Synthesis, Structural Evolution, and Theoretical and Physical Studies of the Novel Compounds M(2)Mo(9)S(11) (M = K, Rb) and Related Metastable Materials Cu(x)()K(1.8)Mo(9)S(11) (x = 0 or 2) Containing Bioctahedral Mo(9) Clusters.

Abstract

The new isostructural K(2)Mo(9)S(11) and Rb(2)Mo(9)S(11) phases were prepared by solid-state reaction at 1500 degrees C in a sealed molybdenum crucible. Both compounds crystallize in the trigonal space group (SG) R&thremacr;c, 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 Mo(9)S(11) units forming an original and unprecedented three-dimensional framework. Extended Hückel tight-binding (EHTB) calculations carried out on K(2)Mo(9)S(11) indicate that such compounds are electron-deficient and may be reduced without altering the arrangement of the Mo(9)S(11) units. This was verified by the insertion of copper into K(2)Mo(9)S(11) by topotactic oxydo-reduction reaction, which leads to the new metastable Cu(2)K(1.8)Mo(9)S(11) compound (SG R&thremacr;c, 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 degrees C, leading to the K(1.8)Mo(9)S(11) phase (SG R&thremacr;c, a = 9.2801(8) Å, c = 35.833(7) Å, Z = 6). The X-ray single-crystal structures of K(1.8)Mo(9)S(11) and Cu(2)K(1.8)Mo(9)S(11) are also described. Electrical resistivity measurements carried out on single crystals of K(2)Mo(9)S(11) and Cu(2)K(1.8)Mo(9)S(11) indicate that the former is metallic whereas the latter is semiconducting, as expected from EHTB calculations. Magnetic and electrical resistivity measurements performed on K(1.8)Mo(9)S(11) reveal a superconducting behavior below 4.5 K.