Effect of high bismuth deficiency on structure and oxide ion conductivity of a Bi_0.55MoO₄ single crystal

We report, for the first time, the ionic conductivity properties of two polymorphic structures of bismuth molybdates: Bi_0.55MoO₄ and Bi₂(MoO₄)₃. A Bi_0.55MoO₄ single crystal was first synthesized, presenting a surprisingly high bismuth deficiency compared to the known stoichiometric phase Bi₂(MoO₄)₃. It is expected to have a good oxide ion conduction due to the presence of Bi deficiency and oxygen vacancies. Bi_0.55MoO₄ crystallizes in the tetragonal system, space group I4₁/a with a = b = 5.239(2), c = 11.567(5) Å, and Z = 4. While Bi₂(MoO₄)₃ crystallizes in the monoclinic space group P2₁/n, with unit cell parameters a = 7.71500(10), b = 11.5101(2), c = 11.1237(2) Å, β = 103.3640(10)°, and Z = 4. Taking an advantage over many other ionic conductors, the crystals do not undergo any clear structural phase transition below their melting points. The oxide ion conductivities of Bi_0.55MoO₄ and Bi₂(MoO₄)₃ were measured to be 6.5 × 10^-5 S cm^-1 and 1.1 × 10^-6 S cm^-1 at a temperature of 400 °C, respectively.