Significant reduction of lattice thermal conductivity observed in CuInTe₂-CuAlTe₂ solid-solution alloys

CuInTe₂ and CuAlTe₂, which are ternary chalcogenide compounds with the same tetragonal structure, are considered as thermoelectric materials owing to high Seebeck coefficients with large bandgaps of ∼1.08 and 1.96 eV, respectively. In this study, the electrical, thermal, and thermoelectric properties of a CuInTe₂-CuAlTe₂ solid solution alloy system were systematically investigated by synthesizing a series of CuIn_1-AlTe₂ (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0) compositions. CuInTe₂ and CuAlTe₂ form the complete solid solutions as reported, and the electrical conductivity and Seebeck coefficient decrease simultaneously to x = 0.8 due to a significant reduction in carrier mobility, thereby reducing the power factor. For CuAlTe₂, the power factor suddenly increased owing to its very high electrical conductivity. On the other hand, the total and lattice thermal conductivity is greatly reduced by additional phonon scattering originating from solid-solution alloying. For instance, the largely reduced lattice thermal conductivity was measured to be 1.8 and 1.9 W m^-1 K^-1 for the sample with x = 0.4 and x = 0.6 at 300 K, whereas those for CuInTe₂ and CuAlTe₂ were 4.8 and 5.6 W m^-1 K^-1, respectively. Nevertheless, the thermoelectric figure of merit zT was significantly reduced by the solid solution alloying due to a significant reduction of power factors despite the reduction in thermal conductivity.