Enhanced thermoelectricity by controlled local structure in bismuth-chalcogenides

TL;DR

This study investigates how Se substitution in LaOBiS$_{2-x}$Se$_x$ improves thermoelectric performance by suppressing local distortions and altering electronic structure, offering a new approach to optimize thermoelectric materials.

Contribution

It demonstrates that controlling local crystal structure via Se substitution enhances thermoelectric properties in bismuth-chalcogenides.

Findings

Se substitution suppresses local distortion

Increased carrier mobility observed

Electronic structure is altered by Se doping

Abstract

Spectroscopic techniques, including photoelectron spectroscopy, diffuse reflectance, and x-ray absorption, are used to investigate the electronic structure and the local structure of LaOBiS$_{2-x}$Se$_x$ thermoelectric material. It is found that Se substitution effectively suppresses local distortion, that can be responsible for the increased carrier mobility together with a change in the electronic structure. The results suggest a possible way to control thermoelectric properties by tuning of the local crystal structure of these materials.

Tables

Table 1. Table 1: Bond distances (R) and mean square relative displacements ( σ 2 superscript 𝜎 2 \sigma^{2} ) of LaOBiS 2 and LaOBiSSe determined by Bi L 3 -edge EXAFS ( T 𝑇 T = 30 K). Maximum uncertainty in distance determination is about ∼ similar-to \sim 0.01 Å, and in the corresponding σ 2 superscript 𝜎 2 \sigma^{2} is about ∼ similar-to \sim 0.001 Å 2 . Bi-Ch 1 i ​ n superscript subscript absent 𝑖 𝑛 1 {}_{in}^{1} and Bi-Ch 2 i ​ n superscript subscript absent 𝑖 𝑛 2 {}_{in}^{2} are two in-plane Bi-Ch in distances while Bi-Ch i i ​ n superscript subscript absent 𝑖 𝑛 𝑖 {}_{in}^{i} is the distance between the BiCh 2 -layers.

		LaOBiS2			LaOBiSSe
	R(Å)	${\sigma }^2$(Å2)			R(Å)	${\sigma }^2$(Å2)
Bi-Sout	2.47	0.003			2.48	0.004
Bi-S/Se${}_{}{}^1{}_{in}{}^{}$	2.68	0.003			2.82	0.003
Bi-S/Se${}_{}{}^2{}_{in}{}^{}$	3.09	0.005			3.04	0.006
Bi-S/Se${}_{}{}^i{}_{in}{}^{}$	3.42	0.006			3.47	0.009

Full text

I INTRODUCTION

Thermoelectric materials with layered structures hold possibillities to modify their physical properties, $e.g.$, by partial substitutions in the block layer and/or conducting layer. Among them, REOBiCh2 (RE = rare earth and Ch = chalcogen) system Mizuguchi1st is acquiring much attention due to its structural similarity with high-$T_{c}$ cuprates and iron-based superconductors in which carrier number and other physical parameters are known to be controlled by combinations of constituting block layers and conducting layers Ce ; Pr ; Nd ; Eu ; Sm ; TM ; Sr ; HEA ; inter ; Se . The parent material, LaOBiCh2 is insulating, and an object of superconductivity research since it becomes superconductor by carrier doping Mizuguchi1st . It was found JAP1 that isovalent substitution of chalcogen atoms in parent compound induces a large improvement of thermoelectric figure of merit given by $ZT$ = ($S^{2}$/$\rho\kappa)T$. The $ZT$ in the LaOBiS2-xSex system has been found NishidaJPSJ to be as high for $\sim$0.36 in x = 1.0 at 650 K.

The thermoelectric $ZT$ consists of several physical parameters, namely Seebeck coefficient ($S$), resistivity ($\rho$), and thermal conductivity ($\kappa$). For $x$ = 1.0, the absolute value of Seebeck coefficient is $\sim$20 % higher while the total thermal conductivity remains similar to that of $x$ = 0.0, instead the resistivity shows a significant decrease JAP1 . It can be seen in Fig. 1 that the electrical resistivity of LaOBiSSe (corresponds to $x$ = 1.0) below room temperature is smaller than that of LaOBiS2 by about a factor of 10. Judging from the reported relationship between the carrier concentration and Seebeck coefficient in LaOBiCh2 Mizuguchicogent , the carrier concentrations in LaOBiS2 and LaOBiSSe are expected to be similarNishidaJPSJ . In a previous study NishidaJPSJ , it has been argued that such a difference in electrical resistivity could be related with large enhancement of carrier mobility by Se substitution, attributed to the chemical pressure effect or more precisely, the misfit strain effect in the layered structure since the ionic radius of Se is larger than that of S. This may cause an enhanced overlap between adjacent orbitals through substitution, resulting in a change in the electronic structure. However, there is no direct experimental evidence providing microscopic mechanism responsible for improving the thermoelectric properties in this system. This is also important due to the fact that REOBiS2 is structurally instable Sugimoto with polymorphism poly1 ; poly2 characterized by coexisting atomic configurations and hence apart from the knowledge of the electronic transport, study of the local structure should provide important feedthrough.

