A New Material with a Composite Crystal Structure Causing Ultralow Thermal Conductivity and Outstanding Thermoelectric Properties: TlAgTe.

A new state-of-the-art thermoelectric material, TlAgTe, which possesses an extremely low thermal conductivity of about 0.25 W m K and a high figure-of-merit of up to 1.1 at 525 K, was obtained using a conventional solid-state reaction approach. Its subcell is a variant of the ZrFeP type, but ultimately its structure was refined as a composite structure of a TlAgTe framework and a linear Te atom chain running along the c axis. The super-space group of the framework was determined to be P6(00γ) s with a = b = 11.438(1) Å, c = 4.6256(5) Å, and that of the Te chain substructure has the same a and b axes, but c = 3.212(1) Å, space group P6(00γ) s. The modulation leads to the formation of Te and Te fragments in this chain and a refined formula of TlAgTe. The structure consists of a complex network of three-dimensionally connected AgTe tetrahedra forming channels filled with the Tl atoms. The electronic structures of four different models comprising different Te chains, TlAgTe, TlAgTe, and 2× TlAgTe, were computed using the WIEN2k package. Depending on the Te content within the chain, the models are either semiconducting or metallic. Physical property measurements revealed semiconducting properties, with an ultralow thermal conductivity, and excellent thermoelectric properties at elevated temperatures.