Electronic and Thermoelectric Properties of Transition Metal

Article pubs.acs.org/JPCC

Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

Electronic and Thermoelectric Properties of Transition Metal Substituted Tetrahedrites Sahil Tippireddy,† Raju Chetty,† Mit H. Naik,‡ Manish Jain,‡ Kamanio Chattopadhyay,§ and Ramesh Chandra Mallik*,† †

Thermoelectric Materials and Devices Laboratory, Department of Physics, ‡Department of Physics, and §Department of Materials Engineering, Indian Institute of Science, Bangalore, India S Supporting Information *

ABSTRACT: A detailed theoretical and experimental study is carried out to understand the effect of transition metal atom (TM) substitution at the Cu tetrahedral site in synthetic tetrahedrite material Cu12−xTMxSb4S13 (TM = Mn, Fe, Co, Ni, and Zn). The samples are prepared by solid state synthesis method with the desired compositions. The X-ray diffraction (XRD) pattern of all the samples reveal tetrahedrite as the main phase with traces of secondary phases, confirmed by Electron Probe Micro Analysis (EPMA). X-ray Photoelectron Spectroscopy (XPS) reveals that TM is in +2 oxidation state for all samples except for Fe which shows a + 3 oxidation state. Ultraviolet Photoelectron Spectroscopy (UPS) measurements show the highest work function for Cu11.5Co0.5Sb4S13, indicating high band degeneracy. Density Functional Theory (DFT) calculations reveal that TM substitution introduces spin polarized states within the band structure, thus, changing the band degeneracy and density of states (DOS) at the Fermi level (EF). It is confirmed from DFT calculations that band degeneracy is highest for Cu11.5Co0.5Sb4S13 among the TM substituted samples, with high DOS at EF, which is experimentally confirmed by magnetic susceptibility analysis. The electrical resistivity (ρ) and Seebeck coefficients (S) of the substituted samples are higher than the pristine compound due to compensation of holes caused by substitution of TM+2 or TM+3 on the Cu tetrahedral site. Because of the high band degeneracy and DOS at EF, a high power factor is achieved for the composition Cu11.5Co0.5Sb4S13, enabling it to attain the maximum figure of merit (zT) among the substituted tetrahedrite compositions. The paper presents a comprehensive study to understand the role of magnetic impurities (TM) in influencing the band structure and, hence, the transport properties in substituted tetrahedrite. where S, κ and ρ represent the Seebeck coefficient, total thermal conductivity, and electrical resistivity, respectively. Equation 1 shows that zT depends directly on Seebeck coefficient (S) and is inversely proportional to electrical resistivity (ρ) and thermal conductivity (κ). The latter consists of lattice (κL) and carrier (κe) components. It, therefore, follows that, for a high zT, one needs a high power factor (S2/ρ) and low κ. However, it is challenging to improve zT, as these properties are coupled to each other, which means improvement of one property is achieved at the cost of another. This is because the charge carriers are the main agents of electric and

1. INTRODUCTION According to the present energy consumption demand, more and better alternate sources of energy are required to sustain consumer needs while being reliable and sustainable. Out of several alternate sources of energy, thermoelectricity is a potential candidate that can convert thermal energy into electrical energy and vice versa. Thermoelectric materials are used in thermoelectric generator (TEG) modules that recover waste heat from heat sources such as in automobiles1−3 and convert it into electricity. The efficiency of a thermoelectric material can be inferred by the figure of merit given by zT =

S2 T ρκ

Received: December 13, 2017 Revised: March 13, 2018

(1) © XXXX American Chemical Society

A

DOI: 10.1021/acs.jpcc.7b12214 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

maximum zT of 0.7 at 665 K was obtained for the composition Cu10.5Ni1.5Sb4S13. For the same composition, a zT of 0.8 at 700 K was achieved by Barbier et al.11 Chetty et al.8 performed thorough studies on Mn-substituted tetrahedrite, Cu12−xMnxSb4S13, with x = 0, 0.5, 1, 1.5, and 2.0. A maximum zT of 0.64 was obtained at 673 K for the composition Cu11.5Mn0.5Sb4S13. A maximum zT of 0.89 at 700 K was also obtained for the same composition by Han et al.12 Chetty et al.9 also studied the thermoelectric properties of Co-substituted tetrahedrite, Cu12−xCoxSb4S13, with x = 0, 0.5, 1, 1.5, and 2. A high power factor of 1.76 mW/m·K2 was obtained for Cu11.5Co0.5Sb4S13, which yielded a maximum zT of 0.98 at 673 K. Kumar et al.13 carried out Cd substitution at the Cu tetrahedral site in the series Cu12−xCdxSb4S13, with x = 0, 0.25, 0.5, 0.75, 1, 1.25, 1.5. A maximum zT = 0.9 at 623 K was found for the composition Cu11.25Cd0.75Sb4S13, mainly due to decreased thermal conductivity. Suekuni et al.14 performed low temperature thermoelectric studies up to 340 K for the compositions Cu10Tr2Sb4S13 (Tr = Mn, Fe, Co, Ni, and Zn). They obtained a maximum zT of 0.17 for the Ni-substituted sample. Suekuni et al.10 also performed theoretical calculations of the electronic and magnetic properties for Cu12−xTMxSb4S13 (TM = Mn, Fe, Co, Ni, Zn). The band structure and PDOS for Cu12−xNixSb4S13 (x = 0, 1, and 2) and Cu11TM1Sb4S13 (TM = Mn, Fe, Co, and Zn) were calculated using spin polarized DFT.15,16 A high DOS at EF was found for the x = 0 compound (pristine tetrahedrite), which consisted of hybridized orbitals of Cu-3d and S-3p. For Cu11TM1Sb4S13, polarized impurity states consisting of TM 3d orbitals were introduced in the valence band and the bandgap, that resulted in the modification of the band structure. Although many of the transport properties of transition metal substituted tetrahedrite were explained based on experimental results and initial theoretical studies, a systematic and coherent explanation is lacking between the experimental findings and theory. Our investigations have, therefore, bridged this gap and achieved a better understanding of the electrical and thermal properties of the TM-substituted tetrahedrite system in terms of band structure and projected density of states (PDOS). This will also enable us to explore new dopants in the future and tune the band structure in a suitable and systematic manner. In this work, we have started with the compositions that have shown the best zT, as reported in the literature: Cu12−xTMxSb4S13, where x = 0.5 for TM = Mn, Fe, Co and x = 1.5 and 1 for TM = Ni and Zn, respectively.4,5,8,9 It should be mentioned here that the characterization and transport data for the pristine, Mn-, and Co-substituted tetrahedrite samples were taken from our group’s earlier work by Chetty et al.,8,9 whereas the Fe-, Ni-, and Zn-substituted samples were prepared for this report. A detailed characterization was carried out to determine the structure, phases, and thermal stability of the samples using XRD, EPMA, DSC, and TGA. The oxidation states of the individual elements were obtained from XPS, which was used to interpret the electrical resistivity results. UPS was carried out to determine the work functions of all the samples that provided insight into understanding the transport properties. In addition, magnetic susceptibility measurements were performed to verify the spin polarization in TMsubstituted samples and determine the density of states at the Fermi level. The electrical resistivity, Seebeck coefficient, and thermal conductivity were measured and understood on the basis of both experimental results and theoretical calculations.

