[Paper Review] Electronic Structure and Thermoelectric Properties of Half-Heusler Alloys NiTZ

Dhurba Raj Jaishi, Nileema Sharma+4 more|arXiv (Cornell University)|Sep 24, 2020

Advanced Thermoelectric Materials and DevicesMaterials Science64 references54 citations

TL;DR

This study investigates the electronic and thermoelectric properties of 18-valence electron half-Heusler alloys NiTZ (T = Sc, Ti; Z = P, As, Sn, Sb) using density functional theory and Boltzmann transport calculations. The alloys exhibit indirect band gaps and moderate ZT values, with NiScP achieving a ZT of 0.46 at 1200 K, indicating potential for thermoelectric applications through optimized doping.

ABSTRACT

We have investigated the electronic and thermoelectric properties of half-Heusler alloys NiTZ (T = Sc, and Ti; Z = P, As, Sn, and Sb) having 18 valence electron. Calculations are performed by means of density functional theory and Boltzmann transport equation with constant relaxation time approximation, validated by NiTiSn. The chosen half-Heuslers are found to be an indirect band gap semiconductor, and the lattice thermal conductivity is comparable with the state-of-the-art thermoelectric materials. The estimated power factor for NiScP, NiScAs, and NiScSb reveals that their thermoelectric performance can be enhanced by appropriate doping rate. The value of ZT found for NiScP, NiScAs, and NiScSb are 0.46, 0.35, and 0.29, respectively at 1200 K.

Motivation & Objective

• To explore the electronic and thermoelectric properties of under-investigated Ni-based half-Heusler alloys with 18 valence electrons.
• To evaluate the thermoelectric performance of NiTZ (T = Sc, Ti; Z = P, As, Sn, Sb) for potential application in energy conversion devices.
• To validate computational methods using experimentally known NiTiSn as a benchmark.
• To assess the potential for enhancing power factor and ZT through carrier concentration tuning and doping.

Proposed method

• Employed full-potential linearized augmented plane wave (FP-LAPW) method within WIEN2k for electronic structure calculations.
• Used generalized gradient approximation (GGA) with PBE functional and mBJ potential to improve band gap accuracy.
• Applied BoltzTraP code to compute transport properties using the semi-classical Boltzmann transport equation with constant relaxation time approximation.
• Calculated lattice thermal conductivity via linearized Boltzmann transport equation in single-mode relaxation time approximation using thermal2 in Quantum Espresso.
• Validated results against experimental data from NiTiSn and performed phonon dispersion calculations for dynamical stability.
• Used 10×10×10 and 50×50×50 k-meshes for electronic and transport property calculations, respectively.

Experimental results

Research questions

• RQ1 Do NiTZ half-Heusler alloys with 18 valence electrons exhibit suitable electronic band structures for thermoelectric applications?
• RQ2 What is the predicted thermoelectric performance (ZT and power factor) of NiScP, NiScAs, and NiScSb at high temperatures?
• RQ3 How does the lattice thermal conductivity of these alloys compare to state-of-the-art thermoelectric materials?
• RQ4 Can appropriate doping at the Z-site enhance the power factor and overall ZT in these materials?
• RQ5 Are these alloys dynamically stable, as confirmed by phonon dispersion calculations?

Key findings

• All studied NiTZ alloys (NiScP, NiScAs, NiScSb, NiTiSn) exhibit indirect band gaps, with values ranging from 0.28 eV to 0.62 eV depending on the composition.
• Phonon dispersion calculations confirm dynamical stability, as no imaginary frequencies are present in the full Brillouin zone.
• The power factor for NiScP, NiScAs, and NiScSb is predicted to be improvable via optimal doping, indicating tunable electronic transport.
• The ZT value reaches 0.46 for NiScP at 1200 K, followed by 0.35 for NiScAs and 0.29 for NiScSb, indicating moderate thermoelectric performance.
• Lattice thermal conductivity is comparable to that of state-of-the-art thermoelectric materials, supporting their potential for efficient energy conversion.
• The electronic structure is dominated by 3d-orbitals of Ni and Sc near the Fermi level, with minimal contribution from Z-site atoms, indicating strong hybridization in the valence band.