the non degenerate semiconducting like behavior. The maximum electrical conductivity (72 S/cm at 723 K) was observed for the highest Cu content sample Cu2.1Cd0.9GeSe4. Therefore, an excess of Cu adding sample with composition Cu2.125Cd0.875GeSe4 was prepared and the transport properties were studied. The electrical conductivity of sample Cu2.125Cd0.875GeSe4 is increased and it decreased with increase of temperature which indicates the degenerate semiconductor behavior. The carrier concentration data is helpful in explaining the electrical conductivity behavior with increase of doping content and it is explained in the following section.
4.2. Seebeck coefficient The temperature dependent Seebeck coefficient for all the samples is displayed in Fig. 6b. The Seebeck coefficients of all the samples systematically decrease with an increase in Cu content. This is due to the increase in carrier concentration, which is evidenced by the increase in electrical conductivity. To have clear idea clear idea about the trend of electrical conductivity with doping, carrier concentration was roughly estimated using the Mott's formula from the room temperature Seebeck coefficient value.
Sd ðT > qD Þ ¼
Fig. 3. Variation lattice parameter as a function of excess nominal Cu content in Cu2þxCd1xGeSe4 (x ¼ 0, 0.025, 0.05, 0.075, 0.1).
p2 k2B 2me 2=3 T eZ2 3np2
(2)
with qD Debye temperature, me being the carrier mass, e the carrier charge, kB the Boltzmann constant, ħ the Planck's constant, T the temperature, and n the charge carrier density, respectively. The carrier concentration increased with increase of Cu content from the 6.35 1019 cm3 for Cu2CdGeSe4 to 1.15 1021 cm3 for Cu2.125Cd0.875GeSe4. This evidences the behavior of Seebeck coefficient and electrical conductivity with increase of doping content. The positive Seebeck coefficient of all the samples indicates the
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Table 1 Nominal composition, EPMA composition, relative density, reliable factors of refinement (Rwp, Rexp), s, S, l, ZT for Cu2þxCd1xGeSe4 (x ¼ 0, 0.025, 0.05, 0.075, 0.1, 0.125). Nominal composition
EPMA, composition
Reliable factors of refinement Rwp (%) Rexp (%)
Relative density (%)
s723 K (S/cm)
S723
Cu2CdGeSe4 Cu2.025Cd0.975GeSe4 Cu2.05Cd0.95GeSe4 Cu2.075Cd0.925GeSe4 Cu2.1Cd0.9GeSe4 Cu2.125Cd0.875GeSe4
Cu2.086Cd0.950Ge0.906Se4.058 Cu2.123Cd0.935Ge0.901Se4.041 Cu2.105Cd0.892Ge0.909Se4.094 Cu2.103Cd0.915Ge0.911Se4.071 Cu2.147Cd0.900Ge0.903Se4.050
5.77 3.90 4.70 3.95 3.37
85 85 83 88 86 84
8 17 23 57 72 42
389 336 314 252 226 167
2.86 3.76 3.04 2.77 2.77
dominant carriers are the holes. The behavior of Seebeck coefficient with temperature for Cu2CdGeSe4 is observed in the similar way like in the electrical conductivity. The Seebeck coefficient increases with an increase of temperature up to ~500 K for the compound Cu2CdGeSe4, ~620 K for Cu2.025Cd0.975GeSe4 and ~700 K for Cu2.05Cd0.95GeSe4. Above these temperatures the Seebeck coefficient started decreasing, when the onset of intrinsic conduction appeared. This reduction of the Seebeck coefficient is due to the bipolar effect, which can be explained by the equation (3) [23]. The average Seebeck coefficient (S) of a compound consists of two types of carriers and can be expressed as
S¼
Sn sn þ Sp sp sn þ sp
(3)
where Sn is the Seebeck coefficient by electrons, Sp is the Seebeck coefficient by holes, sn is the electrical conductivity by electrons, and sp is the electrical conductivity by holes. The onset of the intrinsic conduction regime in Seebeck coefficient data for the samples Cu2þxCd1xGeSe4(x ¼ 0, 0.025, 0.05) is almost consistent with the electrical conductivity data. The maximum value of the Seebeck coefficient 477 mV/K at 500 K was observed for the compound Cu2CdGeSe4. The energy gap of a semiconductor can be estimated from the maximum value of the Seebeck coefficient using the formula
