Structural and Electronic Transport Properties in Sr-Doped BiCuSeO

Article pubs.acs.org/cm

Structural and Electronic Transport Properties in Sr-Doped BiCuSeO Céline Barreteau, David Bérardan,* Emilie Amzallag, LiDong Zhao, and Nita Dragoe †

Institut de Chimie Moléculaire et des Matériaux d’Orsay, University of Paris-Sud, UMR 8182, Orsay F-91405, France CNRS, Orsay F-91405, France

‡

S Supporting Information *

ABSTRACT: We report on the structural and electronic transport properties of BiCuSeO based compounds, that have recently been reported as promising thermoelectric materials with figure of merit ZT > 0.8 at 923 K, and share the same crystal structure as the high-Tc iron based 1111 oxypnictides. We show that the substitution of Bi3+ by Sr2+ induces a strong decrease of the electrical resistivity up to the solubility limit reached for x = 0.35, which originates from the strong increase of the carriers concentration. Two anomalies in the resistivity curves have been observed, one for the undoped compound near 260 K and the other for the doped samples at very low temperature. However, structural and magnetic measurements do not provide indications of structural or magnetic phase transition or superconductivity as it had been previously suggested in BiCu1−xOS. We show that the thermoelectric properties of Bi1−xSrxCuSeO materials can be well understood through the analysis of the electronic band structure and the density of states close to the Fermi level and we provide possible directions toward the enhancement of the thermoelectric figure of merit of these materials. KEYWORDS: thermoelectricity, band structure calculations, oxychalcogenides

■

INTRODUCTION Thermoelectric (TE) systems can be used to convert directly a heat flux into electrical power, which offers promising opportunities for electrical power generation by waste heat harvesting. Therefore, thermoelectricity could constitute a key technology in the greenhouse gas emissions mitigation. The performances of a TE material depend on the dimensionless figure of merit (ZT), defined as ZT = (S2T)/(ρκ), where S, ρ, κ, and T are the Seebeck coefficient, the electrical resistivity, the thermal conductivity, and the absolute temperature, respectively.1 Recently, p-type layered oxyselenides with parent compound BiCuSeO were found to exhibit a very promising figure of merit (ZT > 0.8 at 923 K)2,3 that is much larger than other polycrystalline oxide systems such as p-type Ca3Co4O9 (ZT ∼ 0.2 at 973 K),4,5 or n-type SrTiO3 (ZT ∼ 0.35 at 1273 K),6 ZnO (ZT ∼ 0.4 at 1273 K,)7 or In2O3 (ZT ∼ 0.45 at 1273 K).8,9 It can also be compared with the best p-type intermetallic systems, including PbTe10,11 or skutterudites,12 whose ZT values reach 1.0−1.2 in this temperature range but which require costly protective coatings or the encapsulation in inert atmosphere to be used in real-conditions applications.13 These promising TE properties mostly originate from the very low lattice thermal conductivity, lower than 0.5 W m−1 K−1 at 873K, coupled to a moderate thermoelectric power factor S2/ρ. Oxychalcogenides belong to the larger family of ZrSiCuAsstructure type 1111 compounds. The first synthesis of a 1111 oxychalcogenide sample LaAgSO, which was studied as an ionic conductor, was reported by Palazzi et al. in 1981.14 This material crystallizes in a layered structure, reminiscent of the cuprates one, with conductive AgS layers separated by charge © 2012 American Chemical Society

reservoir LaO layers. Because of the similarity of their crystal structure to that of cuprates, oxysulfides and more generally oxychalcogenides were thoroughly studied during the 1990s with the unachieved goal of finding new high-temperature superconductors. There has been a renewed interest in these materials in the 2000s when it was demonstrated by Hosono’s group that they are promising p-type transparent conductors in the thin film form due to their wide band gap coupled to rather high electrical conductivity with optimized doping, of the order of 1 mΩ cm.15 Very few room temperature thermopower values were reported, which were mainly used to assess the p-type character of the materials. In the 2000s, the electronic band structured of undoped RCuChO (R = Bi, La; Ch = S, Se, Te) has been thoroughly studied, the band gap and the carriers mobility being key parameters for transparent conducting materials.15 However, very few studies have been devoted to the influence of doping on this electronic structure and on the charge carriers concentration. More recently, possible traces of superconductivity have been reported in a BiCuSO sample by magnetic measurement with Tc ∼ 6 K,16 but with a very small volume fraction that did not exclude possible secondary phase contribution. In view of this double interest, promising thermoelectric properties and possible iron-free new superconductors with 1111 crystal structure, we have performed a thorough investigation of the low temperature properties of Received: May 15, 2012 Revised: July 27, 2012 Published: August 2, 2012 3168

