Measured and simulated thermoelectric properties of FeAs2−xSex (x = 0.30–1.0): from marcasite to arsenopyrite structure

Abstract

FeAs_2−xSe_x (x = 0.30–1.0) samples were synthesized as phase pure powders by conventional solid-state techniques and as single crystals (x = 0.50) from chemical vapor transport. The composition of the crystals was determined to be Fe_1.025(3)As_1.55(3)Se_0.42(3), crystallizing in the marcasite structure type, Pnnm space group. FeAs_2−xSe_x (0 < x < 1) was found to undergo a marcasite-to-arsenopyrite (P2₁/c space group) structural phase transition at x ∼ 0.65. The structures are similar, with the marcasite structure best described as a solid solution of As/Se, whereas the arsenopyrite has ordered anion sites. Magnetic susceptibility and thermoelectric property measurements from 300–2 K were performed on single crystals, FeAs_1.50Se_0.50. Paramagnetic behavior is observed from 300 to 17 K and a Seebeck coefficient of −33 μV K⁻¹, an electrical resistivity of 4.07 mΩ cm, and a very low κ_l of 0.22 W m⁻¹ K⁻¹ at 300 K are observed. In order to determine the impact of the structural transition on the high-temperature thermoelectric properties, polycrystalline FeAs_2−xSe_x (x = 0.30, 0.75, 0.85, 1.0) samples were consolidated into dense pellets for measurements of thermoelectric properties. The x = 0.85 sample shows the best thermoelectric performance. The electronic structure of FeAsSe was calculated with DFT and transport properties were approximately modeled above 500 K.