Superconductivity in the AxNbS2 intercalation compounds (A = Cs, Rb)

Four new AxNbS2 phases, where A = Cs, x = 0.34, 0.60; A = Rb, x = 0.33, 0.6, were prepared from Nb and A2CO3, in a CS2/Ar atmosphere at 700 degrees C. The compounds were refined (Rietveld profile analysis) in P63/mmc symmetry. Crystal data were Cs0.34NbS2, a = 3.3452(5) Angstrom, c = 18.424(5) Angstrom; Cs0.60NbS2, a = 3.3539(6) Angstrom, c = 18.324(1) Angstrom; Rb0.33NbS2, a = 3.3425(5) Angstrom, c = 18.075(2) Angstrom; Rb0.6NbS2, a = 3.3462(3) Angstrom, c = 17.986(2) Angstrom. The compounds comprise a 2H-NbS2 host with trigonal prismatic Nb ions and alkali ions intercalated in trigonally distorted octahedral holes between NbS2 layers. Increasing the alkali content effects a decrease in the c lattice parameters due to decreases in interlayer S···S separations. Magnetic susceptibility and resistivity measurements indicate metallic behavior for all compounds below room temperature. The Cs phases are bulk type II superconductors (Cs0.34NbS2, Tc = 3.0 K, Cs0.60NbS2, Tc = 2.0 K). 19 refs., 5 figs., 3 tabs