dx.doi.org/10.1021/cm301492z | Chem. Mater. 2012, 24, 3168−3178

Chemistry of Materials

Article

Figure 1. XRD pattern and Rietveld refinement of Bi0.95Sr0.05CuSeO at room temperature and the structure of BiCuOSe; inset: zoom from 20 to 50°. performed from 300K to 10K in the PPMS chamber using a Keithley 6220 current source and a Keithley 2182A nanovoltmeter. The Hall coefficient was obtained from the linear fit of the Hall resistivity versus magnetic field between −9T and 9T. Seebeck coefficient was measured on bars cut from the pellets, with typical size 10 × 2 × 2 mm3. The measurements were performed by the differential method with two T-type thermocouples by using the slope of V-T curve with gradients up to about 0.2 K/mm, by using a laboratory made system in a He-free cryostat. All electrical characterizations were performed in a direction perpendicular to the pressing direction. Computational Details. All the calculations were performed using CRYSTAL09, an ab initio code for periodic systems, developed in Turin.18,19 The crystalline orbitals are expanded in terms of localized atomic Gaussian basis set, in a way close to the linear combination of atomic orbitals (LCAO) method currently adopted for molecules. The eigenvalues equations are solved at the B3LYP level. The hybrid B3LYP functional uses the Becke’s exchange20 and Lee−Yang−Parr’s correlation functional.21 The number of k points in the first irreducible Brillouin zone at which the Hamiltonian matrix is diagonalized was equal to 75. In order to reduce the computational cost, the Hay and Wadt large core22 and the Durand and Barthelat effective core pseudopotentials23 were used to model the core electrons in selenium and bismuth, respectively. Valence basis sets already optimized in early studies were adopted 8−411d11G for oxygen24 and 86−4111G for copper.25 CRYSTAL09 code carries out the optimization of any parameter (geometrical parameters or atomic positions) relative to the total energy of the system by a conjugated gradient algorithm.26 The calculations for a doped Bi0.9Sr0.1CuSeO composition were performed using a 3 × 3 × 3 supercell including 54 formula units.

Bi(1−x)SrxCuSeO in order to link the structural and physical properties and to determine the influence of the doping on the physical properties and on the electronic structure. The experimental study has been combined with ab initio density functional theory (DFT) calculations to explain the electrical behavior of these materials.

■

EXPERIMENTAL SECTION

Bi(1−x)SrxCuSeO (nominal composition, x = 0−0.40) samples were synthesized by a two steps solid state reaction. Stoichiometric mixtures of Bi2O3 (BHO, 99.5%), Bi (Aldrich, 99.99%), Se (Alfa Aesar, 99.99%), Cu (Merck, 99.9%), and SrO (Alfa Aesar 99.5%) powders were sealed under argon in silica ampules and annealed for 6 h at 573 K. The powder was then grinded and pressed into bars under uniaxial stress (250 MPa), which were heated again for 1 week at 973 K in silica ampules sealed under argon. All handlings were performed in an argon-filled glovebox with less than 0.5 ppm O2 and H2O. The samples were densified using a spark plasma sintering (SPS) system (SyntecSPS-511S) at 950 K with holding time of 10 min in a Ø = 15 mm graphite mold under an axial compressive stress of 100 MPa in an argon atmosphere. The density of the obtained pellets is higher than 95%. A typical SEM micrograph, representative of the samples microstructure, can be seen in the Supporting Information, Figure i. Structure Refinement. Room temperature X-ray diffraction characterization was performed using a Panalytical X’Pert diffractometer by using a Cu−Kα1 radiation, with a Ge(111) incident monochromator and a X’celerator detector. Temperature dependent XRD patterns were recorded using a Bruker D8 Advance diffractometer with an Anton Paar TTK chamber. Rietveld refinement was performed using FULLPROF software.17 Compositions were analyzed by energy-dispersive X-ray spectroscopy (EDX) using a Zeiss SUPRA 55 VP SEM microscope coupled to an EDAX-TSL EDX system. Measurement of Electronic and Magnetic Properties. The electrical resistivity and specific heat were measured using a Quantum Design PPMS (physical properties measurement system) from 300K to 2K under magnetic fields from 0T to 9T. Hall measurements were

■

RESULTS AND DISCUSSION Crystal Structure. BiCuSeO crystallizes in the ZrSiCuAs structure-type with a tetragonal P4/nmm layered structure.27 This structure is composed of stacked “conductive” layers [Cu2Se2]2− and “insulating” layers [Bi2O2]2+. These layers are composed of edge sharing (CuSe4) and (OBi4) tetrahedra 3169



Article

Table 1. Structural Parameters Obtain from Rietveld Analysis at Room Temperature x

a (Å)

c (Å)

z (Bi)

z (Se)

volume cell (Å3)

angle α (Se−Cu−Se) (deg)

angle β (Se−Cu−Se) (deg)

d (Cu−Se) (Å)

d (plan Cu−Se) (Å)

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

3.928 3.928 3.932 3.934 3.938 3.942 3.947 3.949

8.933 8.933 8.955 8.978 9.012 9.035 9.074 9.092

0.140 0.139 0.138 0.137 0.135 0.134 0.131 0.130

0.674 0.674 0.671 0.666 0.665 0.665 0.663 0.662

137.8 137.8 138.4 139.0 139.8 140.4 141.3 141.8

103.2 103.2 104.3 105.6 105.9 105.6 106.3 106.7

112.6 112.7 112.1 111.4 111.2 111.4 111.1 110.9

2.505 2.505 2.490 2.469 2.466 2.475 2.466 2.462

0.174 0.174 0.171 0.166 0.165 0.166 0.163 0.162

Figure 2. Structural parameters for Bi1−xSrxCuSeO in function of the Sr content ((a) lattice constants, (b) atomic positions in the cell, (c) Cu−Se distances (bond and plan), (d) (Se−Cu−Se) angle).

