From semi-metal to superconductor: The New Mo12 Cluster Sulfides Rb1+xMo12S14 (x = 0.08, and 1.65)

International audience; The novel Mo12 cluster sulfides Rb2.65Mo12S14 have been synthesized by direct solid-state reaction at a temperature of about 1500 °C in a welded Mo tubing. The crystal structure of Rb2.65Mo12S14 (space group P-31c: a = 9.2828(6) Å, c = 16.446(5) Å) was solved from X-ray single-crystal data. It is isomorphous with the K1+xMo12S14 compounds in which x ranges from 0 to 1.6. Consequently, the Mo-S network presents interconnected Mo12S14 cluster units delimiting large tunnels that are randomly occupied by a part of the Rb+ ions, the other ones being localized in large voids between two consecutive Mo12S14 units along the [001] direction. By topochemical redox reactions at around 90 °C, we could remove a part of the Rb+ ions from the channels and thus, form the oxidized phase Rb1.08Mo12S14. Electrical resistivity measurements performed on single crystals of Rb2.65Mo12S14 and Rb1.08Mo12S14 shows that the oxidized form is a semimetal while the reduced form is a metal and becomes superconducting below 4.07 K. The evolution of Mo-Mo bond lengths within the trioctahedral Mo12 cluster with respect to the cationic charge x is discussed and, compared with that found in the potassium series.