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

A complete thiospinel solid solution CuCoxTi(2-x)S(4)(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+, Co(2+)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+ + Ti-4(+)reversible arrow Cu2++ Ti3+. The thiospinel CuCO0.6Ti1.4S4, which exhibits the higher ZT of 0.18 at 675 K, opens the route to the research of new Ti-4+-Cu+ based thermoelectric sulfides with different structures. (C) 2018 Published by Elsevier B.V.