Low temperature thermoelectric and magnetoresistive properties of Tl2Cu3FeQ4 (Q = S, Se, Te)

Abstract

Tl₂Cu₃FeQ₄ (Q = S, Se, Te) belong to the ThCr₂Si₂ structure type (space group: I4/mmm), which is a layered structure. Alternating metal–chalcogenide layers are formed from TlQ₈ square prisms and (Cu/Fe)Q₄ tetrahedra; the latter results in a weakly connected Cu-square net. Herein, 25% Fe atoms randomly occupy the Cu-sublattice, which causes magnetic exchange interactions. From the electrical transport property measurements, the sulfide and selenide were found to possess semiconducting characteristics, with room temperature resistivity values of 0.308 and 1.465 × 10⁻⁴ Ω m, respectively. The resistivity values gradually increase as the samples were cooled down towards 2 K. The telluride, however, exhibits metallic character with a resistivity of the order of 10⁻⁶ Ω m within the measured temperature regime. These layered materials can be good choices as thermoelectrics, due to their low thermal conductivity. Their thermoelectric properties were studied below 350 K, and the selenide composition was found to possess a figure of merit of 0.1 at 350 K. Driven by the Fe-atom disorder, these materials cause phase separation within the largely paramagnetic matrix, reminiscent of well-known rare-earth manganites and iron selenide-based superconductors. The selenide and telluride also exhibit huge negative magnetoresistance (MR) below ferromagnetic transition temperature: a maximum of 50.5% at 2 K for selenide and 22% at 20 K for telluride for an applied field of 9 T. Below 40 K, the telluride exhibits a huge asymmetric MR effect similar to spin-valves where exchange bias effects occur. We attribute these unusual MR features to the phase separated regions within the extended lattice owing to the inhomogeneous distribution of Fe atoms within the Cu-rich sublattice.