Phase stability and thermoelectric properties of semiconductor-like tetragonal FeAl(2)

Tetragonal FeAl(2) is a high-pressure phase and is predicted to exhibit semiconductor-like behavior. We investigated the pressure and temperature synthesizing conditions of tetragonal FeAl(2), 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 FeAl(2) at 7.5 GPa and 873 K, using a multi-anvil press and measured its thermoelectric properties. The Seebeck coefficient of tetragonal FeAl(2) showed a large negative value of – 105 μV/K at 155 K and rapidly changed to a positive value of 75 μV/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/mK(2) because the carrier concentration was not tuned. A comparison of the Gibbs free energy of tetragonal FeAl(2), triclinic FeAl(2) and FeAl+Fe(2)Al(5) revealed that tetragonal FeAl(2) became unstable as the temperature increased, because of its smaller contribution of vibrational entropy.