thermal transport in a material. Increasing their concentration would increase both electrical conductivity (σ) and electronic part of thermal conductivity (κe), but with κ being inversely proportional to zT, needs to be decreased in order to increase the efficiency. The problem, therefore, becomes one of decoupling σ and κ, which is usually done by reducing the lattice component of thermal conductivity κL. This is usually achieved by arresting the motion of heat carrying acoustic phonons. However, in some cases, as in tetrahedrites, a higher zT is achieved by decreasing the electronic component of thermal conductivity κe by reducing the charge carrier concentration. Although this approach increases ρ, an optimized ratio of S2/ρκ can be attained to ultimately increase zT. Tetrahedrites are naturally occurring minerals belonging to the family of sulfosalts. The pristine compound of tetrahedrite with nominal composition Cu12Sb4S13 exhibits a relatively high thermoelectric performance in the temperature between 473 and 773 K. A zT of 0.57 at 673 K was obtained by Lu et al.4 and around 0.5 at 673 K by Suekuni et al.,5 owing to high power factor and low thermal conductivity. The tetrahedrite structure was first proposed by Pauling and Neuman (1934) as a derivative of the sphalerite structure.6 The atomic positions were, however, slightly inaccurate, which was corrected by Wuensch (1964).7 A general chemical formula for this class of 3+ 2− materials can be written as A+10B2+ 2 X4 Y13 , where A is either Cu or Ag, B can be a divalent cation such as Mn, Fe, Co, Ni, Cu, or Zn, X can be Sb or As, and Y is S or Se. Tetrahedrites crystallize in body-centered-cubic (BCC) structure with the space group I4̅3m, as shown in the Figure S1 of the Supporting Information (SI). The unit cell contains 58 atoms with two formula units. It can be seen from the Figure S1 that there are two inequivalent Cu atoms (Cu(1) and Cu(2)) coordinated tetrahedrally and trigonally (planar) to S atoms and two inequivalent S atoms (S(1) and S(2)) bonded tetrahedrally and octahedrally with Cu atoms. Sb is trigonally (pyramidal) coordinated to S atoms. Hence, a more formal way of writing the chemical formula in terms of coordination will be (Cu+)tr6 (Cu+)t4(Cu2+)t2(Sb3+)π4[(S2−)t12(S2−)o1], where tr = triangular, t = tetrahedral, π = pyramidal, and o = octahedral coordination. Earlier studies have shown that substitutions at the Cu, Sb, and S sites in tetrahedrite yield better thermoelectric properties due to optimized power factor and reduced thermal conductivity. Previously,4,5,8−10 it was reported that substituting tetrahedrite material with d-block transition metals yielded better thermoelectric properties due to two reasons: (1) The introduction of a divalent cation on the tetrahedral Cu1+ site optimizes the number of charge carriers (holes in this case). (2) The suppression of charge carriers results in reduction of κe according to the Weidmann-Franz relation (κe = LσT), which in turn decreases the overall total thermal conductivity. These studies explored substituting with transition metals (TM) such as Mn, Fe, Co, Ni, and Zn at the Cu tetrahedral site. Various concentrations of TM were used to optimize the electrical resistivity and Seebeck coefficient to achieve the best possible zT.4,5,8−10 Lu et al.4 performed substitutions at Cu tetrahedral site in Cu12−xMxSb4S13, where M = Fe and Zn, with x = 0, 0.5, 1, 1.5. For M = Fe, a maximum zT of 0.8 at 720 K was obtained for x = 0.5, whereas for M = Zn, a maximum zT of around 1 at 720 K was obtained for x = 1. Studies on Ni-substituted tetrahedrite were carried out by Suekuni et al.,5 who synthesized Cu12−xNixSb4S13 with x = 0, 0.5, 1, 1.5, and 2. A B


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2. EXPERIMENTAL DETAILS The starting materials Cu, Sb, S, and TM (TM = Mn, Fe, Co, Ni, Zn) with high purity (∼5 N) were weighted in stoichiometric ratio, transferred into a quartz ampule, and sealed under a vacuum of around 10−4 mbar. The ampules were heated to 973 K, held at that temperature for 3 h, and then cooled to 823 K in 30 h. The furnace was then allowed to cool down to room temperature. The resulting ingots were ground using mortar pestle and cold pressed to form pellets. Subsequently, the pellets were annealed at 773 K for 30 h in evacuated quartz tubes. The annealed samples were reground, loaded into graphite dies with a diameter of 14 mm, and then hot pressed under a pressure of 30 MPa at 823 K for 2 h. The densities (ds) of all the samples were measured using Archimedes’ principle. For Cu11Ni1Sb4S13 and Cu11Zn1Sb4S13, the density was found to be ∼90% of the theoretical density of the pristine compound, ∼95% for Cu11.5Mn0.5Sb4S13 and Cu11.5Fe0.5Sb4S13, and ∼98% for Cu11.5Co0.5Sb4S13 samples. The sintered samples were cut into cuboids of dimensions ∼11 × 3 × 3 mm3 for Seebeck coefficient and electrical resistivity measurements, and into disk shape having a diameter of ∼12.7 mm and a thickness of ∼1 mm for thermal diffusivity measurement. The X-ray diffraction (XRD) patterns of all the samples were obtained by a Rigaku Smart Lab X-ray diffractometer using Cu Kα (λ = 1.5418 Å) radiation. The Rietveld refinement of the XRD pattern was carried out using FullProf software.17 The microstructures and energy dispersive spectroscopy (EDS) analyses of the compounds were performed by a JEOL JXA-8530F electron probe microanalyzer (EPMA). Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) of the samples were done on a TA Instruments DSC 2920 and a Mettler Toledo TGA SDTA 851e, respectively. The X-ray photoelectron spectroscopy (XPS) was carried out for all the samples on a Kratos Axis Ultra with Al Kα as the excitation source. The electrical resistivities (ρ) and Seebeck coefficients were measured in the temperature between 323 and 673 K on a LINSEIS LSR-3 system. A TA DLF2 laser flash system was employed to measure thermal diffusivity (D) and specific heat (Cp). The total thermal conductivity was obtained using the formula κT = DdsCp. The magnetic susceptibility measurement was carried out in a Quantum Design magnetic property measurement system (MPMS) between 5 K and 300 K, in a magnetic field of 1 T. The measurement errors for electrical resistivity, Seebeck coefficient, and thermal conductivity are 10%, 7%, and 5%, respectively.

structure and the projected density of states (PDOS) for all the samples were obtained with contributions from spin up and spin down states. To incorporate the electron correlation of the 3d orbital electrons in the transition metal atoms, the calculations for TM = Mn, Fe, Ni, and Co were performed using the Hubbard U parameter. U was taken as 2 eV for the Cu atoms and 0, 2, 4, and 6 eV for the substituted transition metal atoms.

4. RESULTS AND DISCUSSION 4.1. X-ray Diffraction. The XRD pattern of all the samples indexed with the tetrahedrite cubic phase (ICSD #25707) is shown in Figure 1. The Rietveld refinement of the XRD

Figure 1. XRD powder pattern of all the samples indexed with the tetrahedrite cubic phase marked with secondary phase peaks.

patterns was carried out for all the samples using the pseudovoigt profile function. The zero-point shift, shape parameters of the profile function and asymmetry parameters were refined systematically, along with background correction. The crystal structure was refined by varying the atomic coordinates, lattice parameters, occupancies and isotropic displacement parameters. The transition metal (TM) element was substituted at the 12d tetrahedral site in accordance with various literature reports,5,8,11,20,21 where it was proved that transition metal elements prefer substitution on the 12d site rather than the trigonally coordinated 12e site. Moreover, it was found from theoretical calculations that TM substitution is energetically more favorable at the 12d site as compared to the 12e site, which leads to the conclusion that TM substitutes at the 12d site. The refined XRD patterns for all the samples (including the secondary phase/phases) are shown in the Figures S2−S7 (Supporting Information). The structural details including the atomic coordinates, atomic displacement parameters (Uiso), occupancies and reliability factors, obtained from the Rietveld refinement are given in Tables T1−T6 of the SI. The results of the Rietveld refinement confirm tetrahedrite structure as the main phase with compositions close to the ones obtained from EDS. The details of the refinement analysis are discussed in the SI. The XRD patterns for Cu11.5Fe0.5Sb4S13 and Cu11Ni1Sb4S13 contained minor peaks corresponding to secondary phases of CuSbS2 and NiS and CuSbS2, respectively. The lattice parameter “a” obtained from Rietveld refinement, is

3. THEORETICAL CALCULATION DETAILS The theoretical calculations were performed using the plane wave density functional theory (DFT) package, using Quantum Espresso.18 The electronic band structure and DOS were calculated using spin polarized DFT within the generalized gradient approximation (GGA) to the exchange correlation functional as proposed by Perdew, Burke, and Ernzerhof.19 The energy cut off for plane wave expansion is 750 eV with a Monkhorst−Pack k-point grid of 7 × 7 × 7. The crystal structure was considered as BCC with 58 atoms per unit conventional cell and a lattice parameter of 10.33 Å. The transition metal atom was substituted at the Cu(1) tetrahedral site, which was reported to be energetically more favorable by 0.4 eV per substitutional atom.4 The structure was relaxed until the forces on the atoms were less than 0.01 eV/ Å. The band C


