Electrical and Thermal Transport of Layered Bismuth-Sulfide EuBiS₂F at Temperatures between 300 and 623 K

Abstract

We demonstrate the electrical and thermal transport of the layered bismuth-based sulfide EuBiS2F from 300 to 623 K. Although significant hybridization between Eu 4f and Bi 6p electrons was reported previously, the carrier transport of the compound is similar to that of F-doped LaBiS2O, at least above 300 K. The lattice thermal conductivity is lower than that of isostructural SrBiS2F, which is attributed to the heavier atomic mass of Eu ions than that of Sr ions.Searching for novel thermoelectric materials is the most fundamental issue for the development of thermoelectrics because the maximum conversion efficiency of a thermoelectric device is primarily determined by the material's dimensionless figure of merit, ZT = S 2 T −1  −1 , where T, S, , and  denote the temperature, Seebeck coefficient, electrical resistivity, and thermal conductivity, respectively. 1) Bismuth telluride (Bi2Te3) and its related compounds have been the state-of-the-art thermoelectric materials at around room temperature since the 1950s with a ZT value as high as 1.4 in a nanocrystalline bulk specimen. 2) Compared with tellurides and selenides, little attention has been paid to the thermoelectric properties of bismuth-based sulfides because of their low ZT value. However, recent studies on sulfide systems have clearly shown that sulfides also exhibit attractive thermoelectric properties. 3) Aside from thermoelectricity, the superconductivity of layered bismuth sulfides, such as Bi4S3O4 4) and LaBiS2O1xFx 5) , has led to novel studies on superconductivity (the chemical formula is written according to standard nomenclature 6) ). The crystallographic structures of these compounds are basically composed of alternate stacks of BiS2 and oxide/fluoride layers, as shown in the inset of Fig. 1. The conduction band near the Fermi level is primarily composed of in-plane Bi 6p orbitals. The thermoelectric properties of LaBiS2O are suppressed by F substitution, 7) whereas they are improved by Se substitution. 8) Recently, EuBiS2F has been reported as a possible compound that exhibits charge-density-wave-like order below 280 K and superconductivity below 0.3 K without element substitution. 9) Specific heat measurement showed significant hybridization between Eu 4f and Bi 6p electrons, while density functional theory (DFT) calculation indicates that the valence band maximum consists of Eu 4f orbitals. 9) Given the results shown in the previous study, it was controversial from the viewpoint of thermoelectricity, whether Eu 4f and Bi 6p hybridized orbitals are effective in modulating the Fermi surface and enhancing S, 10) or reducing S owing to the coexistence of electrons and holes, i.e., mixed conduction. In this study, we demonstrate the electrical and thermal transport of EuBiS2F at temperatures between 300 and 623 K.Polycrystalline EuBiS2F was prepared by a solid-state reaction using a sealed silica tube, following the method reported by Zhai et al. 9) The relative density of the sample was calculated to be 90%. The sample purity was examined by ...

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