respectively, and are stacked along the c axis of the tetragonal cell. In the Figure 1, the XRD pattern and the Rietveld refinement are plotted for Bi0.95Sr0.05CuSeO, this pattern being representative of the series. All major peaks can be indexed in the ZrSiCuAs structure-type, and as shown in the inset, very few impurities are present (mainly Bi2O3) and they account for less than 2%. The RBragg, which is less than 2%, and the Rp and the Rwp, which are less than 7 and 9%, respectively, reflect the good agreement between the calculated data (red line) and the experimental data (black line). Similar results were obtained for all samples with x ≤ 0.35, which are all almost single phase. The cell parameters for the undoped sample are a = 3.926 Å and c = 8.925 Å and a peak shift with the chemical composition can be noticed for all samples, when x ≥ 0. However, for x = 0.4, additional peaks are observed and minority phases, Sr2Bi2CuOx or SrCuO3 are clearly visible, which indicates that the samples are no longer single phase. These results suggest that the solubility limit is reached for 0.35 < x < 0.4. EDX measurements have been made on all samples in order to compare the effective and nominal Sr concentrations. The measurements show that the samples of the series are homogeneous and that the actual Sr content is about 2−3% lower than the nominal values in each sample.

The results of the Rietveld refinements are summarized in Table 1 and in the Figure 2 (all the Rietveld refinements can be seen in the Supporting Information, Figure ii−vii). The first plot of this figure represents the evolution of the lattice constants with doping (Figure 2a), which increase linearly up to x = 0.35 and can be described by a Vegard law. Above x = 0.35, apparent parameters seem to decrease, which is consistent with the presence of a larger amount of secondary phases in the x = 0.40 sample. This observation confirms that the solubility limit is reached for x between 0.35 and 0.4. Below x ≤ 0.35, the increase of the cell parameters with Sr content can be explained by the larger ionic radius of Sr2+ (1.18 Å) than Bi3+ (1.03 Å). Moreover, the substitution of Bi3+ by Sr2+ is an aliovalent doping, which modifies the electronic charge of each layers which become [(Bi1−xSrx)2O2](2−2x)+ and [Cu2Se2](2−2x)− using a simple electron count. Therefore, the doping leads to the transfer of holes in the “conductive layer” and to the decrease in the Coulombic attraction between layers. This second effect results in a larger increase of the c axis than that of the a axis. However it can be noticed that although the evolution of the lattice parameter is linear with the doping, the lattice parameters of the undoped samples deviate from the linear trend. Indeed, although EDX measurements for the undoped samples do not show any trace of Sr or other unintentional 3170



Article

doping, the lattice parameters of the undoped samples are almost identical to those of a compound with x = 0.05 rather than the “theoretical” x = 0. This deviation may be explained by some Cu deficiencies which can occur during the synthesis, as will be discussed latter in relation to the electronic properties. The second plot, in Figure 2b, represents the evolution of the z position of Bi and Se in function of the Sr content. This plot shows that the doping slightly influences both atoms positions. Thus, it is noteworthy that substitutions performed in the Bi2O2 layer also influence the structure of the Cu2Se2 layer, this latter one being directly responsible of the electronic properties of these materials (see later). This conduction layer is built of distorted CuSe4 tetrahedra where bond angles, Se−Cu−Se, noted α1 and α2, are not regular (≠ 109.5°). In Figure 2, plots c and d represent the evolution of essential parameters of these tetrahedra, Cu−Se bond distances, bond angles and width of the Cu−Se block. We have seen previously that the evolution of the lattice parameters can be well-described assuming that the interlayer coupling follows an ionic model. Within the conducting layer, a decrease of Cu−Se bond is also expected from a simple electron count as the formal charge of Cu ions should increase, ([Cu2Se2](2−2x)− layers should correspond to [Cu2(1+x)+Se22−](2−2x)−) resulting in an increased Coulombic interaction), which is indeed observed. On the contrary, a simple electron counts within the charge reservoir layer leads to [Bi2−2xSr2xO2](2−2x)+ resulting in a decreased Coulombic attraction, which coupled to the larger Sr2+ radius is consistent with the lattice parameter a increase. Therefore, although the observed evolution of the lattice parameters is consistent with an ionic description of the reservoir layer, this is not the case for the conducting layer (However, the results obtained by ab initio calculations show that the character of the Cu−Se bonds is predominantly covalent, see later). Figure 2c represents the evolution of distance between the copper and the selenium planes (which is referred to as “pnictogen height” in the 1111 iron-based superconductors), and shows that this distance also decreases with the doping despite the strong increase of the lattice parameter c. This decrease is consistent with the calculations performed by Kusainova et al. in [Cu2S2]2− layers sharing the same geometry.28 The decrease of the Cu−Se bond length with doping coupled to the decrease of the “chalcogen height” leads to an evolution of the tetrahedra geometry, with the distortion of the tetrahedra that decreases with doping. However, this distortion stays rather large even for the highest doping level (α < 107°, β ∼ 111°). Electrical Properties. Figure 3 presents the electrical resistivity of undoped BiCuSeO from 300 to 2 K under magnetic field. BiCuSeO exhibits a metallic behavior with a positive trend of ρ(T), which is unexpected for this undoped sample. Indeed, a semiconductor behavior is expected according to the band gap (∼0.8 eV) of this compound which should preclude any carriers activation through the gap.15 However this result is consistent with the structural characterization which shows, for the undoped compounds, a small deviation between theoretical and observed structural parameters. Those observations suggest the occurrence of an unintentional doping of the sample, which may correspond to Cu deficiencies.3 Several samples have been measured, and they all exhibit this metallic behavior. The electrical resistivity shows a broad anomaly around 260 K, which has been reproducibly observed in several samples. This anomaly does not depend on the applied magnetic field,