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to Zn agrees well with that reported for Cu10M2Sb4S13 by Suekuni et al.14 The variation of magnitudes of lattice parameter between the present work and the reported work is mainly due to the difference in the doping content and slightly different stoichiometry. In summary, this systematic variation of lattice parameter with the doping elements indicates the substitution of transition metal atoms on the Cu site. 4.2. Thermal Stability and Phase Transitions. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were done from room temperature to 820 K in order to investigate the thermal stability of all the samples. The DSC curve shown in the Figure S9 (SI) shows a minor peak at around 650 K in Fe-, Ni-, and Zn-substituted samples, which is attributed to the CuSbS2 phase.26 The Ni-substituted sample had an additional minor peak at around 580 K, which was identified as the NiS phase.27 These DSC peaks are consistent with the XRD results. All the samples displayed a sharp endothermic peak at around 810 K corresponding to a weight loss in the TGA curve (Figure S10 of SI). This endothermic peak can be due to complete decomposition of the tetrahedrite phase into CuSbS2 and Cu2S phases with subsequent sulfur volatilization in the samples, which could also be the reason for the corresponding weight loss seen in TGA. These results are in agreement with those reported by Skinner and Makovicky28,29 and Barbier et al.11 4.3. Microstructure and Phase Characterization. The energy dispersive X-ray spectroscopy (EDS) was performed to identify the phases in the samples. The results for the pristine, Cu11.5Mn0.5Sb4S13, and Cu11.5Co0.5Sb4S13 samples are published elsewhere, 8 ,9 and the results for Cu 11.5 Fe 0.5 Sb 4 S 13 , Cu11Ni1Sb4S13, and Cu11Zn1Sb4S13 are presented here. The Figures S11−S13 (SI) show the backscattered electron (BSE) images for the samples Cu11.5Fe0.5Sb4S13, Cu10.5Ni1.5Sb4S13, and Cu11Zn1Sb4S13, respectively. In sample Cu11.5Fe0.5Sb4S13, secondary phases of Cu2−yS and CuSbS2 were found (∼10% and ∼5%, respectively). The sample Cu10.5Ni1.5Sb4S13 showed secondary phases of NiS (2%) and CuSbS2 (1%), while Cu11Zn1Sb4S13 had an almost negligible secondary phase of CuSbS2 (0.3%). The sample Cu11.5Co0.5Sb4S13 had a minor secondary phase of Cu3SbS4 (1%),9 whereas Cu11.5Mn0.5Sb4S13 contained no secondary phase.8 The chemical compositions of all the samples determined by EDS are given in Table 2. All the

given in Table 1 and plotted in Figure S8 (SI) as a function of dopants. The lattice parameter for the pristine composition was Table 1. Lattice Parameter of All the Samples Obtained from Rietveld Refinement sample

dopant ionic radius (Å)

lattice parameter (Å)

Cu12Sb4S13 Cu11.5Mn0.5Sb4S13 Cu11.5Fe0.5Sb4S13 Cu11.5Co0.5Sb4S13 Cu11Ni1Sb4S13 Cu11Zn1Sb4S13

0.6 (Cu1+), 0.57(Cu2+) 0.66 (Mn2+) 0.49 (Fe3+) 0.58 (Co2+) 0.55 (Ni2+) 0.6 (Zn2+)

10.3198(1) 10.3453(1) 10.3302(3) 10.3194(3) 10.3144(2) 10.3391(1)

found to be 10.3198 Å, which is close to the literature value of ∼10.32 Å based on reports by Makovicky et al.22 and Tatsuka et al.,23 Our value is also similar to the value obtained by Barbier et al.11 (10.32 Å). On Mn substitution, the lattice parameter increases to 10.3453 Å due to the higher ionic radius of Mn2+ (0.66 Å) as compared to Cu1+ (0.6 Å) and Cu2+ (0.57 Å). The lattice parameter of Cu11.5Fe0.5Sb4S13 was found to be 10.3302 Å, which is similar to the value of 10.332 Å obtained by Makovicky et al.24 and 10.33269 Å obtained by Friese et al.25 for the composition Cu11.4Fe0.6Sb4S13. The value obtained in the present work is also only slightly larger than the value obtained by Andreasen et al.20 (10.3283 Å) and Nasonova et al.21 (∼10.328 Å), but lower than 10.3424 Å obtained by Tatsuka et al.23 for the composition Cu11.5Fe0.5Sb4S13. It was observed that the lattice parameter for Fe-substituted sample in both the present work and literature is, however, larger than the pristine compound’s which is contradictory, as Fe3+ has a lower ionic radius (0.49 Å) than both Cu1+ (0.6 Å) and Cu2+ (0.57 Å). This increase in lattice parameter with Fe substitution was also observed by Nasonova et al.21 and Tatsuka et al.23 One possible explanation for this anomaly can be understood based on the analysis by Friese et al.25 They explain that initially for the pristine composition there are 10 Cu1+ (0.6 Å ionic radius) atoms and 2 Cu2+ (0.57 Å ionic radius) atoms. Fe substitution at Cu tetrahedral sites induces the replacement of Cu2+ atoms by Cu1+ due to the higher stabilization of (Fe3+ + Cu1+) as compared to (Fe2+ + Cu2+) combination, as reported by Makovicky et al.24 As a result, more Cu1+ atoms having a higher ionic radius than Cu2+ are present than previously, which can lead to a higher lattice parameter. Another possible reason can be inferred from the report by Tatsuka et al.,23 who performed thorough analysis over a wide range of Fe containing tetrahedrite compositions. It was reported that at low levels of Fe substitution, a copper-rich phase can form, with additional Cu atoms added at the interstitial sites in the unit cell. This can also therefore lead to a higher lattice parameter as compared to the pristine compound. However, they assume Fe to be in the +2 oxidation state in their report and thus attribute the increase in lattice parameter to the higher ionic radius of Fe2+ as compared to Cu2+. It therefore follows that a more thorough investigation is required to ascertain the cause of the increase in a with Fe substitution. Further, while going from Co to Ni, the lattice parameter decreases due to a decrease in ionic radius from 0.58 Å (Co2+) to 0.55 Å (Ni2+). The lattice parameter of 10.3144 Å exhibited by the Ni-substituted sample is similar to the one obtained by Suekuni et al.5 (10.31894 Å). However, a increases from Ni to Zn because of the higher ionic radius of Zn2+ (0.6 Å), substituting at the Cu1+ or Cu2+ site. The trend of variation of lattice parameter as a function of dopants from Mn

Table 2. Nominal Composition and Observed EPMA Composition for Main Phase and Secondary Phases nominal composition

observed EPMA composition for main phase

observed EPMA composition for secondary phases

Cu11.5Mn0.5Sb4S13 Cu11.5Fe0.5Sb4S13 Cu11.5Co0.5Sb4S13 Cu10.5Ni1.5Sb4S13 Cu11Zn1Sb4S13

Cu11.6(0.03)Mn0.4(0.006)Sb4(0.01)S13(0.01) Cu11.5(0.24)Fe0.5(0.25)Sb4(0.08)S13(0.12) Cu11.6(0.02)Co0.4(0.006)Sb4(0.01)S13(0.03) Cu10.9(0.17)Ni1.1(0.12)Sb4(0.07)S13(0.06) Cu11(0.23)Zn1(0.18)Sb4(0.15)S13(0.07)

none Cu2−yS, CuSbS2 Cu3SbS4 NiS, CuSbS2 CuSbS2

samples were found to have compositions close to nominal compositions, except for the Ni-substituted sample, which formed Cu11Ni1Sb4S13 as the main phase. This could be due to the partial amount of Ni precipitated as NiS impurity phase, hence, showing lower Ni content in the tetrahedrite-substituted sample. Hereafter, we will consider Cu11Ni1Sb4S13 as the composition for analysis throughout the manuscript. D


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Figure 2. XPS spectra of TM for the compounds (a) Cu11.5Mn0.5Sb4S13, (b) Cu11.5Fe0.5Sb4S13, (c) Cu11.5Co0.5Sb4S13, (d) Cu11Ni1Sb4S13, and (e) Cu11Zn1Sb4S13.

4.4. X-ray Photoelectron Spectroscopy (XPS). The XPS spectrum obtained from each sample was analyzed to determine the oxidation states of the elements present in the compounds. The spectra were first calibrated from C 1s line of binding energy (BE) 284.4 eV, taken from the NIST XPS database. The background in the spectra was subtracted and the peaks were deconvoluted followed by peak fitting with a suitable Gaussian or Lorentzian function. For Mn-, Fe-, Co-, and Ni-substituted samples, additional smoothening of the spectra was done using the Fast Fourier Transform (FFT) method. Figures S14−S16 (SI) show the XPS spectra for Cu, Sb, and S in the substituted tetrahedrite samples and Figure 2a−e shows the deconvoluted spectrum for the TM substituted samples after smoothening and peak fit. Details of the peak assignment for the dopant atoms with the corresponding BEs are listed in Table 3. In all the compounds, Sb and S were found to exist in +3 and −2 states, respectively. The transition metal dopants were found to be in the +2 oxidation state except Fe, which showed peaks corresponding to the +3 oxidation state. This is in accordance with Mössbauer studies done by Makovicky et al.24 on Fe-substituted tetrahedrite, where it was discovered that Fe to exist in the +3 oxidation state for the compositions 0.5 ≤ x ≤ 1 in Cu12−xFexSb4S13. This was also

Table 3. XPS Peaks Corresponding to the Oxidation States of Individual Dopant Elements compound

dopant element

peak

BEa (eV)

oxidation state

Cu11.5Mn0.5Sb4S13 Cu11.5Fe0.5Sb4S13

Mn Fe

Cu11.5Co0.5Sb4S13

Co

Cu11Ni1Sb4S13

Ni

Cu11Zn1Sb4S13

Zn

2p3/2 2p3/2 2p1/2 2p3/2 2p3/2 2p3/2 2p3/2 2p3/2 2p3/2

640.17 710.49 723.29 778.16 785.8 854.28 855.12 856.6 1021.39

+2 +3 +3 +2 +2 +2 +2 +2 +2

a

The binding energy of the XPS peaks are indexed from the NIST database.

