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

Abstract

The novel Mo 12 cluster sulfides Rb 2.65 Mo 12 S 14 have been synthesized by direct solidstate reaction at a temperature of about 1500°C in a welded Mo tubing. The crystal structure of Rb 2.65 Mo 12 S 14 (space group P-31c: a = 9.2828(6) Å, c = 16.446(5) Å) was solved from Xray single-crystal data. It is isomorphous with the K 1+x Mo 12 S 14 compounds in which x ranges from 0 to 1.6. Consequently, the Mo-S network presents interconnected Mo 12 S 14 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 Mo 12 S 14 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 1.08 Mo 12 S 14 . Electrical resistivity measurements performed on single crystals of Rb 2.65 Mo 12 S 14 and Rb 1.08 Mo 12 S 14shows 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 Mo 12 cluster with respect to the cationic charge x is discussed and, compared with that found in the potassium series.