Synthesis and thermoelectric properties of Rashba semiconductor BiTeBr with intensive texture
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Bismuth tellurohalides with Rashba-type spin splitting exhibit unique Fermi surface topology and are developed as promising thermoelectric materials. However, BiTeBr, which belongs to this class of materials, is rarely investigated in terms of the thermoelectric transport properties. In the study, polycrystalline bulk BiTeBr with intensive texture was synthesized via spark plasma sintering (SPS). Additionally, its thermoelectric properties above room temperature were investigated along both the in-plane and out-plane directions, and they exhibit strong anisotropy. Low sound velocity along two directions is found and contributes to its low lattice thermal conductivity. Polycrystalline BiTeBr exhibits relatively good thermoelectric performance along the in-plane direction, with a maximum dimensionless figure of merit (ZT) of 0.35 at 560 K. Further enhancements of ZT are expected by utilizing systematic optimization strategies.
KeywordsBismuth tellurohalides BiTeBr Thermoelectric properties Texture
Solid-state thermoelectric (TE) materials enable the direct conversion of waste heat into electric power and provide a possible solution for increased energy demands [1, 2, 3, 4, 5, 6]. The efficiency of a TE material is generally gauged by its dimensionless figure of merit, ZT = α2σT/(κe + κL), where α denotes the Seebeck coefficient, σ denotes electrical conductivity, κe and κL denote the electronic and lattice contributions to the total thermal conductivity (κ), respectively, and T denotes the absolute temperature . Extant studies aim to obtain higher ZT and mainly focus on two aspects. The first aspect involves engineering the electronic structure to enhance the power factor (PF = α2σ) [7, 8], i.e., by increasing band degeneracy [9, 10], inducing resonant levels  and reducing band effective mass [12, 13]. The other aspect involves obtaining lower κL  by introducing multiscale phonon scattering centers [15, 16, 17] or pursuing new materials with intrinsically low κL [18, 19, 20].
Layered ternary bismuth tellurohalides (BiTeX with X = I, Br, Cl) with giant Rashba-type spin splitting have recently received widespread interest as future spintronic applications and have been explored as topological superconductors [21, 22]. Given the spin–orbital coupling and inversion asymmetry, BiTeX exhibits unique Fermi surface topology, reduced dimensionality for the electronic density of state, and thus unusual relativistic physical properties [23, 24]. A recent study indicated that unique Fermi surface and complex non-parabolic band structures are favorable for high TE performance . Thus, semiconductor BiTeX with Rashba-type spin splitting displays promising TE transport properties.
With respect to the BiTeX system, BiTeI attracted maximum attention in terms of the TE transport investigation, and this is partly due to the strongest Rashba-type spin splitting. Wu et al.  found a two-dimensional thermopower in BiTeI due to the spin-splitting-induced constant density of states, and this exceeds that in spin-degenerate bands. With increase in atomic number of the halogen element, the lattice thermal conductivity of BiTeX decreases due to the higher average atomic mass (κL is approximately 1 W·m−1·K−1 at RT) [27, 28]. Nevertheless, with respect to the aforementioned three types, BiTeI is significantly affected by intrinsic point defects . The electron concentration of single-crystalline BiTeI (4.6 × 1019 cm−3) significantly exceeds the optimized value (approximately 5 × 1018 cm−3)  as estimated from the single parabolic band model [8, 30]. Additionally, additional defect scattering leads to relatively small carrier mobility, and thereby a low power factor (approximately 5 μW·cm−1·K−2) [27, 29]. Wu et al.  indicated that Cu-intercalation in BiTeI substantially alters the equilibria of defect reactions and leads to increases in carrier mobility and consequently an enhanced power factor. Furthermore, the alloying of Br broadens the band gap of BiTeI and this leads to diminished thermally activated minority carriers and improved ZT at high temperatures . In a single-crystalline form, BiTeCl was grown by using a topotactic method and its TE transport properties below room temperature were investigated. A maximum power factor corresponding to 18 μW·cm−1·K−2 and ZT of 0.17 was reported . However, the TE properties of BiTeCl were observed as deteriorating with respect to time, which indicates that the system is not stable . The TE properties of BiTeI and BiTeBr single crystals were investigated below room temperature in which BiTeBr exhibits a value of ZT that is almost twice that of BiTeI .
Thus, BiTeX systems are still not fully examined for their TE properties, especially in polycrystalline form and above room temperature. Additionally, the inter-layer interaction for BiTeX is due to the van der Waals force. Thus, strong anisotropy should exist for their transport properties, and this requires further investigation . When compared to BiTeI, BiTeBr exhibits a higher band gap, and this acts to suppress the thermal activation of minority carrier. In the study, polycrystalline BiTeBr with intensive texture was synthesized by spark plasma sintering (SPS). The anisotropic TE properties along in-plane and out-plane directions were investigated. The results indicate that the in-plane direction of polycrystalline BiTeBr exhibits relatively good thermoelectric performance with a maximum dimensionless figure of merit ZT of approximately 0.35 at 560 K.