observed by Nasonova et al.,21 who also confirmed Fe exists in the +3 oxidation state for lower Fe content (x < 1.2) based on crystallographic analysis, Mössbauer spectroscopy, and magnetization measurements. The reason for Fe to exist in +3 oxidation state was given by Hall et al.30 and Makovicky et al.24 The explanation is that Fe3+ having five electrons in the d orbital (d5) is exactly half filled and thus is more stable than E


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The Journal of Physical Chemistry C Fe2+, which has six electrons (d6). A d6 configuration in tetrahedral coordination can also lead to Jahn−Teller distortion leading to breaking of symmetry and hence a d5 configuration becomes more favorable. Fe possessing a +3 oxidation state (high spin d5) in a similar tetrahedral environment is also thus observed in other Fe-bearing compounds with sulfur coordination such as CuFeS2, KFeS2, TiFeS2, and CsFeS2.31 And more recently reported for Cu 8 Fe 3 Sn 2 S 12 32 and Cu22Fe8Ge4S3233 also containing (Fe3+)S4 tetrahedra. Cu was found to exist in both +2 and +1 oxidation states, although the peaks corresponding to +2 were not prominent in all the compounds. Our results are in agreement with those performed by Chetty et al.,9 for the compositions Cu12−xCoxSb4S13 (x = 0, 0.5, and 1.5). Ideally, for the pristine tetrahedrite composition (Cu+)tr6 (Cu+)t4(Cu2+)t2(Sb3+)π4[(S2−)t12(S2−)o1], there are 10 Cu atoms with +1 oxidation state and 2 Cu atoms with +2 oxidation state. Hence, in XPS, one can expect peaks corresponding to Cu1+ to have a higher intensity than the Cu2+ peaks. The ICu1+/ICu2+ XPS peak intensity ratio should, therefore, be higher in the tetrahedrite system and can vary depending on which Cu atom, the TM preferably substitutes. In other words, TM would usually take part in partial substitution at both Cu1+ and Cu2+ tetrahedral sites, but the probability of substituting at any one of these sites could be higher, resulting in different ICu1+/ICu2+ XPS peak intensity ratios. For Mn-, Co-, and Ni-substituted samples, the ICu1+/ICu2+ XPS peak intensity ratio was found to be quite high, indicating that TM is substituting favorably at Cu2+ tetrahedral site, as the signal coming from Cu2+ would have been reduced. That is to say, the probability of TM substituting at Cu2+ is much higher than it substituting at Cu1+ tetrahedral site. But, on the other hand, for Fe- and Zn-substituted samples, a relatively lower ICu1+/ICu2+ peak intensity ratio was observed, indicating that TM may have preference for substitution at the Cu1+ tetrahedral site. Therefore, it can be concluded that in Fe- and Znsubstituted samples, the probability of TM substituting at Cu1+ is higher or similar to that of occupying Cu2+. As a result, relatively more electrons are supplied to suppress holes as compared to other TM-substituted samples. This fact is also reflected in transport properties where electrical resistivity is found to be heavily influenced by the manner in which TM substitutes at the Cu site, as will be discussed in section 4.7.1. In addition to XPS, ultraviolet photoelectron spectroscopy (UPS) was carried out to probe the valence electrons and extract the work functions of the samples. The Figure 3 shows the raw UPS spectra of the samples as a function of Binding Energy (BE). The Fermi levels with respect to vacuum were found from the difference of the intercept of the trailing edge of secondary electron onset, and the He(I) excitation energy of 21.2 eV, shown in the Figure S17 (SI) for Cu11Zn1Sb4S13. The results are listed in Table 4. It was realized from the data that the work function was highest for Cu11.5Co0.5Sb4S13, which means that it has the lowest Fermi level w.r.t. vacuum. A lower value of Fermi level is an indication of high band degeneracy since carriers can be distributed into more number of bands, which was also observed in band structure calculations. A high band degeneracy should result in a high power factor, which is indeed seen in the experimental data. 4.5. Theoretical Calculations. The projected density of states (PDOS) of all the samples is shown in Figure 4a−f and the band structures in Figures S18−S23 (SI). The Fermi level (EF) is taken as 0 eV and all energies were referenced with respect to EF. The Fermi level for all the compounds lies within

Figure 3. UPS spectra of all the samples as a function of Binding Energy (eV).

Table 4. Work Functions of All the Samples Calculated from UPS Spectra compound

intercept of SE onset at x-axis

Fermi position w.r.t. vacuum (eV)

Cu11.5Mn0.5Sb4S13 Cu11.5Fe0.5Sb4S13 Cu11.5Co0.5Sb4S13 Cu11Ni1Sb4S13 Cu11Zn1Sb4S13

16.64 16.63 16.51 16.60 16.68

4.56 4.57 4.68 4.59 4.51

or close to the valence band, indicating p-type nature of the samples. The PDOS calculations of all the samples reveal that there is high density of states near the Fermi level, indicated by a sharp peak at the top of the valence band. The PDOS also revealed that the valence bands are formed primarily from the hybridization of sulfur 3p and copper 3d orbitals. For the pristine tetrahedrite Cu12Sb4S13, an indirect bandgap of around 1.1 eV was found from the band structure (Figure S18), which is close to the value of 1.2−1.7 eV reported in the literature.10 An equivalent density of states (DOS) is observed for both up and down spins, suggesting that there is no spin polarization (Figure 4a). The band structure calculations showed that there are two unoccupied states (holes) in the valence band, with high band degeneracy near EF for pristine tetrahedrite, consistent with DFT calculations performed by Lu et al.4 It was observed that TM substitution created hybridized impurity levels both within the valence bands and the band gap, thereby modifying the band structure, and altering the band degeneracy near EF, which is otherwise highest for the pristine compound. In the case of Mn-substituted tetrahedrite with U = 0 (Figure 4b), the up spin and down spin Mn 3d orbitals are split in energy with the up spin states being almost fully occupied while the down spin states lie primarily in the next unoccupied band above EF. This indicates a high spin (d5) configuration for Mn2+, where all the five “d” orbitals are occupied with up spin states. These up spin states are hybridized with the host valence states of S and Cu, resulting in a modification of the overall up spin states in the valence band, thus, causing an enlargement of the up spin DOS near the Fermi level. In Cu11.5Fe0.5Sb4S13 (Figure 4c), the exchange splitting of the up and down spin Fe 3d bands for U = 0 is reduced as compared to the Mn case. The Fe 3d up spin bands are again almost completely occupied with energies at about 2.8 eV below the EF, whereas the down spin impurity bands consisting mainly of Fe-3d and S-3p orbitals F


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Figure 4. Projected Density of States (PDOS) for U = 0 eV: (a) Cu12Sb4S13, (b) Cu11.5Mn0.5Sb4S13, (c) Cu11.5Fe0.5Sb4S13, (d) Cu11.5Co0.5Sb4S13, (e) Cu11Ni1Sb4S13, and (f) Cu11Zn1Sb4S13.

band structure in a manner that among the TM substituted samples, the highest band degeneracy near EF was retained by the Co substituted tetrahedrite. For Ni substituted tetrahedrite, the results for the composition Cu11Ni1Sb4S13 (Figure 4e) are given instead, as we could not obtain the desired composition of Cu10.5Ni1.5Sb4S13. Here, the exchange splitting (for U = 0) is again even further reduced, with almost equal up and down spin bands contributing to the states below EF. The down spin impurity bands of Ni-3d, Cu-3d and S-3p orbitals appear 0.2 eV above EF again corresponding to the unoccupied d levels by the down spin states. In the case of Cu11Zn1Sb4S13 (Figure 4f), there is no exchange splitting, as all d orbitals of Zn2+(d10) fill up completely by equal up and down spin states, consisting of Zn-3d and S-3p orbitals that lie at 7 eV below EF. It can be

appear in the band gap at around 0.3 eV from the Fermi level. It can be observed from Figure S20 that Fe substitution leads to creation of three down spin unoccupied bands above EF, which is attributed to the unoccupied two eg and three t2g states. This indicates a high spin (d5) configuration, which is to be expected for Fe3+ in a tetrahedral environment, as confirmed by the XPS results. For TM = Co having U = 0 (Figure 4d), the exchange splitting is further reduced and now both the up spin and down spin bands are occupied in the valence band, with additional down spin bands consisting of hybridized Co-3d and S-3p orbitals seen in the band gap at around 0.75 eV above EF. These bands again correspond to the unoccupied t2g levels by the down spin states, as expected for a Co2+ (d7) configuration. It was realized that the up and down spin Co 3d states modify the G


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Figure 5. Total Density of States (DOS) and TM 3d states for U = 0, 2, 4, 6 eV: (a) Cu11.5Mn0.5Sb4S13, (b) Cu11.5Fe0.5Sb4S13, (c) Cu11.5Co0.5Sb4S13, (d) Cu11Ni1Sb4S13.

