Crystal Structure and Physical Properties of Ternary Phases around

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Crystal Structure and Physical Properties of Ternary Phases around the Composition Cu5Sn2Se7 with Tetrahedral Coordination of Atoms Jing Fan,†,‡,§ Wilder Carrillo-Cabrera,† Iryna Antonyshyn,† Yurii Prots,† Igor Veremchuk,† Walter Schnelle,† Christina Drathen,∥ Lidong Chen,‡ and Yuri Grin*,† †

Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Str. 40, 01187 Dresden, Germany State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, 200050 Shanghai, China § University of Chinese Academy of Sciences, 19 Yuquan Road, 100049 Beijing, P.R. China ∥ European Synchrotron Radiation Facility, 6 Rue Jules Horowitz, BP 220, 38043 Grenoble, France ‡

S Supporting Information *

ABSTRACT: A new monoclinic selenide Cu5Sn2Se7 was synthesized, and its crystal and electronic structure as well as thermoelectric properties were studied. The crystal structure of Cu5Sn2Se7 was determined by electron diffraction tomography and refined by full-profile techniques using synchrotron X-ray powder diffraction data: space group C2, a = 12.6509(3) Å, b = 5.6642(2) Å, c = 8.9319(4) Å, β = 98125(4)°, Z = 2; T = 295 K. Thermal analysis and high-temperature synchrotron X-ray diffraction indicated the decomposition of Cu5Sn2Se7 at 800 K with formation of the tetragonal high-temperature phase Cu4.90(4)Sn2.10(4)Se7: space group I42̅ m, a = 5.74738(1) Å, c = 11.45583(3) Å; T = 873 K. Both crystal structures are superstructures to the sphalerite type with tetrahedral coordination of the atoms. In agreement with chemical bonding analysis and band structure calculations, Cu5Sn2Se7 exhibits metal-like electronic transport behavior.

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INTRODUCTION Searching for high-efficiency low cost materials is crucial for thermoelectric (TE) applications such as waste heat recovery and power generation. Cu-based chalcogenides have attracted a lot of attention lately due to their low thermal conductivity and tunable electrical properties. “Phonon-liquid electron-crystal” type behavior was proposed for the superionic compound Cu2−xSe,1,2 and mobile Cu ions within the Se sublattice result in very low heat capacity and thermal conductivity. Substituting Cu in Cu12Sb4S13 with other transition metals reduces the high carrier concentration to an optimum, and therefore a high thermoelectric figure of merit ZT value was achieved in tetrahedrite Cu12−xMxSb4S13 (M = Zn, Fe, Ni).3,4 Ternary and quaternary chalcogenides with diamond-like crystal structures are another large group of promising thermoelectric materials, as was shown for CuIAIIIX2VI (A = Ga, In; X = Se, Te),5,6 Cu2IAIVSe3VI (A = Sn, Ge),7−9 and Cu2IAIIBIVSe4VI (A = Zn, Cd; B = Sn, Ge).10−14 Strong hybridization between Cu 3d and Se 4p states at the upper part of the valence band leads to 2D or 3D p-type conductive network in Cu2ZnSnSe4 and Cu2SnSe3, respectively. The resulting strongly hybridized bands are highly favorable for thermoelectric performance.7,8 Though it is well-known that Cu and Sn form complex sulfides, so far only a limited number of selenide phases were reported.15 On the basis of intrinsically low lattice thermal conductivity of Cu-based chalcogenides, discovery of new ternary selenides may provide further opportunities for band © 2014 American Chemical Society

structure engineering and tunable electrical properties. According to our previous experience with Cu2SnSe3,16 in the Cu−Sn−Se system it is often difficult to obtain high quality single crystals suitable for structural analysis because of the appearance of twinning. As an alternative, electron diffraction tomography17 combined with high-resolution synchrotron Xray diffraction is a suitable tool for the determination of the crystal structure of new compounds using crystalites with a size of 100−400 nm. Inspired by the good figure of merit ZT of 1.14 reported for Cu2In0.1Sn0.9Se3,7 we explored the Cu−Sn−Se system and found two new phasesCu5Sn2Se7 stable below 800 K and Cu4.90(4)Sn2.10(4)Se7 existing at elevated temperatures. In this study, the crystal structures of both phases are reported and discussed. Electronic structure, chemical bonding, and thermoelectric properties of Cu5 Sn 2Se 7 are also investigated, suggesting its suitability for thermoelectric applications.

