Chemical Bonding Origin of the Thermoelectric Power Factor in Half-Heusler Semiconductors

Intermetallic semiconductors with the cubic Half-Heusler (HH) 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. HHs are often interpreted within Zintl chemistry, making a clear distinction between an electropositive cation (Xn+) and an extended polyanion (YZn–). Based on quantitative real-space chemical bonding analysis, we identify 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 of 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 a simple chemical concept often applied for the design of complex materials.