Here, we have performed a comparative study of the electronic structure and local structure of LaOBiS2 and LaOBiSSe using photoemission, diffuse reflectance, band calculation, and x-ray absorption fine structure, to address possible origin of the enhanced carrier mobility by Se substitution. We have observed that valence band structure is altered by Se substitution with the band gap decreasing by about half, which agrees qualitatively with first principle band structure calculations. According to band structure calculations, the top of valence band mainly consists of in-plane chalcogen p-orbitals which is strongly affected by partial substitution of chalcogen atoms. Calculated conduction band does not show any substantial change with Se substitution, indicating that the large enhancement of carrier mobility below room temperature may not be accounted for the change in the electronic structure. On the other hand, x-ray absorption fine structure shows marked change by substitution, indicating that the local distortion of in-plane Bi-Chalcogen plane in LaOBiSSe compound is drastically suppressed with respect to LaOBiS2. It has been suggested that this suppression of the local distortion might be playing a large contribution in enhanced thermoelectric property of this system.

II METHODS

Polycrystalline LaOBiS2 and LaOBiSSe samples were prepared by solid-state-reaction methodMizuguchi1st ; JAP1 . Same procedure was used for preparing the two samples with the same person as the former reports of crystal structure and thermoelectric propertiesNishidaJPSJ ; JAP2 . Temperature dependence of electrical resistivity was measured by the four-probe methods.

Photoelectron spectroscopy measurements on pelletized samples were performed using Scienta-Omicron R4000 analyzer constructed in Okayama University. Xe I$\alpha$ (h$\nu$ = 8.437 eV), He I$\alpha$ (21.218 eV), and He II$\alpha$ (40.814 eV) lines were used to excite photoelectrons. The energy resolution was set to 10 meV for Xe I and He I and 50 meV for He II, and the spectra were taken by the transmission mode in an ultrahigh vacuum of  8$\times$10*-9* Pa. The Fermi energy ($E_{F}$) of samples were calibrated by that of gold, electronically contacted with samples. During the measurements, we found no signature of charging. Clean surfaces for the measurements were obtained by in situ scraping of samples at  2$\times$10*-8* Pa.

Total diffuse reflectance spectra (R) were measured by a spectrometer with an integrating sphere (Hitachi High-Tech, U-4100). Al2O3 powders were used as the standard reference. Optical absorption coefficient ($\alpha$) was determined from the $R$ value via the Kubelka–Munk equation, $(1-R)^{2}/2R$ = $\alpha$/$s$, where $s$ is the scattering factor.

Bi L3-edge ($E$=13418 eV) x-ray absorption measurements were used to probe the local structure of LaOBiS2 and LaOBiSSe. The measurements were performed at the beamline BM26A of the European Synchrotron Radiation Facility (ESRF) where the synchrotron light was monochromatized using a double crystal Si(111) monochromator. The measurements were made sequentially at low temperature ($T$ = 30 K) in transmission mode. The finely powdered samples of LaOBiS2 and LaOBiSSe were diluted uniformly in boron nitride matrix and pressed into pellets of 13 mm diameter, for obtaining the edge jump to be about 1. The EXAFS oscillations were extracted by the standard procedure based on the cubic spline fit to the pre-edge subtracted absorption spectrum Bunker .

First principles calculations were performed using WIEN2k code WIEN2k where spin-orbit coupling was included. The lattice parameters were taken from x-ray diffraction measurements on LaOBiS2 and LaOBiSSe reported earlier JAP2 . The modified Becke-Johnson (mBJ) potential proposed by Tran and Blaha mBJ1 ; mBJ2 was used as exchange correlation functional, as it has been discussed to be appropriate for BiS2 system Ochi . For simplicity, we assumed the tetragonal crystal structure mono despite a symmetry reduction in both compounds Athauda ; Sagayama ; Nagasaka . 24$\times$24$\times$7 $k$-mesh was used with the $RK_{max}$ parameter being 7.