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EXPERIMENTAL SECTION

Sample Preparation. A polycrystalline sample of nominal composition Cu5Sn2Se7 was synthesized by direct reaction of a stoichiometric mixture of the elements Cu (powder, 99.999%, Chempur), Sn (foil, 99.99%, Chempur), and Se (shots, 99.999%, Received: May 25, 2014 Revised: August 21, 2014 Published: August 25, 2014 5244

dx.doi.org/10.1021/cm501899q | Chem. Mater. 2014, 26, 5244−5251

Chemistry of Materials

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Scherrer geometry, λ = 0.43046 Å, scan step of 0.001°, 1° ≤ 2θ ≤ 44°). Diffraction patterns were collected in the temperature range of 295−900 K. Lattice parameter refinement and full-profile refinement of the crystal structures were performed employing the WinCSD program package.18 Crystallographic data of both new phases are given in Table 1. Finally, the PLATON package was used to check for additional symmetry (ADDSYM module).19

Alfa Aesar). The starting elements sealed in an evacuated fused quartz ampule were heated up to 1173 K and held for 12 h, cooled down to 750 K and heat treated at this temperature for 7 days, and then cooled to room temperature with the furnace. The obtained ingot was ground into fine powder, cold pressed into pellets, and annealed at 750 K for another 7 days. The annealing process is necessary to reduce the amount of binary compounds present as impurities in the reaction products. To confirm the composition of the high-temperature phase, a second sample is annealed at 850 K. In order to obtain densified bulk specimens suitable for thermoelectric property measurements, spark plasma sintering (SPS) was applied on the reground powder (SPS − 515 ET Dr. Sinter setup, SDC Fuji, Japan). SPS consolidation was performed by heating to 790 at 25 K/min under a uniaxial pressure of 60 MPa. This temperature was held for 10 min. The density of the pellet, determined by the Archimedes method, was >92% of its theoretical value. Electron Diffraction Tomography. The selected area electron diffraction mode (SAED, FEI TECNAI 10 microscope, 100 kV; 2k CCD camera, TemCam-F224HD, TVIPS) was used for data collection on an ca. 60 nm thick thin crystallite of Cu5Sn2Se7. The crystal area selected for electron diffraction filled almost the full aperture area (Figure 1a,b). The tilt sequence with steps of 1° was performed

Table 1. Crystallographic Data for Cu5Sn2Se7 and Cu4.9Sn2.1Se7a composition formula mass crystal system space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z; ρc (g cm−3) λ (Å) monochromator T (K) 2θ range (deg) μ (mm−1) F(000) (e) no. of reflections in the 2θ range measured refined parameters refinement method Ri, Rp, Rwp

Figure 1. Area of the Cu5Sn2Se7 crystal (thickness about 60 nm, aperture radius ca. 220 nm) used for electron-diffraction tomography (a, b) and selected area electron diffraction (SAED) pattern along the b* axis (c). Notice the strong reflections of the (pseudocubic) sublattice.

a

Cu5Sn2Se7 1107.89 monoclinic C2 12.6509(3) 5.6642(2) 8.9319(4) 98.125(4) 633.61(4) 2; 5.807

295 9.73 966 775

Cu4.9Sn2.1Se7 1113.50 tetragonal I4̅2m 5.74738(1) 11.45583(3) 378.414(2) 0.875; 5.583 0.43046 Si(111) 873 2.5 to 32 9.33 554 149

56

18 full-profile Rietveld 0.059, 0.069, 0.032, 0.062, 0.124 0.125

Synchrotron X-ray powder diffraction data.

