Electronic structure and chemical bonding in half-Heusler phases

The electronic structure and chemical bonding in half-Heusler phases have been systematically investigated using first-principles, self-consistent tight-binding linear-muffin-tin-orbital calculations within the atomic-sphere approximation (TB-LMTO-ASA). The density-of-states profiles for the mainstream half-Heusler phases exhibit very similar features originating from the cubic, positional-parameter-free AlLiSi-type structural arrangement and resemblances in composition. The electronic structures of these half-Heusler phases are accordingly very suitable for rigid-band considerations and predictions made on this basis are tested against actually calculated data. The nature of the chemical bonding has been systematically explored for the large transition-metal branch of the half-Heusler family using density-of-states, charge-density, charge-transfer, electron-localization-function, and crystal-orbital-Hamilton-population plots. The study has laid stress on the remarkable consistency in bonding behaviour among the considered intermetallic phases even though the properties range from non-magnetic metals and semiconductors via ferromagnetic metals to half-metallic ferromagnetic metals, brought about by large spread in element combinations and valence-electron content. The typical half-Heusler phase exhibits an appreciable covalent contribution to the bonding independent of the electronic state at the Fermi-level. The empirical rule that the (numerous) half-Heusler phases with valence-electron content of 18 are semiconducting and more stable than those (metallic) with a higher or lower valence-electron content is substantiated.