2 Experimental and theoretical methods
In the first set of experiments, BiTeBr1−xCl x (0 ≤ x ≤ 1) were synthesized and their TE transport properties were examined. The results indicate that the specimens with high Cl content are sensitive to moisture and cannot be kept in air for a long period. A similar phenomenon was also reported in a previous study on BiTeCl single crystal . Therefore, in the following study, we only focused on investigating the TE transport properties of more stable BiTeBr and BiTeBr0.75Cl0.25. In the typical synthesis of polycrystalline specimens with nominal composition BiTeBr and BiTeBr0.75Cl0.25, stoichiometric amounts of elemental Bi (piece, 99.999%), Te (piece, 99.99%), BiBr3 (powder, 99.9%) and BiCl3 (powder, 99.8%) were weighed and loaded in quartz tubes in a glove box. The quartz tubes were sealed under partial Ar pressure and then placed into a furnace. The quartz tubes were first heated to 600 °C, kept for 10 h, then cooled down to 400 °C and kept for 7 days. The obtained ingots were manually crushed and then placed into graphite dies with an inner diameter of 8 mm. The dies were placed into a SPS instrument (Fuji, Japan) and compacted at 340 °C for 4 min under 80 MPa in vacuum. Finally, bulk samples with a diameter of 8 mm and a thickness of approximately 11 mm were obtained.
Lattice parameter, orientation factor, Hall carrier concentration, Hall mobility (at 300 K), sound velocity and Debye temperature for sintered BiTeBr and BiTeBr0.75Cl0.25
Orientation factor (F)
Hall carrier concentration (nH)/1019 cm−3
Hall mobility, ab (µH)/(cm2·V−1·s−1)
Sound velocity, ab/(m·s−1)
Sound velocity, c/(m·s−1)
θ D, ab
θ D, c
Density functional theory calculations (DFT) were performed by using the Vienna Ab-initio Simulation Package (VASP)  to investigate the electronic properties. The interactions between the valence electrons and ion cores were described by using the projector-augmented wave method [33, 34]. The exchange and correlation energy were formulated by using the generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof scheme . The plane-wave basis cutoff energy was set as 216 eV by default. The Γ-centered k points with 0.3 nm−1 spacing were used for the first Brillouin-zone sampling. The spin-orbit coupling (SOC) was included in the calculation.
3 Results and discussion
3.1 Microstructure and thermal stability
3.2 Calculated band structure
3.3 Electrical properties
The electrical conductivity along the ab direction is more than twice that in the c direction and indicates strong anisotropy for the sintered specimens. The Seebeck coefficient for these two directions differs slightly and is similar to the case for Bi2Te3 . Additionally, the alloying of Cl in BiTeBr reduces the electrical conductivity. This is due to the enhanced alloying scattering. As seen in Table 1, the Hall electron mobility of BiTeBr0.75Cl0.25 is only half of the value of BiTeBr though their electron concentrations are very close.
The power factor of all the specimens is shown in Fig. 4c. The BiTeBr specimen exhibits a maximum PF (approximately 8 μW·cm−1·K−2) along ab direction, and this exceeds that of state-of-art Cu-intercalated BiTeI . This indicates that BiTeBr exhibits better electrical properties compared with BiTeI, although the Rashba energy of the former is lower . This is only corresponding to the result for the pristine BiTeBr, and thus further enhancements in its power factor might be realized through carrier optimization or band engineering.
3.4 Thermal conductivity and ZT
The dependence of ZT as a function of temperature is shown in Fig. 5c. The ZT along ab direction is evidently exceeds that along c direction due to the higher electrical conductivity. The maximum figure of merit (ZT) of 0.35 is obtained for BiTeBr specimen at 560 K. It is possible to reach a higher peak ZT if the measurement temperature is increased further. However, given the possible thermal instability, this does not extend to higher temperatures. Although Cl alloying in BiTeBr lowers the thermal conductivity, it results in a deterioration in the electrical properties and does not contribute to improving the TE performance. To further enhance ZT of BiTeBr, reducing the carrier concentration to further enhance the power factor is a promising way. Furthermore, isoelectronic alloying by using Sb could be another effective way to suppress the lattice thermal conductivity and thus improve the TE performance.
In summary, polycrystalline BiTeBr-based bulk materials with intensive texture were successfully synthesized by using SPS and their thermoelectric properties above temperature were reported. Intensive texture results in anisotropic electrical and thermal transport properties. Overall, the ZT value along the in-plane direction exceeds that along the out-plane direction due to higher electrical conductivity. Low sound velocity along two directions is found in polycrystalline BiTeBr and contributes to its low lattice thermal conductivity. A maximum figure of merit (ZT) of 0.35 is obtained for BiTeBr specimen at 560 K. An increase in TE performance is expected for the new material system by further optimizing the transport properties.
Open access funding provided by Max Planck Society. This study was financially supported by the European Research Council (ERC Advanced Grant No. 291472 “Idea Heusler”), the National Science Fund for Distinguished Young Scholars (No. 51725102) and the National Natural Science Foundation of China (No. 61534001). The authors thank Horst Borrmann, Steffen Hückmann and Yurii Prots for assistance in the powder XRD measurements, Susann Scharsach and Marcus Schmidt for the DTA/TG analysis, Igor Veremchuk for assistance in using the SPS instrument and transport measurements. C. Fu acknowledges financial support from the Alexander von Humboldt Foundation.
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