Abstract
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 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/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.
Highlights
Thermoelectric materials are functional materials that can directly convert heat into electricity
To obtain the optimal conditions for synthesizing high-quality samples of t-FeAl2 under high pressure, we investigated the pressure and temperature (P-T) synthesizing conditions of t-FeAl2 using the multi-anvil type apparatus and in situ Xray diffraction (XRD) experiments supported by synchrotron radiation
Thermoelectric properties were characterized in the temperature range 10–600 K: S, σ, κ and Hall coefficient (RH) at 10–300 K were measured with a physical properties measurement system (PPMS; Quantum design) using the thermal transport option for thermoelectric properties and the AC transport option for five-wire RH; S at 300–600 K was measured with a ZEM-1 (ULVAC RIKO); σ and RH at 300–600 K were measured with a Resitest 8300 (Toyo Technica Co.)
Summary
We previously reported that novel MoSi2-type tetragonal FeAl2 (t-FeAl2, space group I4/mmm) can be synthesized through the laser-heated diamond-anvil cell (LHDAC) technique at 10 GPa and 1873 K [4]. This t-FeAl2 was theoretically predicted to be stable (or at least metastable) [5,6], but could not be synthesized at ambient pressure [7]. Analog compounds RuAl2 and RuGa2 have been reported as narrow bandgap semiconductors with a high power factor, S2σ [7,13,14,15,16] From these theoretical and experimental works, it is expected that t-FeAl2 behaves like a semiconductor. We present a study of the stability of t-FeAl2 and neighboring phases using first-principles total energy calculations supplemented by a phonon-based calculation of vibrational entropy, which yields the Gibbs free energy under a quasi-harmonic approximation
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