Transition to n-type Thermoelectric Conduction in Ni-doped FeSe₂ Alloys

We investigated the effect of Ni doping on the electrical, thermal, and thermoelectric transport properties of FeSe₂ polycrystalline alloys. FeSe₂ alloys exhibit both n-type and p-type conduction at different temperatures and thus their practicality as a thermoelectric material is somewhat limited. In this study, Fe_1-xNi_xSe₂ polycrystalline samples with x = 0, 0.01, 0.05, 0.075, 0.1, and 0.125 were synthesized, and it was shown that Ni doping in FeSe₂ alloys induces n-type conduction in the entire temperature range. The electrical conductivity is gradually increased with an increase in carrier concentration as Ni doping increases, from 8.52 (x = 0) to 329 S/cm (x = 0.125) at 300 K. The maximum power factor of 0.61 mW/mK² was observed in the Fe_1-xNi_xSe₂ sample at x = 0.01 at 600 K. Other Ni-doped samples exhibited power factors between 0.31 and 0.34 mW/mK² at 600 K. The thermal conductivity gradually decreased from 6.9 (x = 0) to 4.2 W/mK (x = 0.125) with Ni doping because of the additional point-defect phonon scattering of the Ni substitutes. As a result, a zT of 0.11 was observed in x = 0.01 at 600 K, while the zT of the other Ni-doped samples exhibited 0.056-0.069 at 600 K.