Layered XZnBi (X = Rb, Cs) with Pudding-Mold Bands, Complex Fermi Surfaces and Low Thermal Conductivity: A First-Principles Study of Thermoelectric Properties
Authors
Aadil Fayaz Wani, Nirma Kumari, SuDong Park, Byungki Ryu
Categories
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.
Layered XZnBi (X = Rb, Cs) with Pudding-Mold Bands, Complex Fermi Surfaces and Low Thermal Conductivity: A First-Principles Study of Thermoelectric Properties
Categories
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.
Authors
Aadil Fayaz Wani, Nirma Kumari, SuDong Park et al. (+1 more)
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