Phase formation and thermoelectric properties of FeSe2–CoSe2 system

Abstract

The phase formation and thermoelectric transport properties of FeSe2–CoSe2 systems were investigated by synthesizing a series of (Fe1–xCox)Se2 polycrystalline alloys (x = 0, 0.25, 0.5, 0.75, and 1). It was observed that as x increased from 0 to 0.5, lattice parameters increased while the orthorhombic structure of FeSe2 was retained. The bipolar conduction behavior of FeSe2 (x = 0) changed to n-type conduction at all measured temperatures. The electrical conductivity is greatly increased with x, and the largely enhanced power factor of 1.37 mW/mK2 at 600 K is seen for Fe0·5Co0·5Se2 (x = 0.5) compared to 0.37 mW/mK2 for FeSe2. As x is further increased to 0.75 (Fe0·25Co0·75Se2), the cubic phase of CoSe2 started to form and no further enhancement of power factor is seen. Lattice thermal conductivity is gradually decreased to 3.18 and 1.79 W/mK Fe0·75Co0·25Se2 (x = 0.25) and Fe0·5Co0·5Se2 (x = 0.5) at 600 K, compared to 4.36 W/mK for x = 0 (FeSe2). Consequently, the thermoelectric figure of merit zT at 600 K for Fe0·75Co0·25Se2 (x = 0.25) and Fe0·5Co0·5Se2 (x = 0.5) is greatly enhanced to 0.14 and 0.16, respectively, compared to 0.05 for x = 0 (FeSe2).