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

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.