Synthesis, Structural Evolution, and Theoretical and Physical Studies of the Novel Compounds M₂Mo₉S₁₁ (M = K, Rb) and Related Metastable Materials Cu<i>_x</i>K_1.8Mo₉S₁₁ (<i>x</i> = 0 or 2) Containing Bioctahedral Mo₉ Clusters

The new isostructural K2Mo9S11 and Rb2Mo9S11 phases were prepared by solid-state reaction at 1500 °C in a sealed molybdenum crucible. Both compounds crystallize in the trigonal space group (SG) R3̄c, Z = 6, a = 9.271(1) Å, c = 35.985(9) Å and a = 9.356(2) Å, c = 35.935(9) Å for the K and Rb compounds, respectively, in the hexagonal setting. Their crystal structures were determined from single-crystal X-ray diffraction data and consist of interconnected Mo9S11 units forming an original and unprecedented three-dimensional framework. Extended Hückel tight-binding (EHTB) calculations carried out on K2Mo9S11 indicate that such compounds are electron-deficient and may be reduced without altering the arrangement of the Mo9S11 units. This was verified by the insertion of copper into K2Mo9S11 by topotactic oxydo-reduction reaction, which leads to the new metastable Cu2K1.8Mo9S11 compound (SG R3̄c, a = 9.4215(4) Å, c = 35.444(2) Å, Z = 6). The potassium nonstoichiometry of this quaternary phase was confirmed by deintercalation of the copper in a HCl 12 M solution at 80 °C, leading to the K1.8Mo9S11 phase (SG R3̄c, a = 9.2801(8) Å, c = 35.833(7) Å, Z = 6). The X-ray single-crystal structures of K1.8Mo9S11 and Cu2K1.8Mo9S11 are also described. Electrical resistivity measurements carried out on single crystals of K2Mo9S11 and Cu2K1.8Mo9S11 indicate that the former is metallic whereas the latter is semiconducting, as expected from EHTB calculations. Magnetic and electrical resistivity measurements performed on K1.8Mo9S11 reveal a superconducting behavior below 4.5 K.