Na0.74Ta3O6, a low-valent oxotantalate with multiple Ta-Ta bonds

The title compound was prepared in a sealed tantalum tube through the react ion of Ta2O5, tantalum and Na2CO3 in a NaCl flux at 1570 K within 5 d. The crystal structure of Na0.74Ta3O6 (a = 713.5(1), b = 1027.4(2), c = 639.9(1) pm, Immm, Z = 4) was determined by single crystal X-ray means. The structu re is isomorphous with NaNb3O5F [1]. The characteristic structural units ar e triply bonded Tal(2) dumb-bells with eight square-prismatically coordinat ed O ligands. Four Ta2, each octahedrally surrounded by O atoms, are side-o n bonded weakly to the binuclear Ta2O8 complex, thus forming a Ta-6 propell ane-like cluster. The lattice parameters of three additional MxTa3O6 phases , M = Li, Mn, and Yb, are reported.