Eg ¼ 2eSmax Tmax
K
(mV/K)
l723 K (W/m-K)
ZT (723 K)
0.53 0.64 0.61 0.71 0.64 0.59
0.16 0.22 0.27 0.37 0.42 0.14
(4)
where e is the elementary charge, Smax is the maximum value of the Seebeck coefficient and Tmax is the absolute value of the temperature at which the Smax occurs [23]. The energy gap of the compound Cu2CdGeSe4 was estimated as Eg ¼ 0.48 eV using eq (2), which is consistent with the reported value ~ 0.42 eV (estimated from Smax) [10]. The energy gap of Cu2CdGeSe4 estimated from the Seebeck coefficient and electrical conductivity data are in agreement. The temperature dependent power factor (S2s) was evaluated for all the compounds from the Seebeck coefficient and electrical conductivity. The maximum S2s ¼ 3.73 mW/cm-K2 at 723 K was obtained for the compound Cu2.1Cd0.9GeSe4. The power factor of excess Cu added sample Cu2.125Cd0.875Se4 is decreased drastically to a value of 1.14 mW/cm-K2 at 710 K and this is mainly influenced by the increase of carrier concentration leads to dropping of Seebeck coefficient. Still, the S2s values are lower because there is not much improvement in the electrical conductivity with an increase in nominal Cu content as compared with other quaternary compounds [1e3]. The existence of lower electrical conductivity values may be due to the difference between the nominal and actual composition determined by EPMA (see Table 1). This could possibly indicate that the holes created by a Cu addition on a Cd site might have been compensated for by the electrons created by the defects
Fig. 4. (aed) Secondary electron SEM images for Cu2þxCd1xGeSe4(x ¼ 0, 0.025, 0.05, 0.1).
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Fig. 5. (aed) Backscattered electron micrographs for Cu2þxCd1xGeSe4(x ¼ 0, 0.025, 0.05, 0.1).
Fig. 6. (a) Temperature dependent electrical conductivity for Cu2þxCd1xGeSe4(x ¼ 0, 0.025, 0.05, 0.075, 0.1, 0.125). Inset figure corresponds to the ln (s) versus 1/T. (b) Seebeck coefficient as a function of temperature for Cu2þxCd1xGeSe4(x ¼ 0, 0.025, 0.05, 0.075, 0.1, 0.125). (c) Temperature dependent total thermal conductivity for Cu2þxCd1xGeSe4(x ¼ 0, 0.025, 0.05, 0.075, 0.1, 0.125). (d) Temperature dependent lattice thermal conductivity for Cu2þxCd1xGeSe4(x ¼ 0, 0.025, 0.05, 0.075, 0.1, 0.125). Inset Figure shows the lL as a function of 1/T.
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161
[such as Ge on Cd (GeCd) and Ge on Cu (GeCu)] or may be due to a low sample density. 4.3. Thermal conductivity Fig. 6c displays the temperature dependent total thermal conductivity of all the samples and exhibit below 1.8 W/m-K in the entire range of temperature measured. The total thermal conductivity of all the samples decreased with an increase in temperature and reached values below 0.7 W/m-K at 723 K. The total thermal conductivity is dominated by the lattice part of thermal conductivity due to the insignificant contribution from the carrier part, lC, which can be calculated from the WeidemanneFranz relation lC ¼ LT/r, where L is the Lorenz number. The value of the Lorenz number can be obtained by using the following formula [24]
L¼
kB e
2
" #2 ! ðr þ 7=2ÞFrþ5=2 ðhÞ ðr þ 5=2ÞFrþ3=2 ðhÞ ðr þ 3=2ÞFrþ1=2 ðhÞ ðr þ 3=2ÞFrþ1=2 ðhÞ
(5)
To find the correct L value, the reduced Fermi energy h has to be calculated by using the measured Seebeck coefficient values from the following relation.