Microstructure Analysis. Scanning electron microscopy (Philips XL30 microscope with a LaB6 cathode, FEI) was employed to prove the homogeneity of the microstructure. Chemical analyses were carried out on the annealed sample and on the pellet obtained after SPS. For further details, see ref 16. Thermal Analysis. Combined differential thermal and thermogravimetric analyses (DTA/TG) were performed on a Netzsch STA 449C machine in the temperature range of 300−1023 K with a heating rate of 10 K/min, under Ar atmosphere. The sample was loaded in an Al2O3 crucible dried by heat treatment prior use. No oxides were detected by powder X-ray diffraction in the samples after DTA/TG experiments. Physical Properties Measurements. The thermal diffusivity λ was measured on the disk-shaped specimen (laser-flash Netzsch LFA 427 setup) in the temperature range from 300 to 700 K in the He atmosphere. A heat capacity measurement was performed using differential scanning calorimeter DSC 8500 (PerkinElmer) in the temperature range of 300−700 K under Ar atmosphere. Al2O3 was used as a standard. The thermal conductivity κ was then calculated as κ = Cpdλ, where d is the density and Cp is the specific heat capacity. For simultaneous measurement of electrical resistivity ρ and Seebeck coefficient S (ULVAC-RIKO ZEM-3, 300−700 K, He atmosphere), a bar-shaped specimen of 1.5 × 1.5 × 10 mm3 was cut with a diamond wire saw from the disk-shaped specimen. Hall coefficient RH and lowtemperature resistivity ρ between 2 and 300 K were measured employing a PPMS system (Quantum Design). The carrier concentration pH and carrier mobility μH were calculated with the formula pHe = 1/RH and 1/ρ = pHeμH, respectively; here e is the electron charge. Calculation Procedures. Electronic structure calculation and study of chemical bonding were carried out for Cu5Sn2Se7 using the lattice parameters and atomic positions obtained from the crystal structure analysis (Tables 1 and 2). The TB-LMTO-ASA program

manually in a total range from −65° to 52°. For further details, see ref 16. In Figure 2, three projections (along the a, b, and c axes) of the

Figure 2. Projections of the 3D electron diffraction pattern of Cu5Sn2Se7 along the [100]*, [010]*, and [001]* directions. reconstructed reciprocal-space diffraction volume of Cu5Sn2Se7 are shown. After indexing the spots in the reciprocal-space lattice, the intensities were integrated and stored as a standard hkl file (555 reflections measured; 449 unique reflections). Laboratory X-ray Diffraction. Powder X-ray diffraction experiments for phase analysis were carried out on a Huber image plate Guinier camera G670 (Cu Kα1 radiation, λ = 1.54060 Å, germanium (111) monochromator, 6 × 30 min scans, 3° ≤ 2θ ≤ 100.3°) at ambient temperature. Synchrotron X-ray Diffraction. For crystal structure determination, powders of the target phases with particle size between 20 and 50 μm were filled in a quartz capillary (diameter 0.5 mm) and sealed under Ar atmosphere. The diffraction data were collected at the highresolution beamline ID31 of the European Synchrotron Radiation Facility (ESRF) in Grenoble, which was equipped with a multianalyzer stage using nine detectors each preceded by a Si(111) crystal (Debye− 5245



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Table 2. Atomic Coordinates and Isotropic Displacement Parameters (in Å2) for Cu5Sn2Se7a

a

atom

site

x

y

z

Ueq

Cu1 Cu2 Cu3 Sn1 Se1 Se2 Se3 Se4

2b 4c 4c 4c 4c 2a 4c 4c

0 0.1438(2) 0.7120(2) 0.9247(1) 0.8526(1) 0 0.0775(2) 0.7772(2)

0.2641(4) 0.2560(4) 0.2675(4) 0.7756(3) 0.5038(3) 0.5001(3) 0.0291(3) 0.0271(3)

1/2 0.9263(2) 0.6424(2) 0.7763(1) 0.5607(2) 0 0.7106(2) 0.8602(2)

0.015(1) 0.015(1) 0.014(1) 0.017(1) 0.008(1) 0.007(1) 0.010(1) 0.009(1)

