Lattice distortion and anisotropic thermoelectric properties in hot-deformed CuI-doped Bi₂Te_2·7Se_0.3

We investigated anisotropic thermoelectric properties of (CuI)_xBi₂Te_2·7Se_0.3 (x = 0.0, 0.3, 0.6, and 0.9 mol.%) compounds, synthesized by the hot-press and hot-deformation process. In spite of polycrystalline compound, the hot-deformed compounds exhibit preferred orientation along the c-axis, parallel with the applied press direction. The sample of x = 0.3 mol.% shows the maximum power factor (3.8 mW m⁻¹ K⁻² at 300 K) and ZT value (0.97 at 423 K), which is relatively high thermoelectric performance in n-type thermoelectric materials as a mild-temperature operation. Notably, the in-plane lattice thermal conductivity of the x = 0.3% compound with covalent bonding layer has lower value than the one of out-of-plane lattice thermal conductivity with van der Waals bonding layer. From the high resolution transmission electron microscopy and electron diffraction measurements, we observe the lattice distortion of the x = 0.3% compound. Therefore, the unconventional anisotropic lattice thermal conductivity can be associated with the lattice distortion along the in-plane on the compound driven by the CuI doping.