Figure 3. Electrical resistivity of BiCuSeO in function of temperature and magnetic field.

but it is dependent on the thermal history of the sample, with a small hysteresis that appears between cooling and heating. In 1997, Ohtani et al.,29 already observed this hysteresis that could originate from a first order phase transition. However, the origin of this anomaly is not clear. First, our magnetic susceptibility measurements did not reveal any magnetic ordering (a susceptibility measurement can be seen in the Supporting Information, Figure viii), which rules out the possibility of a magnetic origin. Moreover, complementary XRD analyses have been performed at different temperatures from 300 to 150 K and Rietveld refinements have been realized for these data. The results do not show any peculiar evolution of the lattice parameters, except those induced by the thermal contraction, or of Bi or Se positions. The three patterns can be refined in the P4/nmm structure with similar refinement quality and there is no trace of distortion (all the Rietveld refinements can be seen in the Supporting Information, Figure x−xii). For example as present in the Figure 4, the FWHM of the [110] Bragg peak does not change when the temperature decreases, as should be induced by an orthorhombic distortion. Thus, these results do not reveal any distortion which could be interpreted as a structural phase transition. Last, as some layered copper based compounds, and especially copper selenides, are known to exhibit very large Cu ionic conductivity values,30 the anomaly could have been linked to the onset of ionic conductivity in this compound. However, our ionic conductivity measurements (not shown) have shown that the ionic conductivity of BiCuSeO is negligible. Therefore, the origin of this anomaly is still under question. The electrical resistivity of Bi(1−x)SrxCuSeO as a function of temperature is plotted in the Figure 5, for x from 0.05 to 0.35. First, it can be noticed that the broad anomaly present for the undoped sample disappears with the Sr doping. Moreover, the electrical resistivity is significantly reduced with Sr doping from 0.11 Ω cm for BiCuOSe at 300 K to 0.0010 Ω cm for Bi0.65Sr0.35CuSeO at 300 K. This evolution can be easily explained by a simple electrons count. The substitution of Bi3+ by Sr2+ induces carriers in the charge reservoir insulating layer [Bi2O2]2+ which are transferred in the conductive layer [Cu2Se2]2−. This increase in charge carrier concentration leads to a decrease of the electrical resistivity. Beside the 3171



Article

LaFeOAs leads to the emergence of superconductivity,31 and a possible trace of superconductivity has been observed by Ubaldini et al.16 in 2010 in BiCuSO. Therefore, this lowtemperature anomaly could be the signature of a superconducting transition. However, our magnetic susceptibility measurements down to 1.8 K did not evidence any transition (a susceptibility curve can be seen in the Supporting Information, Figure ix). In addition, no transition or anomaly has been observed by specific heat measurements (see later). However, as this anomaly has been reproducibly observed in several distinct samples and as all our samples are almost single phase (more especially, no trace of possibly superconducting cuprate phase has been observed within the detection limit of the XRD equipment), we believe that this behavior is intrinsic. The temperature dependence of the Seebeck coefficient is plotted in the Figure 6. The positive Seebeck coefficients for all

Figure 4. XRD data for BiCuSeO at 300, 260, and 150 K. The 260 and 150 K patterns have been shifted in order to superpose the peaks' maximum values.

decrease in the electrical resistivity in the Sr-doped samples, a new anomaly emerges at low temperature with Sr doping. As plotted in the second part of the Figure 5, an anomaly of the electrical resistivity can be observed around 10 K, with an increased slope of ρ(T). This anomaly is strongly influenced by the applied magnetic field, with a marked upturn that occurs when the field exceeds about 4 T. This anomaly has been observed for every Sr doped samples, but its amplitude depends on the doping level and reaches its maximum for x = 0.15, with a domelike behavior. In the oxypnictides, fluorine doping of

Figure 6. Temperature dependence of the Seebeck coefficient in the Bi1−xSrxCuSeO series.

Figure 5. Temperature dependence of the electrical resistivity for Bi(1−x)SrxCuSeO at 0 T and for Bi0.85Sr0.15CuSeO under magnetic fields. 3172



Article

Figure 7. (a) Evolution of the electrical resistivity in function of Sr fraction at 300 K; (b) evolution of the Seebeck coefficient and the thermoelectric power factor in function of Sr fraction at 300 K; (c) evolution of the Hall resistivity in function of magnetic field at different temperatures; and (d) evolution of both carrier concentration and Hall mobility in function of Sr fraction at 300 K.