Full-profile refinement on synchrotron X-ray powder diffraction data.

package was employed.20 The Barth-Hedin exchange potential21 was used for the LDA calculations. The radial scalar-relativistic Dirac equation was solved to obtain the partial waves. Though the calculation within the atomic sphere approximation (ASA) includes corrections for the neglect of interstitial regions and partial waves of higher order,22 an addition of empty spheres was necessary because the structural pattern formed by tetrahedrons CuSe4 and SnSe4 contains large voids. The following radii of the atomic spheres were applied for the calculations: r(Cu1) = 1.330 Å, r(Cu2) = 1.365 Å, r(Cu3) = 1.355 Å, r(Sn1) = 1.509 Å, r(Se1) = 1.469 Å, r(Se2) = 1.500 Å, r(Se3) = 1.483 Å, and r(Se4) = 1.424 Å. A basis set containing Cu(4s,4p,3d), Sn(5s,5p), and Se(4s,4p) orbitals was employed for a self-consistent calculation with Sn(5p,4f) and Se(4d) functions being down-folded. The electron localizability indicator (ELI, γ) was evaluated in the ELI-D representation according to refs 23−25 with an ELI-D module within the program package TB−LMTO−ASA.20 Estimation of the shapes, volumes, and charges of the atoms from the electron density (QTAIM atoms or atomic basins)26 and localization of the ELI-D maxima as indicators of the direct covalent atomic interactions were performed with the program DGrid.27

Figure 3. Thermal behavior of Cu5Sn2Se7 sample (heating rate 10 K/ min).

(peritectic) decomposition may occur at the temperatures just below the liquidus, e.g., 945K

Cu4.9Sn2.1Se7 ⎯⎯⎯⎯→ Cu 2SnSe3 + Cu 2Se + Se

which is confirmed by a complex shape of the DTA curve around 950 K, followed by a mass loss as well. Because of these two reactions from Cu5Sn2Se7 to Cu4.9Sn2.1Se7 and to Cu2SnSe3, the Cu content in the ternary phases decreases and the Sn content increases. This phenomenon is quite similar to the situation in the system Cu−Ge−S.28 Crystal Structure. The electron diffraction tomography study of Cu5Sn2Se7 suggested a C-centered monoclinic unit cell (reflections hkl are observed only with h + k = 2n, Figure 1c). The resulting unit cell parameters are a = 12.62 Å, b = 5.61 Å, c = 8.97 Å, α = 90.5°, β = 98.4°, γ = 89.9°. The final lattice parameters listed in Table 1 are calculated by least-squares refinement on peak positions obtained from powder X-ray diffraction data. Further analysis of the electron diffraction data suggested space groups C2/m, Cm, or C2. Given the fact that the crystal structure of Cu5Sn2Se7 is a superstructure to the noncentrosymmetric cubic structure (space group F4̅3m, ac = 5.688−5.696 Å29−31), the centrosymmetric space group C2/m was excluded from further consideration. The structure solution for Cu5Sn2Se7 was then achieved in space group C2 with direct methods using the SIR2011 software,32 and the atom positions were refined with the SHELXL software33 employing the intensities of the electron diffraction reflections. Typically for this procedure, the resultant R-value is large (0.444). On the basis of the model suggested by electron diffraction tomography, the final full-profile refinement of the crystal structure of Cu5Sn2Se7 was carried out employing the highresolution synchrotron X-ray diffraction data (Figure 4). In addition to the main monoclinic phase, three additional reflections at 7.37, 9.34, and 14.76° from an unknown phase were observed. According to EDXS analysis, the minority phase is likely to be a binary Cu−Se compound; the small size of the phase grains in the studied sample prevented a more precise analysis of the chemical composition. The additional reflections do not match reported structures in the system Cu−Se. The final atomic coordinates and displacement parameters are summarized in Table 2. The program ADDSYM in the PLATON package did not reveal additional symmetry. The selected interatomic distances and angles within the Cu- and

