Structure and conductivity of Nd6MoO12-based potential electron–proton conductors under dry and wet redox conditions

Abstract

Nominal compositions Nd₆MoO₁₂ (3 : 1) and Nd₁₀Mo₂O₂₁ (5 : 2) were prepared by high-temperature synthesis in air from a mechanically activated mixture of Nd₂O₃ and MoO₃. Phase-pure Nd₆MoO_12−δ and Nd₆MoO_12−α have been obtained at 1600 and 1650 °C, respectively. They are both slightly rhombohedrally distorted derivatives from the cubic fluorite structure. Nd₁₀Mo₂O₂₁ with a lower content of Nd₂O₃ was shown to be a more complex phase based on the rhombohedral phase (R) in the 1600–1650 °C temperature range. As a result of the formation of a more complex rhombohedral phase, the conductivity of Nd₁₀Mo₂O₂₁ changes dramatically in comparison with Nd₆MoO_12−δ. In wet air Nd₆MoO_12−δ is a p-type electronic conductor, whereas proton conductivity dominates in Nd₁₀Mo₂O₂₁ over the entire temperature range studied. The electrical conductivity dependence of Nd₆MoO_12−δ on the oxygen partial pressure shows a V-type behaviour typical of a transition from a p-type to n-type conductivity mechanism at 800 ≤ T ≤ 1000 °C. There is no p-type conductivity contribution in Nd₁₀Mo₂O₂₁ in the same temperature range. The prevalence of electronic conductivity in the samples with nominal composition Nd₆MoO₁₂ in a wide temperature range is due to the fact that Nd and Mo in the fluorite materials are readily reduced. Predominantly Nd³⁺ and Mo⁶⁺ forms exist in more complex rhombohedral phase Nd₁₀Mo₂O₂₁ and it has proton conductivity ∼8.5 × 10⁻³ S cm⁻¹ at 800 °C. Thus, the loss of dimensional stability is more characteristic of fluorites and rhombohedral phases with small rhombohedral distortion (Nd_5.4Zr_0.6MoO_12.3, Nd₆MoO_12−δ) than more complex rhombohedral phases based on (R) (Nd₁₀Mo₂O₂₁). A comparative high-temperature in situ neutron diffraction study under high vacuum of rhombohedral Nd₆MoO_12−δ and cubic fluorite Nd_5.4Zr_0.6MoO_12.3 showed that the former transforms to the high-temperature cubic fluorite type above ∼1140 °C while the latter retains its cubic structure in the studied range up to 1350 °C.