Thermoelectric Properties and Electronic Structures of CuTiS Thiospinel and Its Derivatives: Structural Design for Spinel-Related Thermoelectric Materials.

We report the preparations, thermoelectric and magnetic properties, and electronic structures of Cu-Ti-S systems, namely, cubic thiospinel c-CuTiS ( x ≤ 0.375), a derivative cubic and Ti-rich phase c-CuTiS ( x = 0.5, 0.625), and a rhombohedral phase r-CuTiS. All samples have the target compositions except for r-CuTiS, whose actual composition is CuTiS. All of the phases have n-type metallic character and exhibit Pauli paramagnetism, as proven by experiments and first-principles calculations. The Cu and Ti deficiencies in c-CuTiS and r-CuTiS, respectively, decrease the electron-carrier concentration, whereas the "excess" of Ti ions in c-CuTiS largely increases it. For r-CuTiS, the reduced carrier concentration increases the electrical resistivity and Seebeck coefficient, leading to the highest thermoelectric power factor of 0.5 mW K m at 670 K. For all of the Cu-Ti-S phases, the thermal conductivity at 670 K is 3.5-5 W K m, where the lattice part of the conductivity is as low as 1 W K m at 670 K. As a result, r-CuTiS shows the highest dimensionless thermoelectric figure of merit ZT of 0.2. The present systematic study on the Cu-Ti-S systems provides insights into the structural design of thermoelectric materials based on Cu-M-S (M = transition-metal elements).