High power factor in thiospinels Cu2 T r Ti3S8 (T r = Mn, Fe, Co, Ni) arising from TiS6 octahedron network

Katsuaki Hashikuni, Koichiro Suekuni, Hidetomo Usui, Michihiro Ohta, Kazuhiko Kuroki, Toshiro Takabatake

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    Abstract

    Thermoelectric properties and electronic structures of n-type thiospinels Cu2TrTi3S8 composed of CuS4 tetrahedron and (Tr/Ti)S6 octahedron network have been studied for Tr= Mn, Fe, Co, and Ni. The samples with Tr= Mn, Co, and Ni exhibit metallic behaviors in the electrical resistivity (ρ) and rather large and negative thermopower (S), leading to a high power factor (S2/ρ) of 0.4-0.6 mW/K2 m at 650 K. In addition to the superior electrical properties, relatively low thermal conductivity of ∼2 W/Km gives rise to a dimensionless figure of merit ZT reaching 0.16-0.18 at 650 K. The analysis of the temperature dependent magnetic susceptibility indicates that the Mn, Fe, and Ni ions are in high-spin divalent states while the Co2+ ion is in a low-spin nonmagnetic state. This electronic state for the Co2+ in Cu2CoTi3S8 is consistent with our first-principles electronic structure calculation indicating that the Fermi level lies in the conduction bands composed mainly of Ti-3d, Co-3d, and S-3p orbitals. The Ti-3d and S-3p orbitals forming the octahedron network likely results in high power factors irrespective of Tr elements. The addition of Co-3d orbitals makes a peak with steep slope in the density of states near the Fermi level, leading to the further enhanced power factor.

    Original languageEnglish
    Article number182110
    JournalApplied Physics Letters
    Volume109
    Issue number18
    DOIs
    Publication statusPublished - Oct 31 2016

    All Science Journal Classification (ASJC) codes

    • Physics and Astronomy (miscellaneous)

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