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RESULTS AND DISCUSSION Chemical Composition and Thermal Behavior. The sample with the starting composition Cu5Sn2Se7 annealed at 750 K is homogeneous without impurity (Figure S1a, see Supporting Information). Its composition amounts to Cu5.02(1)Sn2.01(2)Se6.97(3) according to ICP-AES analysis and to Cu5.03(1)Sn2.05(1)Se6.93(1) as per WDXS analysis, being in good agreement with the composition observed in structure solution and refinement. SEM study of the sample annealed at 850 K reveals the impurity phases (Supporting Information Figure S1b) Cu2Se and CuSe according to EDXS results. The main phase in this sample has the composition Cu4.96(1)Sn2.13(1)Se6.91(1) based on WDXS analysis which is within two e.s.d.’s in agreement with Cu4.90(4)Sn2.10(4)Se7 obtained from the refinement of the crystal structure. The DTA curve of the Cu5Sn2Se7 sample exhibits two endothermic features. The strongest one at 960 K (Figure 3) indicates the liquidus. The weak one at 800 K is attributed to (peritectic) decomposition, e.g., 800K

Cu5Sn2Se7 ⎯⎯⎯⎯→ Cu4.9Sn2.1Se7 + Cu 2Se + Se

The subsequent evaporation of Se is in accord with the mass loss at this temperature evidenced by the TG curve. Additionally, after cooling from 900 K to room temperature, the synchrotron X-ray powder diffraction pattern indicates the cubic sphalerite-type structure signaling the formation of HTCu2SnSe3 (Supporting Information Figure S2). Hence further 5246



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Figure 4. Synchrotron X-ray powder diffraction pattern of Cu5Sn2Se7 at 295 K (red dots) with the calculated profile (black line) after Rietveld refinement. The calculated positions of the Bragg peaks are marked with vertical green bars. The difference curve Yobs − Ycalc is shown at the bottom of the plot. The inset shows a magnification of the low-angle region with superstructure reflections. Asterisk indicates reflections of an unknown minority phase.

Figure 5. Synchrotron X-ray powder diffraction pattern of Cu4.9Sn2.1Se7 at 873 K (red dots) with the calculated profile (black line) after Rietveld refinement. The calculated positions of the Bragg peaks are marked with green vertical bars. The difference curve Yobs −Ycalc is shown at the bottom of the plot. The inset shows a magnification of the low-angle region with superstructure reflections. Asterisk indicates reflections of unknown minority phase.

Sn-centered selenium tetrahedra are listed in Table S1 (see Supporting Information). Ternary Cu-based sulfides of composition Cu 5M 2S7 (M = Si, Sn, Ge) are known. Cu5Si2S734,35 was proposed to crystallize in the space group Bb, while no structural solutions were found for Cu5Ge2S728 and Cu5Sn2S7.36,37 So far the corresponding selenides were not reported; Cu5Sn2Se7 studied in this work is the first selenide representative. This phase is isostructural with Cu4NiSi2S7 and Cu4NiGe2S7.38 In both quaternary compounds Ni occupies twofold and Cu occupies fourfold positions of the space group C2. In our model, Cu occupies both, twofold and fourfold, positions. Though the structure model is unknown for Cu5Si2S7 and Cu5Ge2S7, they may have the same structure, since Cu4NiSi2S7 and Cu4NiGe2S7 could be considered as the derivatives of these ternary compounds by substituting one Cu atom with Ni atom. In the temperature range from 295 to 773 K, the synchrotron X-ray diffraction patterns show the monoclinic structure, with reflections shifting toward lower diffraction angles because of the thermal expansion of the lattice. The pattern above 873 K exhibits less reflections than that at 773 K, indicating a structure with higher symmetry (Figure 5). The unknown minority phase observed in Cu5Sn2Se7 is still present at elevated temperatures. The indexing of the observed reflections suggests a tetragonal unit cell. The structure of chalcopyrite CuFeS2 (space group I42̅ d) was first tested as a starting model because it is often found in ternary chalcogenides such as CuMSe2 (M = Al, Ga, In). While not all reflections could be indexed within this space group, they were properly assigned using the space group I4̅2m. Therefore, the tetragonal Cu2(Fe,Zn)SnSe4-type (stannite) structure was employed as a starting model. Refinement of the site occupancies indicated that the 2a site is fully occupied by the Cu atom, and the other two sites 2b and 4d are mixed occupied by Sn and Cu atoms. The final refinement led to an occupation of the 2b site by 0.11(1) Cu + 0.89(1) Sn and 4d site by 0.84(1) Cu + 0.16(1) Sn, therefore resulting in the composition Cu4.90(4)Sn2.10(4)Se7. The final Rp is 0.062, proving

