Polymorphism and Electrical Properties in the New Oxide Bi6Mo2O15

new oxide of composition Bi6Mo2O15 has been isolated in the binary system Bi2O3-MoO3. A wet-chemistry procedure, different from coprecipitation, has been used to prepare a nanosized, very reactive precursor. Annealing at different temperatures and for different time lengths leads to isolated polycrystalline polymorphs. X-ray powder diffraction studies on the hitherto unknown low-temperature form, L-Bi6Mo2O15, show that it crystallizes in the monoclinic system, with unit-cell parameters a = 29.0674(5) A, b = 5.64795(7) A, c = 8.6620(1) A, (beta)= 97.979(1), and V = 1408.3 A3. The high-temperature polymorph, HBi6Mo2O15, belongs to the well-known [Bi12O14] columnar structural type. The relationship between the unit-cell parameters of both phases points at the connection of their structural frameworks. Impedance spectroscopy measurements show that the transition LH -Bi6Mo2O15 is partially reversible, as well as the existence of a second high-temperature phase Hʹ, similar to H. In contrast, the transition HHʹBi6Mo2O15 is not reversible. These materials turn out to be good ionic conductors, with conductivities in the order L < H < Hʹ for all temperatures tested.