Oxide ion conductivity, phase transitions, and phase separation in fluorite-based Bi_{38- x}Mo_{7+ x}O_{78+1.5 x}

We present, for the first time, the ionic conductivity properties of two different, but closely related, bismuth molybdates: Bi₃₈Mo ₇O₇₈ and Bi_37.5Mo_7.5O _78.75. Both are good oxide ion conductors, with the latter being comparable to yttria-stabilized zirconia. We show that the structure of Bi ₃₈Mo₇O₇₈ is more complex than previously reported, and that this compound is a 5×3×6 fluorite superstructure with slight monoclinic distortion. In addition to being a good oxide ion conductor, the material is noncentrosymmetric-polar and second harmonic generation (SHG) active. The second phase, orthorhombic Bi_37.5Mo _7.5O_78.75, reported for the first time, is an excellent oxide ion conductor. The materials have been characterized by impedance spectroscopy, variable-temperature synchrotron, neutron and laboratory powder X-ray diffraction, electron diffraction, and SHG measurements.