Chemical bonding origin of the thermoelectric power factor in Half-Heusler semiconductors

TL;DR

This paper reveals that covalent bonding, rather than Zintl ionic distinctions, significantly influences the thermoelectric properties of Half-Heusler semiconductors, challenging traditional chemical bonding interpretations.

Contribution

It provides a quantitative real-space analysis showing covalency in Half-Heuslers, expanding understanding of their electronic and thermoelectric behavior beyond Zintl chemistry.

Findings

Covalent bonding dominates over Zintl ionic model in Half-Heuslers.

Improved thermoelectric properties correlate with deviations from Zintl behavior.

Covalence impacts band structure and response properties significantly.

Abstract

Intermetallic semiconductors with the cubic Half-Heusler structure (XYZ) have excellent thermoelectric properties. This has been attributed to the high degeneracy of the carrier pockets in the band structure, but large differences are found between different material compositions. Half-Heuslers are often interpreted within Zintl chemistry, making a clear distinction between an electropositive cation ($X^{n+}$) and an extended polyanion ($YZ^{n-}$). Based on quantitative real space chemical bonding analysis, we unravel large degrees of covalent bonding between the formal cation and anion, making the Zintl distinction clearly invalid. This covalence is shown to strongly affect the band structure, thermoelectric properties and response properties in the materials, with improved thermoelectric properties observed for those materials that least follow the Zintl concept. This expands our knowledge of the chemical bonding motifs governing physical properties, and gives a critical view on the simplistic chemical concepts too often applied for design of complex materials.