The impact of a magnetic ion on the thermoelectric properties of copper-rich quaternary selenides

Abstract

Abstract A series of copper-based p-type chalcogenide semiconductors, Cu2+x BGe1−x Se4 (B = Zn, Fe; 0 ⩽ x ⩽ 0.15), was prepared by high-temperature methods, to explore the impact of replacement of the closed-shell ion, Zn2+, with the magnetically active, Fe2+ cation. Powder x-ray diffraction in conjunction with Rietveld refinement reveals that zinc-containing materials are described in the kesterite-type structure (space group: I 4 ˉ ) and contain trace amounts of secondary phases, whereas in the iron analogues, described in the stannite-type structure (space group: I 4 ˉ 2 m ), single-phase behaviour persists to x = 0.1. Excess copper ions lead to the formation of holes and the electrical resistivity of both series is reduced from that of the stoichiometric end members. In the case of the iron-containing materials, this is shown to be due to an increase in the hole mobility, µ. This decrease in resistivity offsets the observed reduction in Seebeck coefficient and both series exhibit an improvement in thermoelectric performance. The lower electrical resistivity of the iron-containing materials, leads to higher figures-of-merit, compared to those of the zinc-containing materials at the same level of copper excess. The maximum figure-of-merit, ZT = 0.3 is attained for Cu2.075FeGe0.925Se4 at the comparatively low temperature of 575 K. This is an increase of ca. 62% from that of the end member phase and ca. 67% higher than that of the zinc analogue at the same level of substitution.