Syntheses and Structural, Physical, and Theoretical Studies of the Novel Isostructural Mo₉ Cluster Compounds Ag_2.6CsMo₉Se₁₁, Ag_4.1ClMo₉Se₁₁, and h-Mo₉Se₁₁ with Tunnel Structures

The new isostructural compounds Ag2.6CsMo9Se11 (1) and Ag4.1ClMo9Se11 (2) were prepared by solid-state reaction in evacuated sealed silica tubes at 1200 °C and 860 °C, respectively. By topotactic reduction−oxidation reaction of Ag4.1ClMo9Se11 with I2, we also obtained the metastable compound h-Mo9Se11 (3). The three compounds crystallize in the hexagonal space group P63/m, Z = 2, (1) a = 10.0472(2) Å, c = 11.9548(2), (2) a = 10.0321(2) Å, c = 11.8734(2) Å, and (3) a = 9.4204(2) Å, c = 12.1226(2) Å. 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 Ag2.6CsMo9Se11 and Ag4.1ClMo9Se11 in which the number of electrons per Mo9 cluster is different indicate that the former is semiconducting whereas the latter is semimetallic.