values of U. This modification was not so strong for Mn case (Figure 5a), where TM consisting of hybridized up spin states lie inside the valence band. For TM = Fe case (Figure 5b), the two eg and three t2g states above EF, which were initially localized (for U = 0), get delocalized and merge at about 0.5 eV above EF for U = 2 eV. Interestingly, on further increasing the U value to 4 eV, the Fe states above EF again become distinct peaks in the DOS profile. The two eg bands form distinct DOS peaks separated by 0.86 eV, lying on either side of the t2g band peak within the band gap. With U = 6 eV, both the eg and t2g down spin states are raised in higher energy and merge into the conduction band. For TM = Co (Figure 5c), the sharp DOS peak at 0.75 eV above EF caused by hybridized Co 3d down spin states, is shifted higher in energy with increasing value of U. The sharpness of the profile too decreases with U, indicating more delocalization of the Co 3d down spin states. For U = 6 eV, it can be seen that the down spin Co states are even more delocalized and get merged into the conduction band. In the case of TM = Ni (Figure 5d), the down spin Ni 3d states above EF are shifted higher only slightly (∼0.08 eV) when going from U = 0 to U = 2 eV. The DOS profile too changes with U = 2 eV TM states being slightly more delocalized than in the U = 0 case. On further increasing the U value to 4 eV, the down spin Ni states are even more delocalized, appearing at around 0.9 eV above EF. But on further increasing the U value to 6 eV, the down spin Ni states are lowered in energy and appear at around 0.07 eV from the Fermi level.

concluded that substituting TM at the Cu tetrahedral site introduces impurity bands within the valence band, which hybridize strongly with Cu and S orbitals. The result is polarization of the band structure and modification of the band degeneracy and DOS near the Fermi level, which ultimately affects the transport properties. These results are in agreement with those obtained by Suekuni et al.10 Figure 5a−d shows the total DOS and TM 3d states with different values of U. It was observed in all systems that the DOS profile for both valence and conduction bands is modified by the Hubbard U parameter (Figure 5). It was seen that the highest intensity peaks in the valence band around 2 eV below EF, for U = 0, are shifted lower in energy by around 0.4−0.5 eV w.r.t. the Fermi level on the application of U. Whereas the conduction band states (for finite U) are shifted by around 0.1−0.2 eV higher w.r.t. EF, than in the U = 0 case. The band gap in all the cases increased with increasing values of U by about 0.1−0.3 eV. In all the cases, it was seen that the DOS profile for both valence and conduction bands (for U = 2, 4, and 6) is smoother as compared to the U = 0 profile, with varying degrees of modification of DOS at EF. It can be inferred that the on-site Coulombic potential experienced by the d electrons of the transition metal atoms (introduced by applying U) causes the states to become more delocalized. The states at EF were also thus affected depending on the contributing TM states, which were more pronounced for Mn and Ni cases. On the other hand, for Co and Fe, the DOS at EF for finite U remained fairly the same as in the case for U = 0. It was also found that the positions and profiles of the TM states lying within the band gap too changed with varying H


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The Journal of Physical Chemistry C In summary, the GGA + U calculations reveal that the TM states are considerably dependent on the Hubbard U parameter, which is affecting the band structure and, hence, the transport properties. It is therefore important to incorporate the on-site Coulombic interaction (Hubbard U) of the electrons to have a clearer picture in understanding the transport properties. However, in the present case, the highest DOS and degeneracy at EF was exhibited by Cu11.5Co0.5Sb4S13, which was unaffected by the application of U. Since bands just above the Fermi level would take part in transport properties, it can be seen from the band structure of all the samples that bands corresponding to the symmetry directions along Γ, P, and H would contribute to the transport. The electrical resistivity and Seebeck coefficient would, therefore, also depend on the dispersive nature of these bands. The degree of dispersion is nothing but the hole effective mass along these directions which was calculated from the formula:

m* =

ℏ2 (d E)/(dk 2) 2

Xo = μoμBg (E F)

Here μo, μB, and g(EF) are the permeability of free space, Bohr magneton, and DOS at the Fermi level, respectively. The second term in the equation 3 is the Curie term where TC is the Curie temperature and the constant C is proportional to the effective magnetic moment. The values of Xo, C, and TC were obtained from the fit and are given in the Table 5. It can be observed that the maximum Xo of 3.9 × 10−2 emu mol−1 G−1 was obtained for Cu11.5Co0.5Sb4S13, confirming the relatively higher DOS at the Fermi level. The minimum value of Xo was shown by Zn-substituted sample (1.6 × 10−2 emu mol−1 G−1), indicating low DOS at EF according to equation 4. The negative values of TC for all the samples indicate antiferromagnetic ordering between the effective spins of the transition metal atoms. The value of TC obtained for Cu11Ni1Sb4S13 in this work (∼−33 K) is comparable to the value of −40 K obtained by Suekuni et al.,.10 The effective magnetic moment in Bohr magnetons was evaluated from the constant C for each sample, given in Table 5. It was found that Cu11Zn1Sb4S13 had nearly no magnetic moment, which was expected as Cu11Zn1Sb4S13 is not spin polarized, as also seen from band structure calculations. The samples Cu11.5Co0.5Sb4S13 and Cu11Ni1Sb4S13 had values close to the theoretical spin only magnetic moments of 3.87 μB and 2.83 μB, respectively. But in the case of Cu11.5Mn0.5Sb4S13 and Cu11.5Fe0.5Sb4S13, the experimental values were lower by nearly 30% from the theoretical value of 5.92 μB (considering high spin state). This can be partly understood by evaluating TC/C = λ, which is related to the exchange energy between the ordered spins. For Cu11Ni1Sb4S13 and Cu11.5Co0.5Sb4S13, λ is 29.89 and 7.35, respectively, whereas Cu11.5Mn0.5Sb4S13 and Cu11.5Fe0.5Sb4S13 yielded λ of 1.24 and 6.98, respectively. A higher value of λ indicates a higher exchange energy, which would result in a more localized nature of the d electrons on TM, thus, localizing the magnetic moment on the TM site, as seen for the Cu11Ni1Sb4S13 and Cu11.5Co0.5Sb4S13 cases. On the other hand, a lower λ in Cu 1 1 . 5 Mn 0 . 5 Sb 4 S 1 3 and Cu11.5Fe0.5Sb4S13 indicates that the d bands of TM exhibit higher covalent admixture with the host valence bands, and therefore, the d electrons are relatively more delocalized. The result is a loss of effective magnetic moment in Cu11.5Mn0.5Sb4S13 and Cu11.5Fe0.5Sb4S13, as shown by the experimental results. 4.7. Transport Properties. 4.7.1. Electrical Resistivity. The electrical resistivity (ρ) as a function of temperature is plotted in the Figure 7. It can be seen that ρ increased with increasing temperature for the pristine, Ni-, Co-, and Mnsubstituted tetrahedrite samples indicating degenerate semiconductor behavior. The pristine sample exhibited the least electrical resistivity owing to high charge carrier concentration and has relatively lower hole effective masses of 1.95me, 1.67me, and 1.96me along Γ, P, and H directions, respectively. The Mnand Co-substituted samples showed ρ values quite close to the parent compound. On the other hand, Fe- and Zn-substituted samples showed much higher ρ values, with Zn-substituted sample having the highest resistivity throughout the temperature range. Also, for Fe- and Zn-substituted samples, the resistivity decreased with increasing temperature exhibiting semiconducting behavior. The presence of secondary phases in Fe-substituted compositions resulted in ρ values higher than the ones reported in the literature.4 In Fe substituted sample, Cu2−yS secondary phase is present in fairly high amounts, whose resistivity is higher than that of the primary

(2)

The band effective masses of all the samples were thus evaluated for both spin up and spin down bands near the Fermi level for U = 0. It was observed that the hole effective mass for the pristine compound at the Γ point was evaluated to be 1.95me, which is larger than the value of 1.3me previously reported.34 In general, the band hole effective masses of all the samples range from 1.5−2.5me for U = 0. It will be seen in the following sections that the magnitude of the band effective mass plays a vital role in understanding the transport properties of the substituted tetrahedrite. 4.6. Magnetic Susceptibility. To ascertain the magnetic nature of the substituted samples and to determine the density of states, the magnetic susceptibility as a function of temperature was measured between 5 and 300 K, as shown

Figure 6. Variation of magnetic susceptibility with temperature.

in the Figure 6. All the curves were fitted with the modified Curie-Weiss law: XM = Xo +

C T − TC

(4)

(3)

where Xo is the Pauli paramagnetic term related to the density of states at the Fermi level via the relation: I


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The Journal of Physical Chemistry C Table 5. Magnetic Susceptibility and Calculated Effective Magnetic Moment of TM-Substituted Samples sample

Xo (×10−2 emu mol−1 G−1)

Curie constant (C)

TC (K)

effective magnetic moment obtained from C (in μB)