power factor (PF), inset plot b, is rather moderate due to the similar evolution of the Seebeck coefficient. The PF reaches an optimum for x = 0.1. This result is in good agreement with the results obtained at higher temperature, where the largest ZT is reached for x = 0.075.2 In the third plot of the Figure 7, plot c, the magnetic field dependence of the Hall resistivity at various temperatures is presented. Those results for Bi0.8Sr0.1CuSeO are representative of the series and the linear regression leads to a positive Hall coefficient for all samples. The Hall carriers concentration pH has been calculated using pH = 1/RHe with RH the Hall coefficient, derived from the linear fit of μH(H), and e the electric charge. In principle, the Hall carriers concentration is related to the carriers concentration through p = pHrH with rH the Hall factor, which depends on the electronic density of states close to the Fermi level and on the carriers scattering parameters. However, as rH is close to one for degenerate compounds with acoustic phonons scattering (which is the main scattering process in these compounds, see later), we have assumed p = pH, which should be correct within at worst 10%

the samples indicate that holes are the majority carriers. The undoped BiCuSeO Seebeck coefficient exhibits large values at room temperature which decrease with the doping from 430 μV K−1 for x = 0 to 50 μV K−1 for x = 0.35, which is consistent with an increased carrier concentration. For doped samples, these values are comparable to those of Cu2ZnSnQ4 (Q = S, Se),32 Cu2ZnSn1−xInxSe4,33 or Bi2Te3.34 It is noteworthy that no anomaly of the Seebeck coefficient has been observed at any temperature, which is very different from the electron-doped oxypnictides that exhibit a large peak of S around 100 K vanishing with further doping,35 and which contrasts with the electrical resistivity behavior with the anomaly around 250 K in the undoped sample. The Figure 7 shows the evolution of the electronic properties of Bi1‑xSrxCuSeO as a function of the Sr doping content. Plots a and b summarize, respectively, the doping level dependence of the electrical resistivity, the Seebeck coefficient and the thermoelectric power factor at room temperature. Both S and ρ decrease with Sr doping. Thus, in spite of the large decrease of the electrical resistivity, the increase of the thermoelectric 3173



Article

error. The results of the Hall effect measurements are summarized in the Figure 7d. In the first part of the plot, the nominal and experimental carrier concentrations are presented as a function of the Sr content. It can be noticed that the doping leads to a large increase of the carrier concentration up to more than 1 × 1021 cm−3 which is consistent with the decrease of the electrical resistivity. A fairly good agreement between experimental and nominal values can be noticed, which indicates a good efficiency of Sr doping in BiCuSeO, close to 100%. To calculate this nominal carrier concentration, it has been considered that each Sr atom induced one hole in the structure, taking into account the actual EDX concentration. The small difference between nominal and experimental carriers concentration is well explained by the n = nH hypothesis. It can also be noticed that the carrier concentration of the undoped sample is 3 × 1018 cm−3, which is consistent with the metallic behavior of the sample and is much higher than the value expected for a 0.8 eV band gap semiconductor at room temperature. This observation confirms a probable unintentional doping during the synthesis BiCuSeO. Assuming that Cu deficiencies are responsible for the doping, and that every Cu vacancy leads to one hole in the valence band, a vacancy atomic fraction as small as 2 × 10−4 may be sufficient to produce this holes concentration. This shows that a really undoped sample is hard to obtain. Figure 7d also shows the Sr fraction dependence of the Hall mobility. The mobility values are small, even in the undoped compound where μ reaches 20 cm2 V−1 s−1. As expected, the mobility is reduced by Sr doping, by about 1 order of magnitude from x = 0 to x = 0.05. This decrease is moderate as compared to the large enhancement of the carriers concentration with doping. Indeed, Sr2+ substitution for Bi3+ takes place in the [Bi2O2]2+ layer, whereas the electrical conductivity mostly originates from the [Cu2Se2]2‑ layer (see later). Therefore, Sr doping does not reduce the mobility much by structural disorder or point defect scattering. The Figure 8 shows the temperature dependence of the Hall mobility as a function of Sr doping. Above 100−120 K, the mobility can be described by a power law μHT−r with r that gradually changes from approximately −3/2 (acoustic phonons scattering of the charge carriers) for low doping level to −0.7 for large doping level (r = −1 corresponding to acoustic phonons scattering of the charge carriers in a strongly degenerate model). Therefore, we can conclude that despite the intrinsically low Hall mobility that could have been a signature of a strong point-defect or impurity scattering, the main carriers scattering process is acoustic phonons scattering whatever the doping level. Band Structure Calculation. The optimized geometry of the BiCuSeO bulk has been calculated using the LCAO-B3LYP method. Lattice parameters are in good agreement with the experimental ones, with a bit larger values. a and c are found to be 3.9961 Å and 9.3410 Å, 1.7 and 4.6% greater than experiment, respectively. When compared to experimental z position of Bi and Se in the perfect bulk (z/c(Bi) = 0.132 and z/c(Se) = 0.680), a contraction by about 6% and 1% for Bi and Se respectively can be observed, but the values are in good agreement with the experimental ones. The atomic valence charges resulting from a Mulliken population analysis are +1.46 for Bi, −1.06 for O, +0.24 for Cu and −0.66 for Se, respectively. These values are quite different from the ones obtained by a simple electron count, namely Bi3+Cu1+Se2−O−2, which are obtained assuming an ionic model.

Figure 8. Temperature dependence of the Hall mobility in the series Bi1−xSrxCuSeO (log−log scale).

