Syntheses, Structures, and Physical Properties of CsRE2Ag3Te5 (RE = Pr, Nd, Sm, Gd–Er) and RbRE2Ag3Te5 (RE = Sm, Gd–Dy)

A new series of quaternary CsRE2Ag3Te5 (RE = Pr, Nd, Sm, Gd–Er) and RbRE2Ag3Te5 (RE = Sm, Gd–Dy), which have been synthesized from the elemental mixtures in ACl flux (A = Rb, Cs) and crystallized in the orthorhombic space group Cmcm, with a = 4.620(2)–4.504(2) Å, b = 16.232(8)–16.027(8) Å, c = 18.84(1)–18.32(2) and Z = 4, are isostructural to RbSm2Ag3Se5. These isostructural ARE2Ag3Te5 feature a three-dimensional tunnel framework constructed by ionically bound RETe6 octahedron and covalently bound AgTe4 tetrahedron in which tunnels are filled by A. Typical semiconducting behavior is revealed by the electrical conductivity, Seebeck coefficient, optical band gap measurement, and the theoretical calculations. The undoped sintered polycrystalline pellets of CsRE2Ag3Te5 (containing 1.1–1.7% CsCl impurity) show very low electrical conductivity (σr.t. = 0.5–2.4 S/cm), very low thermal conductivity (kr,t = 0.66–0.53 W/(m·K)), and moderate Seebeck coefficient (160–200 μV/K at 700 K).