A Ba₆Cu₃₁Te₂₂ (A = K, Rb, Cs) Featuring Polyanionic Copper-Telluride Frameworks with Ultralow Thermal Conductivity

Three polyanionic tellurides, ABa₆Cu₃₁Te₂₂ (A = K, Rb, Cs), were synthesized in salt flux. The isostructural tellurides crystallize in a new structure type, in the cubic Pa3 space group with a Wyckoff sequence of d¹⁰c²b¹ and large unit cell volumes of over 5500 ų. The structures feature a framework of [CuTe₄] tetrahedra and [CuTe₃] trigonal pyramids with disorder in the Cu sites. The polyanionic frameworks have large square antiprism and cuboctahedral voids where Ba and alkali metal cations are situated, forming [BaTe₈] and [ATe₁₂], respectively. The overall compositions are close to being charge balanced. The large [ATe₁₂] cuboctahedra allowed for significant anisotropic displacement of the A cations, as observed from both single crystal X-ray diffraction and heat capacity studies. Alkali cations rattling together with Cu atom displacement and disorder leads to the dispersion of phonons, thus softening the lattice and subsequently reducing the thermal conductivity. Evaluations of the electronic band structure revealed the occurrence of a narrow bandgap together with the presence of a flat band near the valence band maximum, giving rise to the high thermopower. The Cs and Rb analogues show a slope change in the temperature dependence of electrical resistivity around room temperature, which is typical for semimetals or degenerate semiconductors. For the as-synthesized and unoptimized materials, high values of the thermoelectric figure-of-merit of ∼0.2 were observed at 623 K.