They evidence a mostly covalent bonding between copper and selenium, as compared to the Bi−O bonding, which is partly ionic. Moreover, it confirms that there is a charge transfer between the [Bi2O2] layer and the [Cu2Se2] layer. The most striking result is the almost metallic character of the copper (with a formal charge being only +0.24 instead of +1), linked to the strong covalency of the Cu−Se bonding. The covalent behavior of the Cu−Se bonding is confirmed by the very large calculated Cu−Se Mulliken overlap population, as can be seen in Table 2, which also shows a large value for Cu−Cu. Whereas Table 2. Calculated Data element 1

element 2

calcd distance (Å)

Mulliken overlap population

Bi

O Se Se Cu Cu Bi

2.354 3.323 2.609 2.826 2.609 3.323

0.004 0.034 0.114 0.047 0.114 0.034

Cu Se

these results are nicely consistent with the usual description of the structure with strong intralayer covalent bonding, the moderate Bi−Se overlap population was rather unexpected. Moreover, this overlap population is probably underestimated as the c value is overestimated by the calculation, which leads to an overestimated Bi−Se distance as compared to the experimental one. This result contrasts with the two-dimensional description of the structure and is the signature of a moderate covalent bonding between the layers. To correlate the band structure calculations to experimental results, we have performed specific heat measurements. Figure 9 shows the specific heat as a function of the temperature for Bi0.85Sr0.15CuSeO, which is representative of each sample in the series. It is noteworthy that no anomaly can be observed at low temperature. The same behavior occurs for the undoped sample with no anomaly observed around 260 K. The inset shows the low-temperature specific heat data, Cp/T plotted as a 3174



Article

close to the Fermi level, with a shift of εF within the valence band. Therefore, the electrical properties of these materials can be described in the framework of a rigid band model. The main features of the band structure and DOS are consistent with the results previously reported by Hiramatsu et al.15 Bi 6s states are located deep in the valence band (−10 to −12 eV relative to the valence band maximum), as well as O 2p states (−5 to −7 eV). The bottom of the conduction band mainly consists of Bi 6p states. The top of the valence band consists in bonding hybridized Cu 3d−Se 4p states and O 2p states (−5 eV to −7 eV), nonbonding Cu 3d states (−1 to −3 eV), and antibonding hybridized Cu 3d−Se 4p states (close to the valence band maximum). It is noteworthy that in the vicinity of the Fermi level (0 eV to −0.5 eV, see the magnified view), copper and selenium almost equally contribute to the density of states, with about 1 state eV−1 fu−1, and thus to the electronic conduction. The same situation occurs for Bi0.9Sr0.1CuSeO. As it can be seen from the magnified view of the electronic band structure, BiCuSeO is a multiband semiconductor. The top of the valence band consists in a hole pocket located on the Γ-M line. Other hole pockets can be observed on the Γ-X line and at the Z point, which are located about −0.15 eV below the Fermi level, and will contribute to the conduction in the doped compounds and at high temperature. All these bands are moderately dispersive and correspond to moderately light holes. The presence of a hole pocket located at the Z point is surprising taking into account the two-dimensional character of the crystal structure. However, it is consistent with the moderate Bi−Se overlap, and with the moderate contribution of the Bi 6p states to the projected DOS around −0.15 eV relative to the valence band maximum, with about 0.2 states eV−1 fu−1. Therefore, it seems that although the electrical behavior of this material will certainly be anisotropic, the anisotropy is probably moderate because of the moderate effective mass anisotropy originating from the Bi6p contribution to the DOS. These layer-based materials had originally been described as “natural multiple quantum well”36,37 and a large density of states close to the Fermi level and a strong anisotropy could have been expected, similarly to the artificial superlattices.38 However, it is not the case, and although these compounds can be crystallographically described as layered compounds, their electrical transport properties are closer to “classical 3D materials”, with parabolic-like DOS as compared to superlattices step-like DOS. Moreover, this moderate contribution of the Bi6p orbitals to the DOS and thus to the electrical properties can probably explain the different behavior of BiCuSeO based compounds, with low electrical resistivity, and RECuSeO (RE = La to Ho) based compounds, which exhibit much higher electrical resistivity values, of the order of 1 Ω cm.39 A similar trend has been observed with sulfides by Sheets et al.40 with a larger density of states close to the Fermi level in BiCuOS as compared to LaCuOS. The promising thermoelectric properties of BiCuSeO-based compounds, with ZT values higher than 0.8 at 923 K,3 are mostly linked to intrinsically very low lattice thermal conductivity, rather than high power factor. Indeed, as compared for example to state-of-the art skutterudites,12 the lattice thermal conductivity of BiCuSeO2 is more than 4 times lower, but the power factor is also much lower. This moderate value of the power factor mainly originates from three factors. First, the holes mobility is low despite the metallic behavior of the Cu−Se layer, resulting in a high doping level needed to obtain a low electrical resistivity. Although these low mobility

Figure 9. Temperature dependence of the specific heat for Bi0.85Sr0.15CuSeO and Linear fit of Cp/T = f(T2) at low temperature.

function of T2. The data have been fitted using Cp = γT + βT3, where γT is the electronic contribution with γ the Sommerfeld coefficient and βT3 is the lattice contribution. According to the Sommerfeld theory of metals, γ can be written as function of the density of states at the Fermi level as: γ=

