Phase stability and thermoelectric properties of semiconductor-like tetragonal FeAl₂.

ABSTRACT: Tetragonal FeAl₂ is a high-pressure phase and is predicted to exhibit semiconductor-like behavior. We investigated the pressure and temperature synthesizing conditions of tetragonal FeAl₂, supported by in situ X-ray diffractions, using synchrotron radiation during heating the sample under a pressure of 20 GPa. Based on the determined optimal conditions, we synthesized the bulk polycrystalline samples of tetragonal FeAl₂ at 7.5 GPa and 873 K, using a multi-anvil press and measured its thermoelectric properties. The Seebeck coefficient of tetragonal FeAl₂ showed a large negative value of - 105 ?V/K at 155 K and rapidly changed to a positive value of 75 ?V/K at 400 K. Although these values are the largest among those of Fe-Al alloys, the maximum power factor remained at 0.41 mW/mK² because the carrier concentration was not tuned. A comparison of the Gibbs free energy of tetragonal FeAl₂, triclinic FeAl₂ and FeAl+Fe₂Al₅ revealed that tetragonal FeAl₂ became unstable as the temperature increased, because of its smaller contribution of vibrational entropy.

SUBMITTER: Tobita K 

PROVIDER: S-EPMC6764376 | biostudies-literature | 2019

REPOSITORIES: biostudies-literature