Syntheses and structural, physical, and theoretical studies of the novel isostructural Mo9 cluster compounds Ag(2.6)CsMo9Se11, Ag(4.1)ClMo9Se11, and h-Mo9Se11 with tunnel structures.

The new isostructural compounds Ag(2.6)CsMo9Se11 (1) and Ag(4.1)ClMo9Se11 (2) were prepared by solid-state reaction in evacuated sealed silica tubes at 1200 degrees C and 860 degrees C, respectively. By topotactic reduction-oxidation reaction of Ag(4.1)ClMo9Se11 with I2, we also obtained the metastable compound h-Mo9Se11 (3). The three compounds crystallize in the hexagonal space group P6(3)/m, Z = 2, (1) a = 10.0472(2) A, c = 11.9548(2), (2) a = 10.0321(2) A, c = 11.8734(2) A, and (3) a = 9.4204(2) A, c = 12.1226(2) A. Their crystal structures were determined from single-crystal X-ray diffraction data and consist of interconnected Mo9Se11 units forming an original and unprecedented three-dimensional framework in which large tunnels are occupied randomly by a part of the Ag+ and the Cl- ions in 2 and the Cs+ ions in 1, the remaining Ag+ in 1 being localized in mirror planes around the 3-fold axis. First-principle calculations allow the understanding of the variation of the atomic distances. Electrical resistivity measurements carried out on single crystals of Ag(2.6)CsMo9Se11 and Ag(4.1)ClMo9Se11 in which the number of electrons per Mo9 cluster is different indicate that the former is semiconducting whereas the latter is semimetallic.