Layered XZnBi (X = Rb, Cs) with Pudding-Mold Bands, Complex Fermi Surfaces and Low Thermal Conductivity: A First-Principles Study of Thermoelectric Properties

TL;DR

This study uses first-principles calculations to explore how stacking order affects thermoelectric properties of layered XZnBi compounds, revealing their potential as high-performance thermoelectric materials with low thermal conductivity and pudding-mold band structures.

Contribution

It provides a detailed analysis of stacking order effects on thermoelectric performance of XZnBi compounds using first-principles methods and Boltzmann equations, highlighting the importance of electron-phonon interactions.

Findings

Low thermal conductivity below 2 W/m/K.

Maximum ZT of 2.42 at 900 K with CRTA.

Stacking order significantly influences thermoelectric efficiency.

Abstract

Layered Zintl compounds exhibit significant tunability of thermoelectric (TE) parameters facilitated by their multiple elemental combinations and flexibility in stacking order within the layers. In this work, the effect of stacking order on TE properties of theoretically predicted layered Zintl compounds XZnBi (X = Rb, Cs) is studied using 1st-principles calculations and Boltzmann equations. The materials are semiconductors having moderate band gaps ranging from 0.44 to 0.52 eV. There exist six identical hole pockets for valence band maxima due to the crystal symmetry. This leads to high band degeneracy but simultaneously promotes intervalley scatterings. While as for conduction bands, the Fermi surface consists of a single but highly anisotropic, quasi-two dimensional electron pockets with cylindrical shape along z-axis. This kind of Fermi surface is a characteristic of a pudding mold band shape. It facilitates a unique combination of heavy and light electron masses, simultaneously optimizing Seebeck coefficient and electrical conductivity. At first, electronic transport coefficients are calculated using constant relaxation time approximation (CRTA) or electron-phonon coupling matrix elements (el-ph). The calculated relaxation times are then integrated with transport results to get the realistic values of TE parameters. The analysis of three-phonon scattering reveals low thermal conductivity ($k_{l}$) below 2 W/m/K in these compounds. The $k_{l}$ also depends on stacking order with the values of nearly half in AB stacking as compared to that of AA stacking. These combined factors lead to a high ZT at 900 K, reaching to a maximum of 2.42 using CRTA and 0.52 when el-ph are included. The study highlights the potential of XZnBi systems as promising TE materials as well as the critical roles of stacking and el-ph in accurately evaluating TE properties.