Improvement of thermoelectric performance of copper-deficient compounds Cu 2.5+ δ In 4.5 Te 8 ( δ = 0-0.15) due to a degenerate impurity band and ultralow lattice thermal conductivity.

Cu-In-Te ternary chalcogenides have unique crystal and band structures; hence they have received much attention in thermoelectrics. In this work we have observed an enhancement in Hall carrier concentration ( n H ) and ultralow lattice thermal conductivity ( κ L ) when Cu was added to ternary Cu 2.5+ δ In 4.5 Te 8 ( δ = 0-0.15) compounds. The enhancement in n H is attributed to a degenerate impurity band at the G point in the valence band maximum (VBM), while the extremely low κ L results from the increased lattice disorder. We thus obtained the minimum κ L value of only 0.23 W K -1 m -1 in the sample at δ = 0.1 and 820 K, which is in good agreement with the calculation using the Callaway model. The highest thermoelectric figure of merit ZT is 0.84 for the material at δ = 0.1, which is about 0.38 higher than that of the pristine Cu 2.5 In 4.5 Te 8 .