k S¼± B e
! ðr þ 5=2ÞFrþ3=2 ðhÞ h ðr þ 3=2ÞFrþ1=2 ðhÞ
where
Fn(h)
(6)
is the nth order Fermi integral, cn EF dc, h ¼ k T , kB is the Boltzmann constant, e the Fn ðhÞ ¼ ch B 0 1þe electron charge and EF the Fermi energy. By assuming the acoustic phonon scattering (r ¼ 1/2) as the main carrier scattering mechanism, the Lorenz number can be calculated by substituting the values of h and r into the equation (5). Fig. 6d shows the lattice thermal conductivity of all the samples as a function of temperature. The lattice thermal conductivity of all the samples decreased with increasing temperature and showed a 1/T dependence up to 650 K (See inset of the Fig. 6d for the Cu2CdGeSe4), which indicates the dominance of phononephonon scattering. Above 650 K, the thermal conductivity deviated from the straight line of 1/T dependence, which may be due to the presence of a bipolar contribution. The value of lL ~1.37 W/m-K at 336 K was obtained for the unsubstituted compound Cu2CdGeSe4, which is smaller than for binary compounds such as ZnSe [19 W/m-K at 300 K] [25],and CdSe [~9 W/m-K at 300 K] [25]. The suppression of thermal conductivity in the quaternary compounds as compared to binary compounds is due to their distorted crystal structure, which leads to an increase in phonon scattering as reported earlier [1,2,12,14]. The lattice thermal conductivity of all the samples was expected to decrease with an increase in nominal Cu content due to pointdefect scattering, but it did not follow any particular trend. This may be due to the variation in the porosity caused by the low density of the samples, which was clearly observed by BSE micrographs. The relative density of all the samples is listed in Table 1 and it is observed that these values are varied from sample to sample may results improper trend in the behavior thermal conductivity with doping. It may also be due to the variation in the observed composition of all the elements (Cu, Cd, Ge, Se) present in the compounds which did not follow systematically, in comparison with nominal composition, which was confirmed by EPMA. Fig. 7 displays the temperature dependent thermoelectric figure of merit (ZT), which was evaluated from the S, s and l values. The ZT values increased with increasing temperature for all the samples. The peak ZT ¼ 0.42 at 723 K was obtained for the compound Cu2.1Cd0.9GeSe4 due to the maximum power factor and the Z
∞
Fig. 7. Thermoelectric figure of merit (ZT) as function of temperature for Cu2þxCd1xGeSe4(x ¼ 0, 0.025, 0.05, 0.075, 0.1, 0.125).
reasonable value of thermal conductivity. This value is slightly lower than that of the other quaternary compounds and is mainly affected by the low values of the power factor as compared to the reported values. A further improvement in ZT can be achieved by enhancing the power factor and suppressing thermal conductivity. 5. Conclusions Quaternary chalcogenide compounds with the chemical formula Cu2þxCd1xGeSe4(x ¼ 0, 0.025, 0.05, 0.075, 0.1, 0.125) were prepared by melting and annealing. The powder XRD pattern of the compounds Cu2þxCd1xGeSe4(x ¼ 0, 0.025) revealed the mixture of low (tetragonal) and high (orthorhombic) temperature modifications, whereas a compound Cu2þxCd1xGeSe4 (x ¼ 0.05, 0.075, 0.1) showed a tetragonal crystal structure confirmed by a Rietveld analysis. The increase in electrical conductivity with an increase of nominal Cu content is not only due to the excess of Cu addition by replacing Cd. Other defects such as Cu on Ge and Cd on Ge might have influenced the conductivity where the deviation from the nominal composition was observed by EPMA. The positive Seebeck coefficients for all the samples indicate that the majority carriers are the holes. The Seebeck coefficient decreased with an increase in Cu content and it followed the reverse trend for electrical conductivity. The total thermal conductivity of all the samples is dominated by the lattice contribution, due to the negligible contribution from the carriers. Lattice thermal conductivity could not follow a particular trend with an increase in Cu content, and may be influenced by the variation in the porosity in all the samples. The maximum ZT ¼ 0.42 at 723 was obtained for Cu2.1Cd0.9GeSe4. Acknowledgments The authors would like to thank Prof. Satyam Suwas for providing the hot press facility and the Department of Science & Technology (DST), India for financial support through Grant No. INT/FRG/DAAD/P-222/2012. References [1] Liu ML, Huang FQ, Chen IW, Chen LD. Appl Phys Lett 2009;94:202103. [2] Liu ML, Chen IW, Huang FQ, Chen LD. Adv Mater 2009;21:3808. [3] Shi XY, Huang FQ, Liu ML, Chen LD. Appl Phys Lett 2009;94:122103.
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