YCuTe₂: A member of a new class of thermoelectric materials with CuTe₄-based layered structure

Intrinsically doped samples of YCuTe₂ were prepared by solid state reaction of the elements. Based on the differential scanning calorimetry and the high temperature X-ray diffraction analyses, YCuTe₂ exhibits a first order phase transition at ∼440 K from a low-temperature-phase crystallizing in the space group P3m1 to a high-temperature-phase in P3. 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., Cu₂Se or Cu₂Te). 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 YCuTe₂. The maximum thermoelectric figure of merit, zT, is measured to be ∼0.75 at 780 K for Y_0.96Cu_1.08Te₂, which is promising for mid-temperature thermoelectric applications.