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

Highlights: • Large one-dimensional channel crystal structure. • Rb_xMo₁₂S₁₄ compounds are particularly well adapted for topotactic reactions by soft chemistry or electrochemistry. • Evolution of the electrical resistivity with the Rb stoichiometry • Superconductor. -- Abstract: The novel Mo₁₂ cluster sulfides Rb_2.65Mo₁₂S₁₄ have been synthesized by direct solid-state reaction at a temperature of about 1500 °C in a welded Mo tubing. The crystal structure of Rb_2.65Mo₁₂S₁₄ (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 K_1+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 Rb_1.08Mo₁₂S₁₄. Electrical resistivity measurements performed on single crystals of Rb_2.65Mo₁₂S₁₄ and Rb_1.08Mo₁₂S₁₄ 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.