We report the preparations, thermoelectric and magnetic properties, and electronic structures of Cu-Ti-S systems, namely, cubic thiospinel c-Cu 1-x Ti 2 S 4 (x ≤ 0.375), a derivative cubic and Ti-rich phase c-Cu 1-x Ti 2.25 S 4 (x = 0.5, 0.625), and a rhombohedral phase r-CuTi 2 S 4 . All samples have the target compositions except for r-CuTi 2 S 4 , whose actual composition is Cu 1.14 Ti 1.80 S 4 . 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-Cu 1-x Ti 2 S 4 and r-CuTi 2 S 4 , respectively, decrease the electron-carrier concentration, whereas the "excess" of Ti ions in c-Cu 1-x Ti 2.25 S 4 largely increases it. For r-CuTi 2 S 4 , the reduced carrier concentration increases the electrical resistivity and Seebeck coefficient, leading to the highest thermoelectric power factor of 0.5 mW K -2 m -1 at 670 K. For all of the Cu-Ti-S phases, the thermal conductivity at 670 K is 3.5-5 W K -1 m -1 , where the lattice part of the conductivity is as low as 1 W K -1 m -1 at 670 K. As a result, r-CuTi 2 S 4 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).
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry
- Inorganic Chemistry