the validity of the structural solution. The atomic coordinates and displacement parameters as well as site occupancies are listed in Table 3, with selected interatomic distances as well as angles summarized in Table S2 (Supporting Information). Table 3. Atomic Coordinates and Isotropic Displacement Parameters (in Å2) for Cu4.90(4)Sn2.10(4)Se7 at 873 Ka

a b

atom

site

x

y

z

Ueq

Cu1 Cu2b Cu3b Se

2a 2b 4d 8i

0 0 0 0.2570(1)

0 0 1/2 x

0 1/2 1/4 0.3724(1)

0.074(1) 0.058(1) 0.078(1) 0.045(0)

Full-profile refinement on synchrotron X-ray powder diffraction data. Cu2 = Cu0.11(1)Sn0.89; Cu3 = Cu0.84(1)Sn0.16.

The Cu-rich phase Cu4.9Sn2.1Se7, i.e., Cu2.1Sn0.9Se3, adopts the tetragonal structure type of stannite Cu2(Fe,Zn)SnSe4 and is, thus, isotypic to the ternary sulfide Cu2.665Sn1.335S4.39 This tetragonal phase with similar unit cell parameters (a = 5.68 Å, c = 11.37 Å, space group unspecified) was reported earlier for the exact composition Cu2SnSe3.40 According to our recent investigation, the phase Cu2SnSe3 crystallizes in the monoclinic-II structure with its own structure type (space group Cc) at low temperature and in the cubic sphalerite-type structure (space group F4̅3m) at elevated temperature.16 Cu5Sn2Se7 and Cu4.9Sn2.1Se7 are both superstructures to the cubic sphalerite-type Cu2SnSe3. The presence of a common cubic substructure is demonstrated by the appearance of the strong reflections representing a face-centered lattice (space group F4̅3m) with ac ≈ 5.66 Å in the X-ray powder diffraction patterns of the new phases (Figures 4 and 5). The relationship between the parameters of the monoclinic lattice of Cu5Sn2Se7 and the cubic unit cell of Cu2SnSe3 is described as follows: am ≈ √5ac; bm ≈ ac; cm ≈ (5/2)1/2ac. For the tetragonal lattice of Cu4.9Sn2.1Se7, the relationship is more simple: at ≈ ac, ct ≈ 2ac. 5247



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As the decomposition temperature of cubic Cu2SnSe3 (964 K, stable only at elevated temperatures 16 ), tetragonal Cu4.9Sn2.1Se7 (945 K), and monoclinic Cu5Sn2Se7 (800 K, stable) decreases, the occupation of metal positions by Cu and Sn atoms changes from completely disordered via partially ordered to completely ordered, respectively (Figure 6). A

Figure 7. Total electronic density of states of Cu5Sn2Se7 together with the selected partial contributions.