Cu11.5Mn0.5Sb4S13 Cu11.5Fe0.5Sb4S13 Cu11.5Co0.5Sb4S13 Cu11Ni1Sb4S13 Cu11Zn1Sb4S13

2.80 3.70 3.98 3.31 1.60

1.89 1.82 1.15 1.10 0.02

−2.35 −12.72 −8.52 −33.12 −4.05

3.91 3.83 3.05 2.98 0.46

atom of Fe3+ in Cu23Fe1Sb8S26 substituting at Cu1+ site, two of the four holes (per two formula units) are compensated. Similarly, two atoms of Zn2+ in Cu22Zn2Sb8S26 will compensate two holes at a substituted Cu1+ site. The outcome is lower charge carrier concentration in Cu 23 Fe 1 Sb 8 S 26 and Cu22Zn2Sb8S26 as compared to the other samples, and as a result relatively higher electrical resistivity values are observed. Although, this charge carrier reduction should be, in principle, similar for Cu23Fe1Sb8S26 and Cu22Zn2Sb8S26, a significant difference in the electrical resistivity values is observed. The reason for this difference can be understood from the band structure plots in Figures S20 and S23. As mentioned earlier, the effective masses of the samples decide the mobility of charge carriers, which, in turn, determine the electrical resistivity. The effective mass (m*) for Cu11.5Fe0.5Sb4S13 for U = 0, was evaluated to be 1.96me (spin up) and 1.3me (spin down) along Γ direction, and 1.1me (spin up) and 1.7me (spin down) along P direction. These values are lower than that of Cu11Zn1Sb4S13 (2.14me and 1.46me along Γ and P directions, respectively, taking U = 0). Hence, the result is lower mobility of holes in Cu11Zn1Sb4S13, consequently, causing a higher ρ value. To understand the electrical resistivity trend in a better way, Hall measurements were attempted to determine the charge carrier concentration in the pristine and substituted samples. Unfortunately, the Hall coefficient RH was too low and successive measurements on the same sample yielded very different values. Previously, Lu et al.4 also reported that attempts to measure the charge carrier concentrations in Feand Zn-substituted tetrahedrite were unsuccessful. 4.7.2. Seebeck Coefficient. The Seebeck coefficient (S) as a function of temperature is shown in the Figure 8. It is observed that S increases almost linearly with temperature for all the samples, which is typical for a degenerate semiconductor. A positive Seebeck coefficient over the entire temperature range confirms that all the samples were p-type in nature. It was seen

Figure 7. Temperature-dependent electrical resistivity of all the samples.

composition,35 resulting in an overall slight increase in electrical resistivity. Another factor is the density of the samples which was ∼95% for Fe and ∼90% for Zn-substituted samples, whereas the density reported in the literature4 was ≥98%. On the other hand, the Ni-substituted sample exhibited similar resistivity as compared to the value reported in the literature.5 The different behavior in the resistivity values for different TM compositions can be understood in terms of hole compensation in the system due to TM substitution at the Cu tetrahedral site. If we consider two formula units of all the samples, we can rewrite the chemical compositions as Cu23Mn1Sb8S26, Cu23Fe1Sb8S26, Cu23Co1Sb8S26, Cu22Ni2Sb8S26 (composition taken from EDS), and Cu22Zn2Sb8S26. The XPS results, for Cu23Mn1Sb8S26 and Cu23Co1Sb8S26, showed a relatively higher ICu1+/ICu2+ ratio. If Mn2+ and Co2+ would have occupied the Cu1+ site, they would have compensated one hole per two formula units but because of the higher ICu1+/ICu2+ XPS intensity ratio, the TM atoms might have substituted more preferably at Cu2+ site, leading to no hole compensation. The result is a charge carrier concentration similar to the pristine compound’s since holes are relatively less compensated. As a consequence, Cu23Mn1Sb8S26 and Cu23Co1Sb8S26 have ρ values similar to that of the parent compound. A similar argument holds true for Cu22Ni2Sb8S26 as well, but now the dopant concentration is higher and two atoms of Ni2+ reside in the unit cell. This results in the compensation of either two holes (if two Cu1+ sites are occupied), one hole (if one Cu1+ site and one Cu2+ are occupied) or none (if two Cu2+ sites are occupied). Consequently, relatively more number of holes are compensated, leading to reduction in charge carrier concentration, and hence higher values of ρ as compared to Cu23Mn1Sb8S26 and Cu23Co1Sb8S26. In case of Cu23Fe1Sb8S26 and Cu22Zn2Sb8S26, XPS spectra showed a relatively lower ICu1+/ICu2+ intensity ratio, which indicated that the dopant has no particular preference and can occupy both Cu1+ and Cu2+ sites. Since there is one

Figure 8. Seebeck coefficient of all the samples as a function of temperature. J


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The Journal of Physical Chemistry C that Cu11Zn1Sb4S13 exhibited the highest value of 215 μV/K at around 700 K, which is because of low charge carrier concentration, facilitated by the filling of the valence band. Consequently, the Fermi level rose toward the middle of the band gap, which was seen in the band structure calculations (Figure S23), and also confirmed by the electrical resistivity data. Another reason for high Seebeck coefficient in Cu11Zn1Sb4S13 can be attributed to high effective mass. The other samples follow a similar trend to the electrical resistivity, in accordance with the varying charge carrier concentration discussed in the previous section. The sample Cu11.5Mn0.5Sb4S13, however, shows less Seebeck coefficient than Cu11.5Co0.5Sb4S13, as opposed to the electrical resistivity, due to difference in the band effective mass and band degeneracy in the two systems. The Seebeck coefficient for a degenerate semiconductor case can be written from Mott’s relation as S=

8π 2kB2T 3eh2

⎛ π ⎞2/3 md*⎜ ⎟ ⎝ 3n ⎠

Figure 9. Temperature-dependent power factor of all the samples.

in a ratio S2/ρ, similar to the one reported in literature.4 However, the literature value of PF in case of Zn-substituted sample is higher than the one obtained in this work,4 due to lower electrical resistivity and moderate Seebeck coefficient. A possible reason for such behavior could be attributed to the presence of secondary phase and slightly lower value of density of our sample, as compared to that in literature. 4.7.4. Thermal Conductivity. The Figure 10 displays the total thermal conductivity (κT) as a function of temperature. It

(5)

where kB = Boltzmann constant, e = electronic charge, h = Plank constant, n = charge carrier concentration, and m*d = DOS effective mass given by md* = NV2/3mb*

(6)

Here, NV is the total band degeneracy, including the orbital and symmetry degeneracies, and m*b is the band effective mass, which was evaluated from the theoretical band structure calculations. It can be seen that S is directly proportional to md* and inversely proportional to n2/3. Since the charge carrier concentration is expected to be nearly the same in Cu11.5Mn0.5Sb4S13 and Cu11.5Co0.5Sb4S13, the difference in S values can be attributed to different effective masses. The mb* at U = 0, for Cu11.5Co0.5Sb4S13 along Γ and H directions was calculated to be 1.86me (spin up), 2.02me (spin down), and 2.68me (spin up), 2.47me (spin down), respectively, while for Cu11.5Mn0.5Sb4S13, mb* along Γ and H is 1.86 (spin up), 1.63 (spin down), and 1.9 (spin up), 2.36 (spin down) respectively. Moreover, if we consider NV to be the number of degenerate bands within kBT near EF, it was observed from the Figure S21 that NV in the case for Cu11.5Co0.5Sb4S13 is larger than that of Cu11.5Mn0.5Sb4S13 in both up spin and down spin states. As a consequence, m*b for Cu11.5Co0.5Sb4S13 is larger than that of Cu11.5Mn0.5Sb4S13, resulting in a higher Seebeck coefficient. 4.7.3. Power Factor. The power factor (S2/ρ) is plotted as a function of temperature in Figure 9. The power factor (PF) is also affected by the band degeneracy at the Fermi level, which manifests itself as the DOS at EF (g(EF)). From Figure 4d it was observed that g(EF) is relatively large for Cu11.5Co0.5Sb4S13 in both up and down spin states because of the effect of Co 3d orbitals on the band structure. It was also observed that g(EF) remained unaffected with increasing values of Hubbard U (Figure 5c). This coupled with the sample’s lower electrical resistivity over the entire measured temperature range, a maximum power factor of 1.76 mW/m·K2 at 673 K9 was achieved for Cu11.5Co0.5Sb4S13, which is larger than the pristine sample’s. On the other hand, a relatively low power factor was found in Cu11Zn1Sb4S13 owing to its high electrical resistivity. Comparing our values with the ones reported in the literature,4,5 the PF for Fe- and Ni-substituted samples is almost similar. Although the Fe-substituted sample exhibited higher ρ, it was compensated by an increase in S, which resulted

Figure 10. Variation of total thermal conductivity with temperature for all the samples.

is observed that κT of all the samples increase with increase in temperature, with values less than 1.4 W/m·K throughout the temperature range. The electronic part of the thermal conductivity (κe) was calculated from the Weidmann−Franz relation: κe = LσT

