New Quaternary Compounds Resulting from the Reaction of Copper and f-Block Metals in Molten Polychalcogenide Salts at Intermediate Temperatures. Valence Fluctuations in the Layered CsCuCeS3

From the reaction of elemental copper and either lanthanides or actinides in molten alkali metal/polychalcogenide salts, several new quaternary phases have been discovered. Specifically, these phases are ACuM 2 Q 6 (where A = K, M = La, Q = S; A = Cs, M = Ce, Q = S; or A = K, M = Ce, Q = Se) and ACuMQ 3 (where A = Cs, M = Ce, Q = S; or A = K, M = U, Q = Se). The CsCuCe 2 S 6 crystallizes in the orthorhombic space group Immm with a = 5.500(1) Å, b = 22.45(1) Å, c = 4.205(4) Å. The KCuCe 2 Se 6 is isostructural. The CsCuCeS 3 crystallizes in the orthorhombic space group Cmcm with a = 4.024(2) Å, b = 15.154(2) Å, c = 10.353(3) Å. The KCuUSe 3 is isostructural. In ACuM 2 Q 6, the lanthanides bond to a mixture of mono- and disulfides in a bicapped trigonal prismatic geometry; these polyhedra subsequently connect in two dimensions, forming layers equivalent to those seen in the ZrSe 3 structure type with Cu + atoms residing in tetrahedral sites within the layers and alkali cations in the interlayer gallery. The compounds of the formula ACuMQ 3 also possess a layered structure. Here the [MQ 6 ] octahedral units form corrugated, two-dimensional sheets via edge-sharing in the first dimension and corner-sharing in the second. Copper cations are coordinated to tetrahedral sites in the folds of the corrugations, and alkali cations are again in the intergallery region. Details of the synthesis, structure, and properties of these compounds are discussed.