Study of Phase Formation Behavior and Electronic Transport Properties in the FeSe2 -FeTe2 System

In this study, we investigated the phase formation behavior of the FeSe2-FeTe2 system and the evolution of their thermoelectric transport properties. A series of Fe(Se1-xTex)2(x = 0, 0.2, 0.6, 0.8 and 1) compounds were synthesized by conventional solid-state reaction. Single phase orthorhombic structures were formed, except for FeSe0.4Te1.6(x = 0.2). For FeSe0.4Te1.6(x = 0.2), in which a mixture of FeSe2 and FeTe2-based phases was observed. The electrical conductivity of FeSe2 was as low as 31 S/cm at room temperature, and greatly increased to 1295 S/cm in FeSe0.8Te1.2. Then it decreased to 335 S/cm for FeTe2. It was shown that the bipolar conduction of electrons and holes pronouncedly exists in the sample. For FeSe2 and FeTe2, the Seebeck coefficient was positive at room temperature, becoming negative at high temperatures. For the other samples, the Seebeck coefficient was negative at room temperature and the magnitude of Seebeck coefficient increased with temperature. As results, the maximum power factor of 0.19 mW/mK2 was observed for FeSe0.8Te1.2(x = 0.6) at room temperature mostly due to its high electrical conductivity. The maximum power factor observed at 600 K was 0.34 mW/mK2 for FeSe2 (x = 0) with the largest Seebeck coefficient of -101 µV/K. The Hall carrier concentration was measured and compared, and effective masses were calculated to further investigate electrical transport in the samples.