Direct Solid-State Synthesis of the Superconductor NaMo6Se8: Single-Crystal Structure, Electrical and Magnetic Properties

Powder samples and single crystals of the ternary selenide NaMo6Se8 were obtained by direct solid-state reaction at high temperature for the first time. The structure was determined by single-crystal X-ray diffraction. NaMo6Se8 crystallizes in the trigonal space group R-3 (No. 148) with unit-cell parameters a = 9.5564 (2) Å, c = 11.7181 (3) Å and Z = 3. Full-matrix least-squares refinement on F2 using 911 independent reflections for 25 refinable parameters results in R1 = 0.0251 and wR2 = 0.0593. The structure contains Mo6Se8iSe6a cluster units (octahedral Mo6 cluster inscribed in a pseudo S8 cube) separated from each other by large voids which are filled up by eight-coordinate Na atoms. The evolution of the Mo-Mo distances of the octahedral Mo6 cluster with respect to the cationic charge in the selenide Chevrel phases is also discussed. NaMo6Se8 behaves as a p-type metal with relatively low electrical resistivity and becomes superconductor below 9.2 K.