Role of cobalt for titanium substitution on the thermoelectric properties of the thiospinel CuTi₂S₄

A complete thiospinel solid solution CuCo_xTi_2-xS₄ (0 ≤ x < 1) has been synthesized by conventional sealed tube synthesis followed by Spark Plasma Sintering, whereas, for x = 1, the presence of secondary phases is observed. The compounds exhibit a semi-metallic behavior in the whole composition range in spite of the existence of two subdomains characterized by different major redox species: (I) involving Cu⁺, Ti³⁺, Ti⁴⁺, Co²⁺ for 0 ≤ x ≤ 0.5 and (II) involving Cu⁺, Cu²⁺, Ti⁴⁺, Co²⁺for 0.5 < x ≤ 1. The thermoelectric properties show that, in the compositional range x = 0.3–0.8, the figure of merit does not depend significantly on x, ZT ranging from 0.13 to 0.18 at 675 K, whereas for the limit x = 0, it drops down to 0.04. This behavior is interpreted in terms of the predominant influence of the redox couple Ti³⁺/Ti⁴⁺ on the transport properties, as long as Cu⁺ is present, according to the equilibrium Cu⁺ + Ti⁴⁺⇌ Cu²⁺ + Ti³⁺. The thiospinel CuCo_0.6Ti_1.4S₄, which exhibits the higher ZT of 0.18 at 675 K, opens the route to the research of new “Ti⁴⁺-Cu^+” based thermoelectric sulfides with different structures.