Probing local distortion around structural defects in half-Heusler thermoelectric NiZrSn alloy

Hidetoshi Miyazaki,Kosuke Watanabe,Selen Gunaydin,Osman Murat Ozkendir,Yoichi Nishino,Kazuo Soda

doi:10.1038/s41598-020-76554-9

Abstract

The half-Heusler NiZrSn (NZS) alloy is particularly interesting owing to its excellent thermoelectric properties, mechanical strength, and oxidation resistance. However, the experimentally investigated thermal conductivity of half-Heusler NZS alloys shows discrepancies when compared to the theoretical predictions. This study investigates the crystal structure around atomic defects by comparing experimental and theoretical X-ray absorption fine structure (XAFS) spectra of the crystal structure of a half-Heusler NZS alloy. The results of both Zr and Ni K-edge XAFS spectra verified the existence of atomic defects at the vacancy sites distorting the C1b-type crystal structure. We concluded that the distortion of the atoms around the interstitial Ni disorder could be the probable reason for the observed lower thermal conductivity values compared to that predicted theoretically in half-Heusler alloys. Our study makes a significant contribution to the literature because the detailed investigation of the lattice distortion around atomic defects will pave the way to further reduce the thermal conductivity by controlling this distortion.

Highlights

In the present scenario, significant reduction or ideally complete elimination of the heat losses—within mechanical and electrical devices, in general—are among the most important factors that are imperative for technological advancement
The composition of the NZS alloy used in the X-ray absorption fine structure spectroscopy (XAFS) measurements (Ni34Zr33Sn33) and the composition used in the band structure calculations were relatively consistent with each other
The atoms around the interstitial Ni disorder are displaced from the equilibrium position, and the Zr atom next to the interstitial Ni disorder undergoes the highest displacement, which is about 0.9% of the equilibrium position

Summary

Introduction

Significant reduction or ideally complete elimination of the heat losses—within mechanical and electrical devices, in general—are among the most important factors that are imperative for technological advancement. The experimental thermal conductivity in the NZS alloy, which contributes to the high thermoelectric properties of the material, is approximately half of the theoretically calculated value that assumes a perfect crystal[22,23,24,25] for the calculations. This inconsistency has been hypothesized to be present owing to the existence of atomic defects at the vacancy s ites[26,27,28]. These distortions were predicted by the ab initio band structure calculations and experimentally validated using the XAFS measurements

Methods

Results

Conclusion