Thermoelectric transport and the role of different scattering processes in the half-Heusler NbFeSb

TL;DR

This study uses advanced ab initio methods to analyze thermoelectric transport in NbFeSb, revealing the significant roles of polar optical phonons and impurity scattering, and introduces a computationally efficient approach applicable to various materials.

Contribution

It presents a novel, efficient ab initio computational method for thermoelectric properties that accounts for multiple scattering processes with high accuracy.

Findings

Polar optical phonon and impurity scattering significantly affect transport.

The method achieves over 10x reduction in computational cost.

Computed power-factor aligns well with experimental data.

Abstract

We perform an ab initio computational investigation of the electronic and thermoelectric transport properties of one of the best performance half-Heusler (HH) alloys, NbFeSb. We use Boltzmann Transport equation while taking into account the full energy/momentum/band dependence of all relevant electronic scattering rates, i.e. with acoustic phonons, non-polar optical phonons (intra- and inter-valley), polar optical phonons (POP), and ionized impurity scattering (IIS). We use a highly efficient and accurate computational approach, where the scattering rates are derived using only a few ab initio extracted matrix elements, while we account fully for intra-/inter valley/band transitions, screening from both electrons and holes, and bipolar transport effects. Our computed thermoelectric power-factor (PF) values show good agreement with experiments across densities and temperatures, while they indicate the upper limit of PF performance for this material. We show that the polar optical phonon and ionized impurity scattering (importantly including screening), influence significantly the transport properties, whereas the computationally expensive non-polar phonon scattering part (acoustic and non-polar optical) is somewhat weaker, especially for electrons, and at lower to intermediate temperatures. This insight is relevant in the study of half-Heusler and other polar thermoelectric materials in general. Although we use NbFeSb as an example, the method we employ is material agnostic and can be broadly applied efficiently for electronic and thermoelectric materials in general, with more than 10x reduction in computational cost compared to fully ab initio methods, while retaining ab-initio accuracy.