(7)

L is the temperature-dependent Lorenz number and T is the temperature. The temperature-dependent Lorenz number was evaluated from the following relation: ⎛ ⎛ kB ⎞ 2 ⎜ r + L=⎜ ⎟⎜ ⎝e ⎠⎜ r+ ⎝

( (

7 2 3 2

)Fr+5/2(η) )Fr+1/2(η)

⎡ r+ ⎢ −⎢ ⎣ r+

( (

5 2 3 2

2⎞

)Fr+3/2(η) ⎤⎥ ⎟ ⎟ )Fr+1/2(η) ⎥⎦ ⎟⎠ (8)

K


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The Journal of Physical Chemistry C where η is the reduced Fermi energy that is obtained from Seebeck coefficient values via the relation: ⎛ kB ⎜ r + S=± ⎜ e r+ ⎝

( (

5 2 3 2

)Fr+3/2(η) )Fr+1/2(η)

⎞ ⎟ − η⎟ ⎠

calculations revealed that the inherent low thermal conductivity arises from strong anharmonic out of plane vibrations of triangular coordinated Cu atoms, induced by the lone pair electrons on Sb atoms. It was found that transverse acoustic (TA) branches became harmonically unstable near the N and P symmetry points of the zone boundary.4 And the Gruneisen parameter was calculated to be more than 10 for the TA branches at the zone boundary, indicating high anharmonicity resulting in strong phonon scattering. Consequently, an intrinsically low value of the lattice thermal conductivity is exhibited by the tetrahedrite system. The κe values for Cu11.5Fe0.5Sb4S13, Cu11Ni1Sb4S13, and Cu11Zn1Sb4S13 are low as compared to Cu11.5Co0.5Sb4S13 and Cu11.5Mn0.5Sb4S13, with Cu11Zn1Sb4S13 having the least electronic thermal conductivity over the whole temperature range. This follows from the electrical resistivity trend seen in the Figure 7 and is in accordance with the charge carrier compensation by the TM atom. It was observed that κT was dominated by the electronic component κe in the pristine, Cu11.5Co0.5Sb4S13, and Cu11.5Mn0.5Sb4S13 samples, owing to high charge concentration. On the other hand, in the case of Cu11.5Fe0.5Sb4S13, Cu11Ni1Sb4S13, and Cu11Zn1Sb4S13, κT was dominated by the lattice component κL since the charge carriers (holes) are relatively more compensated, and therefore, the contribution from κe is reduced. The majority of the thermal energy in these TM-substituted tetrahedrite samples is therefore transported by the lattice component of thermal conductivity κL, unlike in the pristine case. Comparing the total thermal conductivity values obtained in this work with the literature, it was found that Fe-substituted sample showed higher values of κT than that reported in the literature.4 Although κe in Cu11.5Fe0.5Sb4S13 was suppressed because of low electrical conductivity, κL was quite large, resulting in an overall higher κT. It was observed that κL in Cu11.5Fe0.5Sb4S13 was in fact the highest among the substituted samples. The reason is that the charge carrier concentration was reduced significantly on Fe substitution, which lead to lower electron−phonon scattering and, hence, better phonon mobility in the sample. Another reason could be the enhanced covalent admixture of the Fe atoms with the host atoms as seen in the magnetic susceptibility analysis. Consequently, the phonon movement is further mediated because of the stronger covalent nature in Fesubstituted sample. In the case of Cu11Ni1Sb4S13, κT was similar to the value reported in the literature.5 The κL is relatively quite high among the samples, which can also be attributed to reduced electron−phonon scattering. For Cu11.5Mn0.5Sb4S13, although κe was relatively higher, κL was also found to be reasonably high among the other samples. This is due to the relatively higher density (∼95%) of the Mn-substituted sample as compared to other samples and thus very low porosity.8 The thermal conductivity can be related to the porosity by the equation: κT = κo(1 − P2/3), where κo is the thermal conductivity of the material without pores and P is the pore fraction in the material. A very low pore fraction, as observed in Cu11.5Mn0.5Sb4S13 thus leads to higher κ due to reduced scattering of the phonon by the pores. On the other hand, Cu11Zn1Sb4S13 displayed lower value of κL due to the higher porosity in the sample as observed from the EPMA image (Figure S13), which would lead to enhanced scattering of the phonons. The κT is, therefore, lower than reported in the literature4 due to both low κe and κL. In the case of Cosubstituted and pristine sample, the charge carrier concentration is relatively quite high. The majority of thermal

(9)

where F(η) is the reduced Fermi integral given by Fn(η) =

∫0

∞

xn dx 1 + e x−η

(10)

and η = EF/kBT. Assuming that the main scattering mechanism is acoustic phonon scattering, the value of r is taken as −1/2. The Lorenz number at each temperature value is therefore obtained by substituting η and r in equation 8. The variation of κe with temperature is plotted in the Figure 11. The lattice part

Figure 11. Electronic component of thermal conductivity (κe) as a function of temperature calculated from κe = LσT, where L is the temperature-dependent Lorenz number.

of the thermal conductivity (κL) was obtained from the equation κT = κe + κL, by subtracting κe from κT, which is plotted in the Figure 12 as a function of temperature. It was observed that κL showed low values in the range 0.31 − 0.76 W/m·K for all the samples throughout the measured temperature range. The origin of this intrinsically low thermal conductivity in tetrahedrites was explained by phonon dispersion calculations performed by Lu et al.4 The phonon

Figure 12. Lattice component of thermal conductivity (κL) as a function of temperature calculated from κL = κT − κe. L


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by the DFT calculations, which also indicated a high DOS at EF. Analysis of the magnetic susceptibility and UPS data confirmed experimentally the high DOS for Co substituted sample. It was found that Ni and Co substituted samples yielded magnetic moment values very close values to the calculated ones, whereas the experimental values for Fe and Mn substituted samples deviated from the calculated values. It was confirmed that the magnetic ordering of the spins is antiferromagnetic in TM substituted samples, with different strengths of the exchange energy between the spins, resulting in a reduction of effective magnetic moment in Mn and Fe substituted samples. The electrical resistivity (ρ) and Seebeck coefficient (S) as a function of temperature of the TMsubstituted samples was explained based on the compensation of holes by TM atoms and different values of effective masses (m*). A high power factor was achieved for the composition Cu11.5Co0.5Sb4S13, due to high band degeneracy and high DOS at EF induced by the Co 3d states, which was unaffected by the application of Hubbard U. The total thermal conductivity of all the substituted samples was lower than that of the pristine compound, because of the decrease in carrier thermal conductivity. The variation of κe with TM was in accordance to the resistivity trend, following the compensation of holes. The lattice part of the thermal conductivity dominated in Cu11.5Fe0.5Sb4S13, Cu11Ni1Sb4S13, and Cu11Zn1Sb4S13 samples due to the suppression of κe. A coherent explanation was thus given behind the highest figure of merit in the sample Cu11.5Co0.5Sb4S13 based on both theoretical calculations and experimental results. In conclusion, this work highlights the importance and effect of TM substitution on the band structure by introducing polarized impurity states in the valence band, which modified the DOS at EF. Another modification was the dispersion of the bands at EF, which resulted in higher effective masses than the pristine compound. Hence, it was realized that the nature of band structure and PDOS was important for analyzing the properties of substituted tetrahedrite and enables choice of a suitable dopant in future studies. This study also paves a way to explore transition metal atoms as substituents/ dopants for other Cu-based chalcogenides having similar tetrahedral coordination. A systematic study of the influence of TM substitution on their band structure would help in optimizing or tuning the thermoelectric properties.

conduction in these samples is done by electrons rather than phonons, leading to higher κe. The electron−phonon scattering too becomes quite significant in these samples. As a result the κL is lower than other substituted samples. 4.7.5. Thermoelectric Figure of Merit. The Figure 13 shows the zT of all the samples over the measured temperature,

Figure 13. Thermoelectric Figure of Merit (zT) as a function of temperature for all the samples.

evaluated using equation 1. A maximum zT of around 0.98 at 673 K was obtained for Cu11.5Co0.5Sb4S13 due to high power factor (1.76 mW/m·K2 at 673 K) and reasonably low total thermal conductivity (1.2 W/m·K at 673 K). This value is comparable to the highest value of zT obtained in TMsubstituted tetrahedrites. In the case of Cu11.5Mn0.5Sb4S13, a maximum zT of 0.64 was achieved at 673 K. Comparing with other reports, this value is lower than the maximum zT of 0.89 at 700 K (for the composition Cu11.5Mn0.5Sb4S13) by Han et al.12 and 1.13 (at 575 K for the composition Cu11Mn1Sb4S13). For Cu11Zn1Sb4S13, a maximum zT of 0.71 at 673 K was achieved in the present work, which is lower than the maximum zT achieved in literature for the same composition by Lu et al. (∼1 at 720 K4). The least zT (0.6 at 673 K) over the entire measured temperature range was shown by Cu11.5Fe0.5Sb4S13, owing to low power factor. This value is lower than the reported value of 0.8 at 720 K by Lu et al.4 for the composition Cu11.5Fe0.5Sb4S13. Lastly, in the case of Cu11Ni1Sb4S13, a maximum zT of around 0.76 at 673 K was achieved, which is larger than the reported value of 0.7 at 665 K5 by Suekuni et al. (for the composition Cu10.5Ni1.5Sb4S13). However, a maximum zT of 0.8 at 720 K was obtained by Barbier et al.11 for the composition Cu10.4Ni1.6Sb4S13.