π 2kB2 D(εf )(1 + λ) 3

where D(εf) is the density of states at the Fermi level and λ the electron−phonon coupling constant, which we set here to zero. The obtained values of γ and of D(εf) are plotted in the Figure 10 for each composition. The density of states at the Fermi level exhibits a dome-like shape with a maximum reached for x = 0.2 with about 3.6 states eV−1 fu−1. Figure 11 shows the electronic band structure and projected DOS of undoped BiCuSeO. The calculations performed for Bi0.9Sr0.1CuSeO show very similar results for the bands located

Figure 10. Sr fraction dependence of γ and D(EF). 3175



Article

Figure 11. Electronic band structure (right) and projected DOS (left) for the undoped BiCuOSe (the bottom part is a magnified view close to the Fermi level).

within the Cu−Se layer. As the strontium substitution only leads to a moderate improvement of the geometry of the tetrahedra, the mobility could most probably be enhanced by a fine-tuning of the crystal structure toward more regular CuSe4 tetrahedra using other doping elements. Second, we have

values could originate from the polycrystalline nature of our samples, similar low values have also been observed in epitaxial thin films.41 One possible explanation could be the large distortion of the CuSe4 tetrahedra, which could be detrimental to the Cu 3d − Se 4p orbital mixing and to the holes mobility 3176



Article

Notes

shown that the main carriers scattering process in Bi1−xSrxCuSeO is acoustic phonons scattering, which does not favor large values of the Seebeck coefficient as compared for example to ionized impurities scattering.42 Therefore, the Seebeck coefficient at constant carriers concentration could be improved by increasing the scattering parameter r toward ionized impurity scattering values. Last, although BiCuSeO is a multiband material with hole pockets located at degenerated directions of the Fermi surface, the Seebeck coefficient is moderate, due to the relatively small hole effective mass, that should be enhanced. Therefore, we can conclude that the efforts devoted to the increase of the thermoelectric figure of merit of BiCuSeO based materials should rather be directed toward the improvement of the power factor by a fine-tuning of the electronic band structure, of the crystal structure and of the holes scattering process, rather than toward a reduction of the lattice thermal conductivity which is already small.

■

CONCLUSION

■

ASSOCIATED CONTENT

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the ANR through the project OTHer (ANR 2011 JS08 012 01). The authors thank O. Rouleau for his help with temperature-dependent XRD, P. Ribot for EDX measurements, S. Franger for ionic conductivity measurements, and F. Bouquet for his help with specific heat measurements.

■

In summary, our structural study has shown that the solubility limit for Sr doping in the BiCuSeO structure is reached for x = 0.35. The evolution of the lattice parameters and the band structure calculations suggest that the structure has to be described by two different models, with ionic interlayer interactions but covalent (or iono-covalent) intralayer interactions due to a large hybridization of Cu 3d and Se 4d orbitals. Sr doping also induces a strong decrease of the electrical resistivity, 100 times smaller for x = 0.35 as compared to the undoped sample. This decrease can be correlated with the increase of the charge carrier concentration, which, at the same time, leads to the decrease of the Seebeck coefficient. Therefore, the power factor values of the doped samples are not improved that much as compared to the large increase of the electrical conductivity. It means that the very promising ZT values of BiCuSeO compounds are mainly due to the very low lattice thermal conductivity. The moderate Seebeck coefficient values originate from several factors: moderate effective mass of the holes, acoustic phonons scattering being the main scattering parameter, and large carriers concentration requested to obtain moderate electrical resistivity. Therefore, we can conclude that in order to improve the ZT of those compounds, the efforts should be focus on the improvement of the power factor rather than on the decrease of the lattice thermal conductivity. However, two questions have been raised that are still unsolved. Undoped BiCuSeO shows a broad anomaly around 260 K, which disappears with doping and which is dependent on the thermal history of the sample. However, no trace of structural transition has been noticed. For the doped samples, another anomaly has been observed at low temperature, which resembles a superconducting transition. However, no signature of superconductivity has been observed in magnetic measurements.