III RESULTS AND DISCUSSION

Firstly, we focus on the effect of Se substitution on the electronic structure. Figure 2 shows photoemission spectra of LaOBiS2 and LaOBiSSe measured using several ultraviolet photon energies at $T$ = 300 K. Spectra in Figs. 2(a) and 2(b) are normalized by total intensity over the shown area, while those in 2(c) are normalized by the peak height. The overall width of the valence band and the energy position of the top of the valence band in LaOBiS2 is consistent with our earlier soft x-ray photoemission work Nagira . Through S substitution with Se, we have observed that the top of the valence band becomes closer to $E_{F}$ at all the photon energies. Inset of Fig. 2(c) shows the near-EF spectra divided by Fermi-Dirac function of $T$ = 300 K convoluted by experimental energy resolution. We have found that the EFs of both sample are located at the bottom of the conduction band, that would be consistent with the sign of the Seebeck coefficient. Therefore the magnitude of the band gap can be evaluated for both samples, which is roughly 1.1 and 0.6 $\pm$ 0.2 eV for LaOBiS2 and LaOBiSSe respectively. The reduction of the band gap due to Se-substitution is also verified by ($\alpha$h$\nu$/s)2 measurements MiuraOptical on LaOBiS2 and LaOBiSSe shown in Figs. 2(d) and (e), where direct-transition-type optical absorption was observed for 1.0 $\pm$ 0.1 eV for LaOBiS2 and 0.5 $\pm$ 0.1 eV for LaOBiSSe.

In order to discuss the origin of the observed change in the electronic structure, we have performed band structure calculations including spin-orbit interaction. Upper panels of Figs. 3(a) and (b) show calculated partial density of states (pDOSs) for LaOBiS2 (a) and LaOBiSSe (b), where Sin denotes in-plane S atom and Sout is out-of-plane S atom. Following earlier experimental and theoretical works JAP2 ; GWang , we have assumed that Se atoms occupy only in-plane S sites. Bottom of Figs. 3(a) and (b) are experimental spectra (red lines), overlapped with simulated photoemission spectra (blue dashed lines) where each pDOS was multiplied by photoionization cross section Goldberg ; Lindau and extended 1.3 times in energy for comparison. For calculated results, the position of $E_{F}$ was set at the bottom of the conduction band, in order to see the correspondence with the photoemission results in Fig. 2. The simulated spectra successfully captures the characteristics of Chalcogen-substitution induced change in the valence band. Namely, (i) the peak structure $\sim$2.5 eV in LaOBiS2 splits into $\sim$2.5 eV and $\sim$2 eV in LaOBiSSe (ii) the top of the valence band becomes closer to $E_{F}$ and the band gap decreases by Se-substituion. Thus it can be said that the first principles calculations would give reliable prediction in these materials.

According to the calculations shown in Figs. 3(a) and (b), the changes in the experimental spectra are mainly attributed to the pDOS of in-plane chalcogen atom. As shown in Fig. 3(c), it is expected that the top of the valence band is largely affected by the substitution of S with Se while the bottom of conduction band is less affected although the dispersion becomes slightly steeper in LaOBiSSe (Estimated effective masses (m∗) for the bottom of the conduction bands up to 0.1 eV are 0.16 and 0.11 for LaOBiS2 and LaOBiSSe, respectively). Such a change in the band structure would contribute thermoelectric property especially at high temperature, whereas it would not fully explain the reduction of the resistivity in Fig. 1 since the chemical potential seems to be pinned at the bottom of the conduction band and the population of Fermi-Dirac function at binding energy of 0.5-1.0 eV hardly changes below room temperature (see inset of Fig. 2(c)). Therefore, chalcogen substitution certainly modifies the band structure but the change would not fully account for the improvement of thermoelectric property. Next we focus on the Se-substitution effect on the local structure.

Figure 4 shows Fourier transforms (FT) of $k$2-weighted EXAFS oscillations extracted from the Bi L3-edge x-ray absorption spectra measured on LaOBiS2 and LaOBiSSe samples at $T$ = 30 K. The inset includes EXAFS oscillations ($k^{2}$-weighted) as a function of photoelectron wave vector $k$. The FTs are performed using a gaussian window with the $k$ range being 3.5-15 Å*-1* and corrected for the phase shifts. The EXAFS oscillations and the FT peak structures are largely different revealing distinct local structure of the two compounds.

