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

Abstract

Twenty-seven new members of the A2Cu2n Ln4Q7+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 A2Cu2n Ln4Q7+n compounds can potentially find application as high-temperature thermoelectric materials and other semiconductors.

Chemical Bonding Tuned Lattice Anharmonicity Leads to a High Thermoelectric Performance in Cubic AgSnSbTe3

“…The authors have cited additional references within the Supporting Information. [71][72][73][74][75][76][77][78][79][80][81][82][83][84][85][86][87][88][89][90]…”

“…Experimental and theoretical calculation methods are available in Supporting Information linked to this article. The authors have cited additional references within the Supporting Information [71–90] …”

Structures and Magnetic Properties of K2Pd4U6S17, K2Pt4U6S17, Rb2Pt4U6S17, and Cs2Pt4U6S17 Synthesized Using the Boron–Chalcogen Mixture Method

“…Another great synthetic advantage of the BCM method is the in situ alkali polysulfide flux formation, which can be achieved by adding an alkali carbonate and an appropriate amount of boron and sulfur to the reaction mixture. Alkali polychalcogenide fluxes have been paramount in the formation of many chalcogenide phases but are highly moisture- and air-sensitive as well as commercially unavailable, which necessitates their synthesis “in-house” via a combination of the alkali metal and chalcogen in liquid ammonia. − The ability to create an alkali polysulfide flux in situ thus negates the need for premade reagents and reduces the number of highly moisture- and air-sensitive reagents.…”