YCuTe_2: a member of a new class of thermoelectric materials with CuTe_4-based layered structure

Intrinsically doped samples of YCuTe_2 were prepared by solid state reaction of the elements. Based on the differential scanning calorimetry and the high temperature X-ray diffraction analyses, YCuTe_2 exhibits a first order phase transition at ∼440 K from a low-temperature-phase crystallizing in the space group P^(bar)3 with combining P^(bar)3m1 to a high-temperature-phase in P^(bar)3 with combining macron]. Above the phase transition temperature, partially ordered Cu atoms become completely disordered in the crystal structure. Small increases to the Cu content are observed to favour the formation of the high temperature phase. We find no indication of superionic Cu ions as for binary copper chalcogenides (e.g., Cu_2Se or Cu_2Te). All investigated samples exhibit very low thermal conductivities (as low as ∼0.5 W m^(−1) K^(−1) at 800 K) due to highly disordered Cu atoms. Electronic structure calculations are employed to better understand the high thermoelectric efficiency for YCuTe_2. The maximum thermoelectric figure of merit, zT, is measured to be ∼0.75 at 780 K for Y_(0.96)Cu_(1.08)Te_2, which is promising for mid-temperature thermoelectric applications