In LaOBiS2 structure, the Bi atom is coordinated with one sulphur atom in axial direction (Sout), four in-plane sulphur atoms (Sin), and the Sin atom of the adjacent BiS2 layer. Information on all these Bi-S distances is contained in the main peak structure between 1.5 and 3.5 Å in the FTs of Bi L3-edge EXAFS. The contributions of distant atoms are mixed with multiple scatterings and visible beyond the first peak structure. To determine the local structure parameters, the EXAFS oscillations were modeled by the general equation Bunker based on single-scattering approximation. Following earlier EXAFS study and theoretical calculations paris ; paris2 ; poly1 , we used two different in-plane Bi-Sin distances for LaOBiS2. The model contains four shells including one out-of-plane Chout atom (Bi-Chout distance), four in-plane Chin atoms at two different distances (two Bi-Chin distances) and one Bi-Chin distance between the two BiCh2-layers. For LaOBiSSe, considering the fact that Se preferentially occupies the Sin site JAP2 , Se occupation at Sout site was neglected. We have fixed the passive electrons reduction factor, $S_{0}^{2}$ = 0.95 for the analysis of the Bi L3-edge EXAFS, while the photoelectron energy zero, $E_{0}$ was set to zero after analysing different scans. The only parameters refined here are the local bondlengths and the associated $\sigma^{2}_{i}$, measuring the mean square relative displacement (MSRD) of the considered bond distances. The FEFF8 code Feff was used for the calculation of scattering amplitudes and the WINXAS package winxas was used for the EXAFS model fits. The fit $k$-range was 3.5-15 Å*-1* while $R$-range was 1.5-3.5 Å. Therefore, the number of independent points 2$\Delta k\Delta R/\pi$ was $\sim$14 for the model fits in which the fit parameters were 8 to 10. The model fits are included in Fig. 4 as solid lines.

The local structure parameters obtained for two compounds are summarized in table 1. The results on LaOBiS2 are consistent with earlier EXAFS work on the local structure of F-doped systemparis and PDF analysis Athauda showing Bi-Sin bondlength splitting. This confirms that BiS2 layer is largely distorted in LaOBiS2. On the other hand, LaOBiSSe contains two Bi-Sein distances separated by $\sim$0.2 Å. From table 1, it is apparent that local distortion in LaOBiSSe is substantially lower with respect to that in LaOBiS2. Therefore, it is lilkely that structural instability in LaOBiS2, characterized by polytypism, is partially suppressed in LaOBiSSe. Apart from other factors like microstructure, we think this could be the main origin of the change in the resistivity and hence the thermoelectric properties through chalcogen substitution, especially at lower temperature range.

The present EXAFS and earlier experimental works Athauda ; Sagayama ; paris indicate that Bi atoms in LaOBiS2 are in off-center position in Sin square lattice. Such a lowering of the structural symmetry has been intensively studied in post-transition metal oxides and chalcogenides Walsh , where it turned out that the lone pair becomes active when the energy level of chalcogen $p$-orbitals are close enough to that of $s$-orbital in post-transition metal. If the hybridization between those two are strong enough, the post-transition metal atoms choose to take off-center position, forming $s$-$p$ hybridization to obtain energy gain in the electronic system. We think this idea can be also applicable in the current BiCh2 system, as is the case of Bi2O3, Bi2S3, and Bi2Se3 Walsh2 . Indeed, the overall energy position of calculated Se 4$p$ pDOS is higher compared with that of S 3$p$ pDOS in Figs. 3(a) and (b). Therefore, although detailed bond analysisWalsh2 on this series of material should be important to conclude, the manipulation of the lone pair activity by chalcogen subsituion could reduce the local distortion in the conducting layer and recover the conductivity. In the parent compound LaOBiS2, the contribution of electrons to thermal conductivity ($\kappa_{e}$) is small compared with that of phonons ($\kappa_{ph}$) JAP1 . The drastic decrease of resistivity increases $\kappa_{e}$ but it seems to be cancelled by the decrease of $\kappa_{ph}$ with help of possible rattling motion of Bi Lee . As a result, the $ZT$ value would have benefitted from chalcogen substitution. Such a control of the local structural distortion in the compounds with the lone pair (such as Bi, Pb, Sn) can be benefical for further improvement of thermoelectric property.

IV CONCLUSION

In summary, we have studied Se-substitution effect on the electronic- and the local structure of LaOBiCh2 system. It turned out that the top of the valence band is significantly influenced by the substitution where pDOS of in-plane chalcogen atom is dominant. We have also observed that the local structural distortion in the Bi-chalcogen plane is suppressed by Se-substitution, which is in accordance with the lone pair activity of the system. It is suggested that controlling the local structural distortion can help enhancing the functionality of thermoelectric materials.

Acknowledgements.

We thank ESRF staff for support in the EXAFS data collection. The authors would like to thank S. Onari for the use of workstation. K. T. and T. W. would like to acknowledge the hospitality at the Sapienza University of Rome. This research was partially supported by the Program for Promoting the Enhancement of Research University from MEXT, the Program for Advancing Strategic International Networks to Accelerate the Circulation of Talented Researchers from JSPS (R2705), and JSPS KAKENHI (Nos. 15H03691, 16H04493). This work is a part of the executive protocol of the general agreement for cooperation between the Sapienza Unversity of Rome and Okayama University, Japan.