In-gap electronic states responsible for the excellent thermoelectric properties of Ni-based half-Heusler alloys

The half-Heusler compounds NiXSn (X = Ti, Zr, Hf) are regarded as examples of materials possessing excellent thermoelectric properties. We have studied their electronic structures by photoemission spectroscopy excited by soft and hard X-ray synchrotron radiation. Although the overall features of the experimental valence band spectra are well explained by a calculated density of states, in-gap states close to the Fermi energy emerge due to atomic disorder. These electronic states were further confirmed by core-level photoemission and appear to be key for excellent thermoelectric behavior. (C) 2008 The Japan Society of Applied Physics.