Effect of Vacancies on Magnetism, Electrical Transport, and Thermoelectric Performance of Marcasite FeSe_2−δ (δ = 0.05)

The marcasite structure FeSe 2−δ was synthesized using a simple solvothermal method. Systematic study of the electrical transport properties shows that the transport is dominated by variable-range hopping (VRH), with a changeover from Mott VRH at higher temperature to Efros-Shklovskii VRH for temperatures lower than the width of the Coulomb gap. This also confirms the presence of a Coulomb gap in the density of states at the Fermi energy. We observe that Yttrium doping increases the electrical conductivity dramatically without significantly reducing the Seebeck coefficient. This results in remarkably high power factors for thermoelectric performance in the regime where the mean hopping energy shifts from defect dominated to Coulomb repulsion dominated. High resolution transmission electron microscopy, in combination with theoretical calculations, proves the narrowing of the band gap by introducing Se vacancies. This leads to a good conductivity and is responsible for the excellent thermoelectric performance. The formation of nanoclusters, resulting from Se vacancies, is responsible for a dense system of stacking faults and the generally reported weak ferrimagnetism. This also determines the transition between the different electrical transport mechanisms and contributes to the improved thermoelectric performance.