Norihiko Okamoto

Abstract:

One of the iron aluminides, usually denoted as Fe₂Al₅, is the major constituent phase of the coating layer in hot-dip aluminized steel sheets and has an orthorhombic crystal structure consisting of a tunnel-like framework and guest aluminum atoms encapsulated in the framework [1,2]. Fe₂Al₅ can be categorized as inclusion compounds like clathrate and skutterudite compounds, in which guest atoms are rattling inside a cage framework [3,4]. Since it exhibits very low lattice thermal conductivity and relatively large Seebeck coefficient [5], Fe₂Al₅ is considered as a potential thermoelectric material. In this study, to elucidate the origin of the low lattice thermal conductivity, crystal orientation dependence of the transport properties of Fe₂Al₅ was investigated by using single crystals grown by the Bridgman method. Seebeck coefficient, electrical resistivity and thermal diffusivity were measured along the a-, b- and c-axes of single-crystal samples. The lattice thermal conductivity was estimated with the Wiedemann-Franz law and the Lorenz number of 2.44×10^-8 (V²/K²) [6]. The experimental lattice thermal conductivity along the c-axis, which is the principal axis of the tunnel framework, is exceptionally lower than that estimated with the phonon dispersion relationships deduced by first-principles calculations under harmonic approximation. This implies that the anharmonicity of guest vibration along the c-axis plays a dominant role in reducing the lattice thermal conductivity. Such anharmonicity of the guest vibration was confirmed not only by the calculation of the potential energy surface around the guest site but also by internal friction measurement, which suggested frequent atomic jump of guest atoms along the c-axis within the tunnel framework. Besides, Fe₂Al₅ exhibits n-type conduction along the b-axis while p-type conduction along the a- and c-axes at room temperature. Such largely anisotropic electron/hole transport properties were consistently explained in terms of the anisotropic electronic band structure but do not correspond to the anisotropic crystal structure unlike in the case of lattice thermal conductivity.