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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.jpcc.7b12214. Figure S1: Crystal structure of Cu12Sb4S13 displaying the coordination environment of each atom. Figures S2−S7: Rietveld refinement for the samples Cu 12 Sb 4 S 13 , Cu11.5Mn0.5Sb4S13, Cu11.5Fe0.5Sb4S13, Cu11.5Co0.5Sb4S13, Cu11Ni1Sb4S13, and Cu11Zn1Sb4S13, respectively. Figure S8: Variation of Lattice parameter with Dopant atom. Figure S9: Differential Scanning Calorimetry (DSC) plot of the samples. Figure S10: Thermogravimetric Analysis (TGA) plot of the samples. Figures S11−S13: The Backscattered Electron (BSE) images of Cu11.5Fe0.5Sb4S13, Cu11Ni1Sb4S13, and Cu11Zn1Sb4S13, respectively. Figures S14−S16: XPS spectra for Cu, Sb, and S, respectively. Figure S17: The intercept of the trailing edge of secondary electron onset in Cu11Zn1Sb4S13. Figures S18−S23: The band structures

5. CONCLUSIONS The tetrahedrite compound Cu12Sb4S13 was successfully substituted with transition metal (TM) at the Cu tetrahedral site. The composition of the substituted tetrahedrite given by Cu12‑xTMxSb4S13(TM = Mn, Fe, Co, Ni, Zn) was selected based on earlier studies, which have reported the highest zT for that particular dopant series. A detailed characterization of the samples showed tetrahedrite as the main phase with traces of secondary phases in Fe, Co, Ni, and Zn substituted samples. Theoretical calculations revealed that degeneracy of the bands in Cu12Sb4S13 was modified on TM substitution because polarized TM 3d bands were introduced in the valence band. The high band degeneracy in Cu11.5Co0.5Sb4S13 was confirmed M


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(12) Han, Y.; Wu, Y.; Li, G.; Pan, F. S.; Fan, R.; Liu, X. Thermoelectric Properties of Mn Doped Cu12‑xMnxSb4S13 Tetrahedrites. Mater. Sci. Forum 2016, 847, 161−165. (13) Kumar, D. S. P.; Chetty, R.; Rogl, P.; Rogl, G.; Bauer, E.; Malar, P.; Mallik, R. C. Thermoelectric Properties of Cd Doped Tetrahedrite: Cu12‑xCdxSb4S13. Intermetallics 2016, 78, 21−29. (14) Suekuni, K.; Tsuruta, K.; Ariga, T.; Koyano, M. Thermoelectric Properties of Mineral Tetrahedrites Cu10Tr2Sb4S13 with Low Thermal Conductivity. Appl. Phys. Express 2012, 5, 051201. (15) Hohenberg, P.; Kohn, W. Inhomogeneous Electron Gas. Phys. Rev. 1964, 136, 864−871. (16) Kohn, W.; Sham, L. J. Self-Consistent Equations Including Exchange and Correlation Effects. Phys. Rev. 1965, 140, A1133− A1138. (17) Carvajal, J. Fullprof: A Program for Rietveld Refinement and Pattern Matching Analysis, Abstracts of the Satellite Meeting on Powder Diffraction of the XV Congress of the Iucr (1990). FULLPROF: a program for rietveld refinement and pattern matching analysis, Abstracts of the Satellite Meeting on Powder Diffraction of the XV Congress of the IUCr 1990, No. 1990, 127. (18) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G.; Cococcioni, M.; Dabo, I.; et al. Quantum Espresso: A Modular and Open-Source Software Project for Quantum Simulations of Materials. J. Phys.: Condens. Matter 2009, 21, 395502. (19) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (20) Andreasen, J. W.; Makovicky, E.; Lebech, B.; Moller, S. K. The Role of Iron in Tetrahedrite and Tennantite Determined by Rietveld Refinement of Neutron Powder Diffraction Data. Phys. Chem. Miner. 2008, 35, 447−454. (21) Nasonova, D. I.; Presniakov, I. A.; Sobolev, A. V.; Verchenko, V. Y.; Tsirlin, A. A.; Wei, Z.; Dikarev, E. V.; Shevelkov, A. V. Role of Iron in Synthetic Tetrahedrites Revisited. J. Solid State Chem. 2016, 235, 28−35. (22) Makovicky, E.; Skinner, B. J. Studies of the Sulfosalts of Copper. VII. Crystal Structures of the Exsolution Products Cu12.3Sb4S13 and Cu13.8Sb4S13 of Unsubstituted Synthetic Tetrahedrite. Can. Mineral 1979, 17, 619−634. (23) Tatsuka, K.; Morimoto, N. Tetrahedrite Stability Relations in the Cu-Fe-Sb-S System. Am. Mineral 1977, 62, 1101−1109. (24) Makovicky, E.; Forcher, K.; Lottermoser, W.; Amthauer, G. The Role of Fe2+ and Fe3+ in Synthetic Fe-Substituted Tetrahedrite. Mineral. Petrol. 1990, 43, 73−81. (25) Friese, K.; Grzechnik, A.; Makovicky, E.; Balic-Zunic, T.; KarupMoller, S. Crystal Structures of Iron Bearing Tetrahedrite and Tennantite at 25 and 250 °C by Means of Rietveld Refinement of Synchrotron Data. Phys. Chem. Miner. 2008, 35, 455−465. (26) Bouyrie, Y.; Candolfi, C.; Ohorodniichuk, V.; Malaman, B.; Dauscher, A.; Tobola, J.; Lenoir, B. Crystal Structure, Electronic Band Structure and High-Temperature Thermoelectric Properties of TeSubstituted Tetrahedrites Cu12Sb4‑xTexS13(0.5 ≤ x ≤ 2.0). J. Mater. Chem. C 2015, 3, 10476−10487. (27) Bishop, D. W.; Thomas, P. S.; Ray, A. S. A-B Phase ReTransformation Kinetics in Nickel Sulphide. J. Therm. Anal. Calorim 1999, 56, 429−435. (28) Skinner, B. J.; Luce, F. D.; Makovicky, E. Studies of the Sulfosalts of Copper III; Phases and Phase Relations in the System CuSb-S. Econ. Geol. Bull. Soc. Econ. Geol. 1972, 67, 924−938. (29) Makovicky, E.; Skinner, B. J. Studies of the Sulfosalts of Copper VI; Low-Temperature Exsolution in Synthetic Tetrahedrite Solid Solution, Cu12+xSb4+yS13. Can. Mineral 1978, 16, 611−623. (30) Hall, A. J.; Cervelle, B.; Levy, C. The Effect of Substitution of Cu by Zn, Fe and Ag on the Optical Properties of Synthetic Tetrahedrite, Cu12Sb4S13. Bull. Soc. Fr. Mineral Cristallogr. 1974, 97, 18−26. (31) Hoggins, J. T.; Steinfink, H. Empirical Bonding Relationships in Metal-Iron-Sulfide Compounds. Inorg. Chem. 1976, 15, 1682−1685.

of up and down spin for U = 0: (a) Cu12Sb4S13, (b) Cu 1 1 . 5 Mn 0 . 5 Sb 4 S 1 3 , (c) Cu 1 1 . 5 Fe 0 . 5 Sb 4 S 1 3 , (d) Cu 11.5 Co 0. 5 Sb 4 S 13 , (e) Cu 1 1 Ni 1 Sb 4 S 13 , and (f) Cu11Zn1Sb4S13. Figure S24: Specific heat with temperature of Fe-, Co-, Ni-, and Zn-substituted samples. Figure S25: Diffusivity as a function of temperature of Fe-, Co-, Ni-, and Zn-substituted samples. Tables T1−T6: The structural details and reliability factors for the samples: Cu 12 Sb 4 S 13 , Cu 11.5 Mn 0.5 Sb 4 S 13 , Cu 11.5 Fe 0.5 Sb 4 S 13 , Cu11.5Co0.5Sb4S13, Cu11Ni1Sb4S13, and Cu11Zn1Sb4S13, respectively (PDF).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Manish Jain: 0000-0001-9329-6434 Ramesh Chandra Mallik: 0000-0002-8383-7812 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge Department of Science and Technology (DST), India, for their financial support through Grant Number SB/EMEQ-243/2013.

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