Ternary selenides A2Sb4Se8 (A = K, Rb and Cs) as an n-type thermoelectric material with high power factor and low lattice thermal conductivity: importance of the conformationally flexible Sb-Se-Se-Sb bridges.

Changhoon Lee,Won-Joon Son,Ji-Hoon Shim,Myung-Hwan Whangbo,Sujee Kim

doi:10.1039/d0ra01751e

Abstract

We investigated the thermoelectric properties of the layered ternary selenides A2Sb4Se8 (A = K, Rb and Cs) and the lattice thermal conductivity of K2Sb4Se8 on the basis of DFT calculations, to find that these selenides are a high-performance n-type thermoelectric material. The Seebeck coefficients and power factors calculated for the electron carriers of A2Sb4Se8 (A = K, Rb and Cs) are greater than those of the well-known thermoelectric materials Bi2Te3 and PbTe. The lattice thermal conductivity κlatt of K2Sb4Se8 is comparable to that of PbTe, well-known for its low lattice thermal conductivity. In terms of both electronic and phonon structures, the structural parts of the A2Sb4Se8 (A = K, Rb and Cs) phases crucial for their thermoelectric properties are the conformationally-flexible Sb–Se–Se–Sb bridges that interlink between their structurally rigid units.

Highlights

Thermoelectricity enables a direct conversion between thermal and electrical energies, so materials exhibiting thermoelectricity have received much attention for power generation and cooling.[1,2] In particular, thermoelectric power generation is potentially important for waste heat collection and efficient energy utilization, its application is limited due to the low efficiency of thermoelectric devices
We examine the Seebeck coefficients and power factors of A2Sb4Se8 (A 1⁄4 K, Rb, Cs) and the lattice thermal conductivity of K2Sb4Se8 on the basis of DFT calculations, to predict that the ternary selenides A2Sb4Se8 (A 1⁄4 K, Rb, Cs) are a high-performance thermoelectric material comparable in efficiency to, or better than, the wellknown thermoelectric materials Bi2Te3 and PbTe
Since the conduction band minimum (CBM) has more valleys than does the valence band maximum (VBM), the carrier concentration should be enhanced near the CBM,[24] so that the electrical conductivity is higher for the electrons than for the holes

Summary

Introduction

Thermoelectricity enables a direct conversion between thermal and electrical energies, so materials exhibiting thermoelectricity have received much attention for power generation and cooling.[1,2] In particular, thermoelectric power generation is potentially important for waste heat collection and efficient energy utilization, its application is limited due to the low efficiency of thermoelectric devices. The K2Sb4Se8 crystal should be least rigid along the interlayer direction because there is no covalent bonding between adjacent Sb4Se8 layers In this low-dimensional structure of K2Sb4Se8 as determined by Xray diffraction, the unit cell parameters are large so that the lattice thermal conductivity would be low with short mean path for acoustic phonons.[12] it is worthwhile investigate the possibility that K2Sb4Se8 as well as its two isostructural selenides, Rb2Sb4Se8 and Cs2Sb4Se8, are a high-performance thermoelectric material. Since the CBM has more valleys than does the VBM, the carrier concentration should be enhanced near the CBM,[24] so that the electrical conductivity is higher for the electrons than for the holes This explains why the n-type power factor is considerably higher than the p-type power factor despite that the calculated Seebeck coefficients for the hole and electron carriers are similar. The lattice thermal conductivities of Rb2Sb4Se8 and Cs2Sb4Se8 should be similar to that of K2Sb4Se8 because Rb2Sb4Se8 and Cs2Sb4Se8 are very similar to K2Sb4Se8 in atomic and electronic structures

Discussion

Concluding remarks