form the intermediate range of DOS (−6 eV < E < −3 eV) with minor Cu(s) contributions, indicating the Cu−Se bonding. The range below the Fermi level is composed of mainly d contributions of copper and p states of Se. The s states of Se contribute to the DOS mainly below −12 eV. Strong p−d overlap of Cu(d) and Se(p) states is a common feature in Cubased chalcogenides and facilitates the electronic transport in Cu5Sn2Se7 as supported by the electrical properties. Assuming complete ionic interaction, the electron imbalance in the title compound is described as Cu1+5Sn4+2Se2−7. In such a case the Cu and Sn atoms would deliver their valence electrons completely to selenium to fulfill the octet rule. The integration of the electron density within the atomic basins (Figure 8, top) yields the following effective charges: Cu10.4+, Cu20.3+, Cu30.3+, Sn11.3+, Se10.8−, Se20.3−, Se30.7−, Se40.7−. The distribution of the charges is in accordance with the electronegativity of the constituting elements, but the absolute values of charges are rather small with respect to the ionic balance above. Thus, the charge transfer may represent only a part of the atomic interactions in Cu5Sn2Se7. The covalent part of the interactions was revealed applying the electron localizability indicator. The ELI-D distribution reveals maxima between Se and Cu (Figure 8, bottom), confirming the covalent bonding within the Cu−Se framework. The larger part of basin populations of these attractors is originating from the QTAIM basins of selenium, revealing the polar character of the Cu−Se bonds. The ELI-D attractors are also observed on the Sn−Se contacts, suggesting the polar bonding character of the atomic interaction also in this region of the crystal structure. The ELI-D distribution in the penultimate shells of Cu species slightly deviates from a spherical one. This indicates a participation of the electrons of this shell in the bonding within the valence region (cf. DOS above). Superposition of the QTAIM and the ELI-D results leads to the conclusion that the atomic framework in Cu5Sn2Se7 is formed by covalent polar Cu−Se and Sn−Se bonds. Physical Properties. XRD patterns of Cu5Sn2Se7 before and after SPS are the same, confirming that the crystal structure did not change. The positive Hall coefficient demonstrates that holes are the majority charge carriers (Figure 9). The charge carrier concentration is 3.0 × 1021 cm−3 at 300 K, which corresponds to a value of 1.9 holes per unit cell and [Cu1+]5[Sn4+]2[Se2−]7 × 0.95 p+, in good agreement with the electron count given above. Besides, the charge carrier concentration changes only slightly from 2 to 300 K, indicating

Figure 6. Crystal structures of monoclinic Cu5Sn2Se7 (a), tetragonal Cu4.9Sn2.1Se7 (c), and cubic Cu2SnSe3 (b). Projection of the crystal structure of Cu5Sn2Se7 along the b axis with the unit cells of Cu4.9Sn2.1Se7 (purple dashed line) and Cu2SnSe3 (green dashed line) emphasizes their relationship (d).

combination of large radii ratio and Cu/Sn ordering results in the distortion of the MSe4 tetrahedra in Cu5Sn2Se7, indicated by the wide range of angles 104.09(7)°−114.35(9)°. Electronic Structure and Chemical Bonding. The band structure calculations of Cu5Sn2Se7 reveal a distribution of the electronic density of states (DOS) similar to that in Cu2SnSe3.16 In its isostructural compound Cu4NiSi2S7, nickel delivers two valence electrons, so that the complete electronic balance is achieved: [Cu1+]4[Ni2+]1[Si4+]2[Se2−]7. When Cu occupies nickel sites in Cu5Sn2Se7, the holes in the band structure appear: [Cu1+]5[Sn4+]2[Se2−]7 × 1p+. In agreement with the electron count above, its electronic DOS reveals a gap above the Fermi level EF with bonding states partially nonoccupied (Figure 7). The electronic DOS of Cu5Sn2Se7 contains three regions. The low-energy region (−8 eV < E < −7 eV) is mainly formed by s states of Sn and p states of Se with small Se(s) contributions, suggesting bonding interaction between selenium and tin. The p states of Se and d states of Cu 5248



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Figure 9. Temperature dependence of the Hall coefficient RH, carrier concentration pH, electrical resistivity ρ, and Hall mobility μH for Cu5Sn2Se7.