* Supporting Information S

Additional figures (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

■

REFERENCES

(1) Snyder, G. J.; Toberer, E. S. Nat. Mater. 2008, 7, 105. (2) Zhao, L. D.; Berardan, D.; Pei, Y. L.; Byl, C.; Pinsard-Gaudart, L.; Dragoe, N. Appl. Phys. Lett. 2010, 97, 092118. (3) Liu, Y.; Zhao, L. D.; Liu, Y.; Lan, J.; Xu, W.; Li, F.; Zhang, B. P.; Berardan, D.; Dragoe, N.; Lin, Y.-H.; Nan, C.-W.; Li, J.-F.; Zhu, H. J. Am. Chem. Soc. 2011, 133, 20112. (4) Wang, Y.; Sui, Y.; Cheng, J.; Wang, X.; Su, W. J. Phys. D: Appl. Phys. 2008, 41, 045406. (5) Masuda, Y.; Nagahama, D.; Itahara, H.; Tani, T.; Seoc, W. S.; Koumotoa, K. J. Mater. Chem. 2003, 13, 1094. (6) Ohta, S.; Nomura, T.; Ohta, H.; Hirano, M.; Hosono, H.; Koumoto, K. Appl. Phys. Lett. 2005, 87, 092108. (7) Ohtaki, M.; Tsubota, T.; Eguchi, K.; Arai, H. J. Appl. Phys. 1996, 79, 1816. (8) Bérardan, D.; Guilmeau, E.; Maignan, A.; Raveau, B. Solid State Commun. 2008, 146, 97. (9) Guilmeau, E.; Bérardan, D.; Simon, C.; Maignan, A.; Raveau, B.; Ovono Ovono, D.; Delorme, F. J. Appl. Phys. 2009, 106, 053715. (10) Dughaish, Z. H. Physica B 2002, 322, 205. (11) Wood, C. Rep. Prog. Phys. 1988, 51, 459. (12) Tang, X.; Chen, L. D.; Goto, T.; Hirai, T. J. Mater. Res. 2001, 16, 837. (13) Saber, H. H.; El-Genk, M. S.; Caillat, T. Energy Convers. Manage. 2007, 48, 555. (14) Palazzi, M. C. R. Acad. Sci., Paris 1981, 292, 789. (15) Hiramatsu, H.; Yanagi, H.; Kamiya, T.; Ueda, K.; Hirano, M.; Hosono, H. Chem. Mater. 2008, 20, 326. (16) Ubaldini, A.; Giannini, E.; Senatore, C.; Van der Marel, D. Physica C 2010, 470, S356. (17) Rodriguez-Carvajal, J. Physica B 1993, 192, 55. (18) Dovesi, R.; Orlando, R.; Civalleri, B.; Roetti, C.; Saunders, V. R.; Zicovich-Wilson, C. M. Z. Kristallogr. 2005, 220, 571. (19) Dovesi, R.; Saunders, V. R.; Roetti, C.; Orlando, R.; ZicovichWilson, C. M.; Pascale, F.; Civalleri, B.; Doll, K.; Harrison, N. M.; Bush, I. J.; D’Arco, P.; Llunell, M. CRYSTAL09 User’s Manual; University of Torino: Torino, Italy, 2009. (20) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (21) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (22) Hay, W. R.; Wadt, P. J. J. Chem. Phys. 1985, 82, 284. (23) Durand, P.; Barthelat, J. C. Theor. Chem. Acc. 1975, 38. (24) Valenzano, L.; Torres, F. J.; Doll, K.; Pascale, F.; ZicovichWilson, C. M.; Dovesi, R. Z. Phys. Chem. 2006, 220, 893. (25) Doll, K.; Harrison, N. M. Chem. Phys. Lett. 2000, 317, 282. (26) Schlegel, H. B. J. Comput. Chem. 1982, 3, 214. (27) Kusainova, A. M.; Berdonosov, P. S.; Akselrud, L. G.; Kholodkovskaya, L. N.; Dolgikh, V. A.; Popovkin, B. A. J. Solid State Chem. 1994, 112, 189. (28) Vajenine, G.; Hoffmann, R. Inorg. Chem. 1996, 35, 451. (29) Ohtani, T.; Tachibana, Y.; Fujii, F. J. Alloys Compd. 1997, 262− 263, 175. (30) Chatov, V. A.; Iorga, T. P.; Inglizyan, P. N. Fiz. Tekh. Poluprovodn. 1980, 14, 807. (31) Kamihara, Y.; Watanabe, T.; Hirano, M.; Hosono, H. J. Am. Chem. Soc. 2008, 130, 3296. (32) Liu, M. L.; Huang, F. Q.; Chen, L. D.; Chen, I. W. Appl. Phys. Lett. 2009, 94, 202103.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 3177



Article

(33) Shi, X. Y.; Huang, F. Q.; Liu, M. L.; Chen, L. D. Appl. Phys. Lett. 2009, 94, 122103. (34) Nassary, M. M.; Shaban, H. T.; El-Sadek, M. S. Mater. Chem. Phys. 2009, 113, 385. (35) Pinsard-Gaudart, L.; Berardan, D.; Bobroff, J.; Dragoe, N. Phys. Status Solidi 2008, 2, 185. (36) Hiramatsu, H.; Kamihara, Y.; Yanagi, H.; Ueda, K.; Kamiya, T.; Hirano, M.; Hosono, H. J. Eur. Ceram. Soc. 2009, 29, 245. (37) Ueda, K.; Hiramatsu, H.; Ohta, H.; Hirano, M.; Kamiya, T.; Hosono, H. Phys. Rev. B 2004, 69, 155305. (38) Ohta, H.; Mune, Y.; Koumoto, K.; Mizoguchi, T.; Ikuhara, Y. Thin Solid Films 2008, 516, 5916. (39) Ohtani, T.; Hirose, M.; Sato, T.; Nagaoka, K.; Iwabe, M. Jpn. J. Appl. Phys. 1993, 32, 316. (40) Sheets, W. C.; Stampler, E. S.; Kabbour, H.; Bertoni, M. I.; Cario, L.; Mason, T. O.; Marks, T. O.; Poeppelmeier, K. R. Inorg. Chem. 2007, 46, 10741. (41) Hiramatsu, H.; Ueda, K.; Ohta, H.; Hirano, M.; Kikuchi, M.; Yanagi, H.; Kamiya, T.; Hosono, H. Appl. Phys. Lett. 2007, 91, 012104. (42) Pichanusakorn, P.; Bandaru, P. R. Mater. Sci. Eng., R 2010, 67, 19.

3178


Structural and Electronic Transport Properties in Sr-Doped BiCuSeO

Recommend Documents