Homologous Alkali Metal Copper Rare-Earth Chalcogenides A₂Cu_2nLn₄Q_7+n (n = 1, 2, 3)

Twenty-seven new members of the A2Cu2nLn4Q7+n (A = Cs, Rb; Ln = La–Nd, Sm, Gd–Yb; Q = S, Se) homologous series were synthesized in one of three structural types (indicated by n = 1, 2, 3). All the compounds contained 3D frameworks with alkali-metal-containing tunnels. For each increment in n, one Cu2Q was added, which was incorporated into the framework as an edge-sharing tetrahedron by replacing a square planar chalcogenide site. High-throughput DFT calculations predicted many of the phases to be thermodynamically stable. These predictions were compared with the synthesis results for the phases formed in each composition space. In the syntheses, heavier lanthanides showed a preference to start forming the n = 3 ACu3Ln2Q5, which is consistent with the predictions. RbCuNd2Se4 and RbCuTb2Se4 were found to be thermally stable under vacuum at temperatures up to 1000 °C. Optical measurements revealed band gaps of 1.55(5) and 1.62(5) eV for CsCuCe2Se4 and RbCuTb2Se4, respectively, and a work function of 4.83(5) eV for CsCuPr2Se4. Additionally, some n = 3 ACu3Ln2Q5 compounds exhibit a negative phonon mode because of a copper atom coordination, which may distort to a trigonal planar geometry at sufficiently low temperatures. The dynamic instabilities and the predicted distortion in the copper tetrahedra for the n = 3 ACu3Ln2Q5 compounds were found to have a linear relationship with the atomic number of the lanthanides and the electronegativity of the lanthanides. The A2Cu2nLn4Q7+n compounds can potentially find application as high-temperature thermoelectric materials and other semiconductors.