Figure 8. Chemical bonding in Cu5Sn2Se7: (top) selected QTAIM atomic basins for Cu3, Sn, and Se3 atoms; (bottom) ELI-D distribution around the Cu−Se and Sn−Se contacts.

heavily doped behavior. The Hall mobility of Cu5Sn2Se7 is 11 cm2 V−1 s−1 at 300 K, being better than some other Cu-based selenides such as Cu2SnSe37,41 and Cu2ZnGeSe4,42 despite its quite high carrier concentration and resultant strong carrier− carrier scattering. The strong band formed by overlap between Cu(d) and Se(p) states should facilitate the electronic transport. The electrical resistivity and Seebeck coefficient are quite small in the investigated temperature range and increase with rising temperature (Figure 10). At 300 K, the electrical resistivity of Cu5Sn2Se7 is 1.5 × 10−6 Ω m. Above 600 K, an obvious change in the slope of the Seebeck coefficient is observed, probably due to the volatility of Se in the dynamic He atmosphere. The total thermal conductivity of Cu5Sn2Se7 at 300 K is 5.3 W m−1 K−1, almost two times higher than that of Cu2SnSe3.7,16 To obtain the lattice contribution κL, the electronic component κe of the thermal conductivity was estimated by the Wiedemann−Franz law (κe = LT/ρ; κ = κL + κe) and then subtracted from the total thermal conductivity. The Lorenz number L of 2.45 × 10−8 V2 K−2 at the metallic limit was used. The estimation results in a quite small lattice thermal conductivity of 0.42 W m−1 K−1 at 300 K. Cu5Sn2Se7 could indeed have a quite low lattice thermal conductivity, in particular due to the inhomogeneity of chemical bonding, as it was found for the intermetallic clathrates.43,44 Since the charge carrier holes contribute to most of the heat transfer,

Figure 10. Temperature dependence of electrical resistivity ρ (a); Seebeck coefficient S (b); thermal conductivity κ (c); and thermoelectric figure of merit ZT (d) for Cu5Sn2Se7.

further reduction in total thermal conductivity may be achieved by tuning the carrier concentration. As expected for a metallic material, the thermoelectric figure of merit ZT for Cu5Sn2Se7 is low, reaching the maximum value of 0.13 at 700 K (Figure 10). While, inspired by the existence of Cu4NiSi2S7 and Cu4NiGe2S7, if Cu5−xMxSn2Se7 (M = transition metal) could be synthesized, this kind of substitution may not only allow for tuning of the charge carrier concentration but also band structure modification in the vicinity of the Fermi level. Probably a better thermoelectric performance could be achieved.

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CONCLUSIONS The new ternary selenides Cu5Sn2Se7 and Cu4.9Sn2.1Se7 have been synthesized and characterized. The phase Cu5Sn2Se7 crystallizes in a monoclinic structure, being the first 5249



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representative of a new structure type. The phase Cu4.9Sn2.1Se7 adopts the tetragonal structure of the stannite type. Both crystal structures are superstructures to the sphalerite type with tetrahedral coordination of all atoms. Electron count and calculated band structure both reveal extra holes in the electronic density of states of Cu5Sn2Se7, further confirmed by its metal-like behavior in electronic transport. Similarly to other Cu-based chalcogenides, Cu 3d and Se 4p states in Cu5Sn2Se7 dominate the DOS around the Fermi level, exhibiting a feature that facilitates the electronic transport, and thus the phase Cu5Sn2Se7 exhibits in particular a good Hall mobility. The electronic structure causes high carrier concentration and high charge carrier thermal conductivity, which is not beneficial for good thermoelectric performance. Substitution of Cu by transition metals like Ni probably could be utilized to reduce the carrier concentration and enhance thermoelectric properties.

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ASSOCIATED CONTENT

S Supporting Information *

Scanning electron micrographs of Cu5Sn2Se7 samples annealed at 750 and 850 K. Selected interatomic distances and angles of Cu5Sn2Se7 and Cu4.90(4)Sn2.10(4)Se7. Synchrotron X-ray powder diffraction pattern of Cu5Sn2Se7 after cooling to 295 K. Crystallographic data in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. H. Borrmann, Mr. S. Hückmann, Mrs. S. Scharsach, Mrs. S. Kostmann, Mrs. M. Eckert, Mr. R. Koban, Dr. G. Auffermann, Dr. R. Gumeniuk, and Dr. L. Akselrud for experimental assistance and fruitful discussions.

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REFERENCES

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Crystal Structure and Physical Properties of Ternary Phases around

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